Employment 2. Surgical Anatomy of
Internal and External Skull’s Base, Twelve Pairs of
Cranial Nerves. Blood Supply of the Brain, its Tunics and Sinuses. Intermeningeal
Spaces, Circulation of Liquor. Clinical Anatomy of
Skull’s Base Fractures. Topographical Anatomy of
Temporal Region. Cranio-cerebral Topography. Decompressive and Osteo-Plastic Trepanation
of the Skull. Antrotomy.
NORMA
BASALIS
This view
is obtained when the skull is turned upside down, thus exposing the external
surface of its base. The anterior part of this aspect is occupied by the bony
palate, which is formed by the palatine processes of the maxillae and the
horizontal plates of the palatine bones. In the median plane anteriorly, the incisive
fossa receives the openings of the lateral incisive canals, which transmit
the terminal parts of the greater palatine vessels to the nose and the
descending terminal branches of the long sphenopalatine nerves. Anterior and
posterior median incisive canals are sometimes present. The greater palatine
fossa, which transmits the greater palatine vessels and nerves, is found in
the posterolateral corner near the last molar. The lesser palatine fossae lie
immediately behind the greater. Behind and above the hard palate are the choanae
(the posterior bony apertures of the nose). These are separated from each
other by the vomer and are bounded laterally by the medial pterygoid plate. The
pterygoid plates are a pair of large lateral and medial processes
projecting downward from the roots of the greater wings of the sphenoid bones.
Between these processes is an interval known as the pterygoid fossa, which
opens posteriorly and is about half an inch in width. The free border of the
medial plate ends below in a hook called the hamulus. This gives
attachment at its tip to the pterygomandibular ligament, and by its posterior
border to the upper fibers of the superior constrictor muscle of the pharynx.
The tensor palati tendon twists around its lateral and anterior aspects. The
lateral plate gives origin to the lateral pterygoid muscle on its lateral
surface and to the medial pterygoid on its medial surface. Lateral to the
structures just described is the roof of the infratemporal fossa.
Posterolateral to the plates, the foramen ovale is found, which is quite
large and transmits the mandibular nerve, the accessory meningeal artery and
some small veins that connect the cavernous venous sinus with the pterygoid venous plexus. Some
lymph vessels from the meninges also pass through this foramen, as does the
lesser superficial petrosal nerve at times. Posterolateral to the foramen ovale
is the foramen spinosum, which transmits the middle meningeal vessels.
The zygomatic arch is a prominent feature of this aspect. At its caudal
end is found the articular fossa, which receives the articular process of the
mandible. The foramen lacerum is a large and jagged aperture located at
the base of the medial pterygoid plate. The carotid canal, which is
posterolateral to the foramen lacerum, is a tunnel in the petrous portion of
the temporal bone through which the internal carotid artery travels on its way
to the cranial cavity. From its opening the canal leads upward for a short
distance, bends to become horizontal and runs in a medial direction and forward
to open into the foramen lacerum. The canal is in immediate relationship to the
middle and the internal ears. The thumping sounds that one hears in the head
during moments
FIG
The top of the skull, viewed from above (norma
verticalis).
of
excitement or after violent exertion are due to the beating of the internal
carotid artery against the bone that separates it from the internal ear.The jugular
foramen is a large opening with uneven margins situated directly behind the
carotid canal. The largest structure in this foramen is the internal jugular
vein. Other structures associated with it will be reviewed when the interior of
the base of the skull is discussed. The jugular foramen is opposite the
external auditory meatus, and that part of the bone which bounds the foramen
forms the floor of the middle ear. It is important to keep this relationship in
mind since, in diseases of the middle ear, infection
may pass through the bone and attack the internal jugular vein. Directly
lateral to the foramen is the styloid process. Two ligaments (the
stylohyoid and the stylomandibular) and 3 muscles (the styloglossus, the
stylohyoid and the stylopharyngeus) are attached to this process. The
stylohyoid ligament runs from its tip to the hyoid bone, and the
stylomandibular ligament extends from the front of it to the posterior border
of the mandible. The stylomandibular ligament is a thickened part of the fascia
that covers the anteromedial aspect of the parotid gland. The stylo-mastoid
foramen is found immediately at the base of the styloid process and is the
FIG
The external surface of the base of the skull (norma
basalis).
foramen
that transmits the facial nerve from the brain to the exterior of the skull.
The stylomastoid branches of the posterior auricular vessels are also transmitted
by this foramen. The mastoid process can be palpated under cover of the
lobule of the auricle but is not recognizable as a bony structure until the end
of the 2nd year. The mastoid foramen, which is variable in size and
position, is found posterior to the mastoid process. It transmits a vein to the
transverse sinus and a small branch of the occipital artery to the dura mater.
The foramen magnum, the largest bony foramen in the skull, is the
opening through which the medulla oblongata, or lowest subdivision of the
brain, becomes continuous with the spinal cord. Its level is approximately the
same as that of the mastoid process on the side of the head, and it is opposite
a point on the back of the neck midway between the external occipital protuberance
and the spine of the second cervical vertebra. The occipital condyles are
the large, smooth and rather oblong protuberances that lie at the margins of
the foramen magnum. They articulate with the atlas, and nodding movements of
the head take place at the joints between the atlas and the condyles.
FIG. X-ray study of the front of
the skull (posteroanterior projection): (1) parietal bone, (2) coronal suture,
(3) frontal sinuses, (4) crista galli, (5) sphenoid bone and sinus, (6) zygoma,
(7) lesser wing of sphenoid bone.
The anterior condylar canal is above the
lateral margin of the anterior part of the condyle. It is usually hidden by the
condyle, and the skull must be tilted before the opening can be seen. The
anterior condylar canal is smaller than the jugular foramen and is the opening
that transmits the hypoglossal nerve. The posterior condylar canal, when
present, passes above the posterior part of the condyle and opens into the posterior
fossa. It transmits an emissary vein that connects the sigmoid venous sinus
with the suboccipital venous plexus. Behind the foramen magnum a bony crest is
noted, known as the external occipital crest, which ends in an elevation
called the external occipital protuberance (inion). From the region of
the midpoint on this crest the inferior nuchal line curves laterally on
each side, but the line is often poorly defined and difficult to see. The superior
nuchal line curves laterally on each side from the
external occipital protuberance and separates the scalp area above from
the area for the neck muscles (nuchal area) below.
BASE OF THE
SKULL AND THE CRANIAL FOSSAE
The base of
the skull on its inner surface shows a natural subdivision into 3 cranial
fossae: anterior, middle and posterior. Since the anterior fossa is on a
higher plane than
FIG. The upper
surface of the base of the skull. The anterior fossa is on a higher
plane than the middle fossa, and the middle is higher than the posterior; in
this way three terraces are formed.
the
middle, and the middle is higher than the posterior, there is a natural
tendency toward the formation of 3 terraces. The anterior cranial fossa is
limited posteriorly by the posterior edges of the lesser wings of the sphenoid
and in the median part by the anterior edge of the optic groove of the
sphenoid. It lodges the frontal lobes of the brain and the olfactory bulbs and
tracts. The floor of the fossa is depressed in its median part, where it
constitutes the roof of the nasal cavity. The median part is formed by the
cribriform plate of the ethmoid bone, through which the crista galli, or
cock's comb, rises. It is an upward continuation of the nasal septum and gives
attachment to the anterior end of the falx cerebri. The foramen caecum is
a small pit found directly in front of the crista. In early life the superior
longitudinal sinus communicates with the veins of the nose through this
foramen, but in the adult it is usually closed, hence its name—caecum (blind).
The cribriform plate is perforated like a sieve by numerous olfactory nerves,
which are clothed in an arachnoid sheath and arise from the olfactory cells in
the nasal mucosa. At the side of the cribriform plate the anterior and
the posterior ethmoidal foramina are found. They mark the medial ends of
two short canals that lead from the orbital cavity and open at the side of the
cribriform plate; they transmit the anterior and the posterior ethmoidal
arteries and the anterior ethmoidal nerve. The anterior ethmoidal artery and
nerve, after passing through the foramina, run on the cribriform plate and then
descend into the nose through the nasal slit which is found at the side
of the front of the crista galli. Anterolateral to the median area, the roof of
the frontal sinus and the roof of the orbit are found. Fractures of the
anterior fossa may involve the cribriform plate and be accompanied by
lacerations of the meninges and the mucous membrane of the roof of the nose.
Such an injury gives rise to epistaxis, accompanied or followed by a discharge
of cerebrospinal fluid. There may result some loss of smell due to laceration
of the olfactory nervfcs as they pass upward from the nose and, if dural injury
is present, it affords a route whereby infection can travel to the intracranial
region from the nose. Meningitis or abscess in the frontal lobe may be a
sequela of this type of fracture. If the cribriform plate does not heal after
fracture and if a dural laceration remains unrepaired, there may be a continuous
discharge of cerebrospinal fluid from the nose, known as cerebrospinal
rhinorrhea. When the fracture involves the orbital plate of the frontal bone,
subconjunctival hemorrhage is a characteristic feature, and the hemorrhage may
seep within the orbit, producing an exophthalmos. The frontal sinus may also be
involved. The middle cranial fossa is shaped like a butterfly, having a
small median and two lateral expanded concave parts. The median part is formed
by the upper surface of the body of the sphenoid. The sella turcica is
the saddle-shaped area that accommodates the pituitary gland. Anteriorly is the
ridge known as the tuberculum sellae, on either side of which is an anterior
clinoid process. Immediately anterior to this process the optic foramen is
situated at the end of the optic groove. The posterior part of the sella
turcica is formed by the crest of the dorsum sellae, ending laterally in the posterior
clinoid process The lateral part of the floor of the middle cranial fossa
is formed by the greater wing of the sphenoid, the upper aspect of the petrous
part of the temporal and a portion of the squamous part of the temporal bone.
These lateral parts lodge the temporal lobes of the brain. The superior
orbital fissure transmits to the orbital cavity the oculomotor, the
trochlear, the ophthalmic division of the trigeminal and the abducens nerves,
some filaments from the cavernous plexus of the sympathetic system and the
orbital branch of the middle meningeal artery. From the orbital cavity this
fissure also transmits the ophthalmic veins and a recurrent branch of the
lacrimal artery to the dura mater.On either side of the sella is the carotid
groove for the internal carotid artery. Three foramina run almost parallel
with this groove. These are, from anterior to posterior and from medial to
lateral: the foramen rotundum for the passage of the maxillary nerve,
the foramen ovale for the mandibular nerve, the accessory meningeal
artery and the lesser petrosal nerve, and the foramen spinosum for the
passage of the middle meningeal vessels and a recurrent branch of the
mandibular nerve. Medial to the foramen ovale is the foramen lacerum, a
short, wide canal rather than a foramen, its lower part being filled by a layer
of fibrocartilage. Its upper and inner parts transmit the internal carotid
artery, which is surrounded by a plexus of sympathetic nerves. The petrous
portion of the temporal bone forms a large and important part of the floor of
the fossa. The highest part of this bone is known as the arcuate eminence and
marks the position of the superior semicircular canal. Lateral to the eminence
and immediately adjoining the squamous portion of the bone, the tegmen
tympani is found. This is a very thin plate of bone which roofs the
tympanic antrum, the tympanic cavity and the auditory tube. The important
relationship of the thin tegmen tympani intervening between the inferior
surface of the temporal lobe of the brain and the tympanic cavity cannot be
overemphasized. This bone is the only barrier which exists between a diseased
middle ear and the membranes of the brain or the brain itself. The hiatus for
the greater superficial petrosal nerve is a small slit seen lower down
on the anterior surface and about midway between the apex of the petrous
temporal and the side of the skull. It communicates with the facial canal in
the interior of the bone and transmits a slender nerve from which it takes its
name. This nerve has its origin from the facial in the substance of the
temporal bone and runs in a medial direction forward to the foramen lacerum.
The trigeminal impression is found at the upper aspect of the apex of
the petrous temporal and is represented by a slightly hollowed-out area. In it
is lodged the trigeminal ganglion, which extends forward over the upper and the
lateral parts of the foramen lacerum. The middle fossa is the commonest site of
fracture of the skull because of its position and because it is weakened by
numerous foramina and canals. Frequently, the tegmen tympani is fractured, and the tympanic membrane torn. Then blood and
cerebrospinal fluid are discharged from the external auditory meatus and appear
at the ear. The facial and the auditory nerves may be involved. At times the
walls of the cavernous sinus are lacerated, and
cranial nerves 3, 4 and 6, which lie in relation to its lateral wall, may also
be injured. Fractures involving the middle cranial fossa may also pass through
the sphenoid bone or the base of the occipital bone and cause bleeding into the
mouth. The posterior cranial fossa is the largest and deepest of the
cranial fossae and lodges
the
hind brain (cerebellum, pons and medulla oblongata). Its floor is formed by the
basilar, the condylar and the squamous parts of the occipital bone; its lateral
wall, by the posterior surface of the petrous and the medial surface of the
mastoid part of the temporal bone. The foramen magnum is the most
prominent feature of the fossa. At the anterolateral boundary of the foramen
the anterior condylar canal is found which transmits the hypoglossal nerve.
This nerve arises by several roots of origin, and the canal is frequently
divided into two parts by a small bar of bone. The foramen magnum transmits a
number of structures, the most important being the medulla oblongata, the
meninges, the vertebral arteries and the ascending parts of the accessory
nerves. This foramen marks the lowest part of the posterior cranial fossa.
The clivus
is the broad, sloping surface that exists between the anterior margin of
the foramen magnum and the root of the dorsum sellae; it is related to the pons
and the medulla oblongata. The internal auditory meatus is found at the
posterior aspect of the petrous temporal and runs laterally into the bone.
Through it pass the motor and the sensory roots of the facial nerve, the auditory
nerve, the internal auditory branch of the basilar artery and the auditory vein
which joins the inferior petrosal sinus. The jugular foramen is situated
between the lateral part of the occipital and the petrous part of the temporal
bone. It is a large aperture with irregular margins and transmits three sets of
structures. At times small spicules of bone project from its margin and may
divide it partly or completely into corresponding compartments. The
anteromedial compartment transmits the inferior petrosal sinus and a meningeal
branch of the ascending pharyngeal artery. The middle compartment transmits the
glossopharyngeal, the vagus and accessory nerves. The posterolateral
compartment is larger than the other two and transmits the sigmoid sinus as it
becomes the internal jugular vein, and a meningeal branch of the occipital
artery. The inferior petrosal sinus, which passes through the anterior part of
the foramen, becomes the internal jugular vein immediately outside of the
skull. The transverse groove begins at the side of the internal
occipital protuberance and sweeps around the cranial vault to the lateral end
of the upper margin of the petrous temporal. It then joins the sigmoid
groove, which curves downward and descends along the side wall of the skull
and extends in a medial direction to end at the jugular foramen. The right
transverse groove is wider than the left because it usually receives the
sagittal sinus. The mastoid foramen is an aperture of variable size
which leads from the exterior of the skull into the sigmoid groove on the side
wall of the posterior cranial fossa. Through it a mastoid vein and the mastoid
emissary vein and the mastoid branch of the occipital artery pass. The aqueduct
of the vestibule (aqueductus vestibuli) is found about Vi
inch lateral to the internal auditory meatus. Fractures of the posterior
fossa are probably more important than such injuries in the other fossae, since
it is here that a small fissure fracture may prove to be fatal. The bone is
thin in places and, since there is no outlet for the escape of blood or
cerebrospinal fluid as in the anterior and the middle fossae, these fractures
may be overlooked. Some days after the injury, blood may be noted over the
mastoid process. Fractures of the base of the skull involving the hypoglossal
canal may be manifested by paralysis of one side of the tongue.
FIG.The two methods used to expose the brain, its coverings and vessels.
Trephine operation: (A) skin flap formed, turned down and trephine in place;
(B) removal of trephined "button" of bone; (C) incision into dura
mater; (D) dural flap formed and reflected; (E) closure of dura. Osteoplastic
craniotomy: (1) soft tissues incised, bone exposed and trephine openings made;
craniotome divides the bone; (2) bone is fractured at the base of the flap; (3)
dura is divided and underlying structures exposed; (4) dural closure.
FIG.Subtemporal decompression:
(A) Line of incision and amount of bone to be removed; the incision is placed
about three fifths of an inch in front of the tragus and extends upward and
slightly backward for about
SURGICAL
CONSIDERATIONS
TREPHINING
OPERATIONS Two methods are usually employed to expose the brain: trephining and
osteoplastic resection In trephining
operations a circular disk of a cranial bone is removed by use of a
trephine. The main indications for such operations are hemorrhage, abscess,
fracture, evacuation of cerebrospinal fluid, or as a preliminary step to
further brain surgery. A U-shaped or linear incision is made. If the former is
used, its convexity is placed toward the crown of the head and the pedicle
toward the base. The size of the flap is much larger than the bone which is to
be removed. The incision passes through the skin, the superficial fascia, the muscle
and the periosteum to the bone, and hemostasis is accomplished as the operation
proceeds. With the trephine site cleared, a piece of bone is removed and, if a
larger opening is needed, it may be obtained by removing pieces of bone from
the circumference with a rongeur forceps. The dura is exposed and can be
opened, but any large dural vessels should be ligated first. The necessary
operative procedure is carried out, and the dural flap is sutured back into its
normal position. The bone may or may not be replaced, and the wound is closed
in layers.
OSTEOPLASTIC
CRANIOTOMY Osteoplastic craniotomy implies the raising of a portion of skull
which may be replaced when the operation is completed. Lateral, frontal,
transfrontal, occipital and suboccipital osteoplastic flaps have been
described, depending upon the area to be operated The
incision passes through all the soft tissues down to the bone. Vessels are
clamped and ligated, and the periosteum is detached for a short distance along
the line of the contemplated bone incision. Openings are made along the bone
margins by means of a drill, a burr or a small trephine, and the bone that
intervenes is divided by a saw or rongeur forceps. The base of the pedicle is
steadied, usually with the left hand. The upper portion of the flap is grasped
with a cranial claw forceps, and with a quick jerk the bone is fractured. The
flap thus created is turned back, and the dura is opened by means of a similar
but smaller flap. The necessary operative procedure is carried out, the dura is
closed by fine interrupted sutures, and the bone flap with its attached soft
parts is replaced and sutured into position.
SUBTEMPORAL
DECOMPRESSION A subtemporal decompression is
really a craniectomy, which implies the removal of a portion of the skull,
leaving a permanent gap. Such a procedure is necessary in about 10 per cent of
all cases of severe head injuries where it is desired to give the brain room
for expansion. The permanent bone defect should be covered over, if possible,
by muscle so that a herniation of the brain does not result. Since the temporal
muscle is conveniently situated, the decompression is usually made subjacent to
it, hence the name "subtemporal decompression." In this operation the
skin incision, beginning at the zygoma, is placed three fifths of an inch in
front of the tragus and extends upward and slightly backward for about
INTRACRANIAL
HEMORRHAGE A line known as the eye-ear line, or Reid's base line, is
utilized in cranial topography. It extends from the lower margin of the orbit
to the upper border of the external auditory meatus. Some anatomists prefer to
refer to a horizontal plane known as the
Extradural
Hemorrhage. Extradural
hemorrhage is usually caused by an injury to the middle meningeal artery or one
of its branches. This vessel arises from the internal maxillary artery and
enters the cranium via the foramen spinosum. It passes upward and forward for a
short distance over the great wing of the sphenoid and soon divides into
anterior and posterior branches, which ramify upon the dura and supply the
greater part of its lateral and superior surfaces. The anterior branch, which
is the larger, continues obliquely forward over the great wing to the
antero-inferior angle of the parietal bone, in which it forms a deep groove. It
ascends in this groove behind the anterior margin of that bone almost as far as
the sagittal suture. The posterior branch passes upward and backward over the
squamous portion of the temporal bone. The artery is accompanied by its two
venae comites. The anterior branch is found readily through an opening
which is made 1 1/2 inches behind the external angular process of the
frontal bone and a similar distance above the upper border of the zygoma. This
is the branch that is damaged most frequently and, since it is closely related
to the motor area of the cortex, injury to it might produce a loss of power in
the muscles of the opposite side of the body. The posterior branch can
be reached through a trephine hole
Subdural
Hemorrhage. When subdural
hemorrhage is present, it might become necessary to explore through a trephine
opening to locate the point of hemorrhage. After the skull has been opened, the
dura is tense and plum colored, signifying extravasated blood beneath it. The
exploratory trephine hole is enlarged, the dura is opened, and necessary
hemostatic measures are carried out.
FIG. Extradural hemorrhage: (A)
cranial topography, location of the anterior and posterior branches of the
middle meningeal artery; (B) incision for ligation of the middle meningeal
artery; (C) extradural hematoma located and vessel clamped.
MENINGES
FIG
Diagrammatic frontal section through the scalp, the skull, the meninges and the
brain. The arachnoid villi invade the dura, and the
subarachnoid space is trabeculated. The 4 intracranial spaces should be noted.
FIG The 4 membranes formed by the infolding of the dura mater
(falx cerebri, falx cerebelli, tentorium cerebelli and diaphragma sellae). The
venous sinuses and the cranial nerves are also shown.
The brain and the spinal cord are surrounded by
three enveloping membranes, which are known from inside out as the pia mater,
the arachnoid mater and the dura mater. Their names suggest their qualities:
the dura is tough and firm, the arachnoid resembles a spider's web, and the pia
represents a very thin, clinging, skinlike structure that hugs the surface of
the brain and follows its irregularities. The dura and the arachnoid do not dip
into the fissures but fit the brain as a child's mitten fits its hand; on the
other hand, the pia mater dips into each fissure and fits the brain very much
as a glove fits the hand, since each finger has its own indentation (see venous
sinuses of the dura mater.The dura mater is
the most external membrane of the brain; it consists of two layers that are
firmly blended with each other except in certain locations. The more
superficial of these layers is the endocranium, which is a periosteum
(endoperiosteum). Through the openings in the skull it is continuous with the
external periosteum (pericranium). The endoperiosteum is the layer that is intimately
related to the bones of the skull and in no way takes part in the formation of
the falx cerebri or the tentorium cerebelli. Bulging arachnoid granulations
(enlarged villi of the arachnoid projecting through the layers of the dura
mater) project from each side of the median sagittal plane and produce the pits
found on the parietal bone. The middle meningeal vessel ascends in the dura and
produces a groove in the parietal bone. The deeper or inner layer of dura is
smooth and lined by endothelial cells. It resembles a serous membrane and is
separated from the superficial layers by a small amount of fibrous tissue. The
venous sinuses and the meningeal vessels separate the 2 layers of dura. By a
process of infolding and reduplicating itself, the inner layer of dura forms 4
membranes that subdivide the cranial cavities. These membranes are the falx
cerebri, the falx cerebelli, the tentorium cerebelli and the diaphragma sellae.
The sickle-shaped falx cerebri is placed vertically between the 2
hemispheres of the cerebrum and is a reduplication of the inner layer of the
dura. It consists of 2 layers of serous dura. Its upper border is convex and
attached to the crista galli in front; it extends back to the internal
occipital protuberance and between these two points is attached to the internal
surface of the skull. Its lower border is attached to the tentorium cerebelli
behind but otherwise remains free to project between the cerebral hemispheres
in front of the tentorium. The falx is narrow in front and becomes wider as it
is traced backward. The superior sagittal sinus appears in its upper border;
its lower border contains the inferior sagittal sinus and aids the tentorium in
the support of the straight sinus. The falx cerebelli passes vertically
from the tentorium to the foramen magnum and separates the 2 cerebellar
hemispheres. It attaches posteriorly to the internal occipital crest, where it
encloses the occipital sinus. Construction of the falx cerebelli is exactly the
same as that of the falx cerebri.
FIG. Coronal section through the foramen magnum, showing the
relationships of the meninges, the venous sinuses and the blood vessels.
The tentorium cerebelli is a tentlike fold
of a double layer of serous dura mater, forming a partition between the cerebellum
and the posterior part of the cerebral hemispheres. It forms a roof for the
cerebellum and a floor for the occipital lobe and the posterior part of the
temporal lobe of the cerebrum. Anteriorly, a wide gap known as the tentorial
notch permits the passage of the midbrain. Because of this arrangement the
tentorium possesses a free inner and an attached outer border. This outer
border has 3 attachments: to the margins of the groove of the transverse sinus
of the occipital bone; to the margins of the groove for the superior petrosal
sinus on the petrous portion of the temporal bone; to the posterior clinoid
process of the sphenoid bone. The free border runs forward to the anterior
clinoid process, and the upper layer of the tentorium becomes continuous with
the falx cerebri in the median plane. The diaphragma sellae is also a
fold of inner layer of dura mater with a foramen in its center. Its lateral
border is attached to the clinoid processes; its medial border forms the
boundary of the foramen of the diaphragma sellae and also surrounds the
infundibulum. The superior surface of the diaphragm is in relation to the base
of the brain; its inferior aspect is related to the hypophysis, which is bound
by it to the hypophyseal fossa. The arachnoid mater, a
delicate membrane enveloping the brain and medulla spinalis, lies between the
pia mater internally and the dura mater externally. It does not dip into
the various sulci on the surface of the brain, but is carried into the
longitudinal fissure by the falx cerebri. Over the convolutions the arachnoid
and the pia are in close contact but are separated at the sulci by the
subarachnoid space, which contains the cerebrospinal fluid and is crossed by a
gauzy retinaculum of cobweblike fibers connecting the two membranes. At the
base of the brain this network is much reduced and the two membranes are widely
separated to form the so-called subarachnoid cisternae. The three main
cisternae are: 1. The cisterna cerebromedullaris (cisterna
magna) is a cavity resulting from the arachnoid's bridging the inferior surface
of the cerebellum and the dorsal surface of the medulla oblongata. It is
continuous below with the spinal subarachnoid space. Cerebrospinal fluid passes
directly into this cistern from the fourth ventricle by means of the foramen of
Magendie (median aperture). 2. The cisterna ponds, a space lying in
front of the pons and the medulla oblongata, is continuous with the
subarachnoid space about the medulla and has been referred to as "Hilton's
water bed," since it forms a water cushion to protect the brain. The roots
of the lower 8 th cranial nerves traverse this cavity.
3. The cisterna interpeduncularis, a wide cavity formed by the arachnoid
as it extends across and between the two temporal lobes, encloses the cerebral
peduncles and contains the arterial circle of Willis. Some consider it part of
the cisterna basalis, which connects it to a smaller cisterna in front of the
optic chiasma. The arachnoid granulations are seen best in old age, where they
produce pitting of the parietal bone. When hypertrophied, they are called pacchionian
bodies. Although they appear to originate in the dura, they are really
villous processes of the arachnoid that push the dura mater ahead of them. They
serve as channels for the passage of cerebrospinal fluid into the venous system
and at times may become large enough to produce pressure signs. The pia
mater is the innermost of the three meninges and is in reality the membrane
of nutrition. It is closely attached to the surface of the brain and dips into
the depths of all the sulci, carrying branches of the cerebral arteries with
it. The larger blood vessels of the brain lie in the subarachnoid space, but
the smaller ones ramify the pia and proceed into the substance of the brain
proper. At certain locations the pia mater sends strong vascular duplications
into the brain; these spread over the cavities of the third and the fourth
ventricles and are known as the choroid telae. The choroid tela of the 3rd
ventricle extends into each lateral ventricle. The blood vessels on the border
projecting into the lateral ventricle are enlarged into a plexus known as the
choroid plexus of the lateral ventricle, from which the greater amount of
cerebrospinal fluid is formed.
INTRACRANIAL
SPACES
The 4 intracranial
spaces are: the extradural (exterior to the dura); subdural (beneath the dura);
subarachnoidal (beneath the arachnoid); the intracerebral (within the brain
tissue proper). 1. The extradural space is only a potential one because
the dura touches the internal surface of the skull. The meningeal vessels are
in this space, and if they are injured, bleeding takes place between the dura
and the skull. If this bleeding is permitted to continue, the dura is slowly
stripped away from the bone. Bleeding into this space is usually arterial and
therefore rapid and often fatal. If the arachnoid is intact and the
subarachnoid space has not been entered, there is no blood in the cerebrospinal
fluid. 2. The subdural space is situated between the dura and the
arachnoid. Hemorrhage into this space may result from injury to large arteries
such as the middle cerebral or internal carotid, but this is rare, rapidly
fatal and of no practical importance. It is much more important to consider
subdural hemorrhage as venous, since the large sinuses, such as the
superior longitudinal and lateral, may be torn when the dura is injured. If the
arachnoid is also torn, as it may be over the great cisterns at the base of the
brain, blood escapes into the subarachnoid space and will appear in the
cerebrospinal fluid. 3. The trabeculated subarachnoid space is situated
between the arachnoid and the pia mater; cerebrospinal fluid circulates here.
The space is not wide over the convexity of the brain, but is quite extensive
at the base of the skull where the cisternae are formed. These form a
"water bed" of subarachnoid fluid upon which the brain floats. Only
the anterior third of the brain rests directly upon bone (the orbital plates of
the frontal bone). 4. Involvement of the intracerebral "space" is
really involvement of brain substance proper. Theoretically, the subpial space
is that potential interval that exists between the pia and the brain and is of
no practical importance. Attempts to strip the pia mater from the brain are
often unsuccessful, since brain tissue comes away with the intimately attached
pia. Bleeding into the brain proper may be traumatic in origin or may be the
result of spontaneous rupture of an artery in its interior. Since the pia is
frequently torn with these hemorrhages, frank blood appears in the
cerebrospinal fluid.
VENTRICULAR
SYSTEM AND CEREBROSPINAL FLUID
The
circulation of the cerebrospinal fluid is associated with (1) the ventricular system
and (2) the subarachnoid space. The spinal fluid is formed in the ventricular
system and absorbed in the subarachnoid space. The ventricular system (Fig.
24) is composed of four ventricles, two of which are lateral.
Normally, these spaces communicate freely with each other through well-defined
openings. Each lateral ventricle is situated within a cerebral hemisphere and
is subdivided into an anterior horn (in the frontal lobe), a body (in the
parietal lobe), a posterior horn (in the occipital lobe), and a descending horn
(in the temporal lobe). Each communicates with the third ventricle by a single
opening known as the foramen of Monro. This foramen has a V-shaped arrangement
of two limbs, each draining its respective lateral ventricle. It is situated in
the anterior horn and is the only means of exit for the lateral ventricles. The
3rd ventricle empties into the 4th by means of the infant.
FIG.The ventricular system.
The horns and the body of the first and the second lateral ventricles are
pictured in relation to the brain.
A spina
bifida is also present. aqueduct of Sylvius, which is
about 1\2 inch long and quite narrow, being only slightly larger than the lead
of a pencil. This aqueduct, passing through the midbrain, enters the anterior
part of the 4th ventricle; it is the only source of exit for the 3rd and both
lateral ventricles. Because of its location and its small size, it is the
weakest and most important point of the entire ventricular system. The 4th
ventricle is situated in the posterior cranial fossa, the cerebellum forming
its
roof
and the pons and the medulla its floor. It connects with the subarachnoid space
by three openings: the two lateral foramina of Luschka and a median foramen of
Magendie. The two lateral foramina of Luschka open into the cisterna lateralis,
and the foramen of Magendie into the cisterna magna (cisterna
cerebellomedullaris). In this way the ventricular system becomes connected with
the subarachnoid space. The fluid, having gained entrance into this space and the
cisternae, circulates freely around the cerebrum and the cerebellum, finally
passing down the spinal subarachnoid space.
FIG. 25. Photograph
and roentgenogram of a hydrocephalic
Cerebrospinal
fluid is formed by the choroids plexus, mainly in the lateral ventricles; from
this point it passes through the foramen of Monro into the 3rd ventricle and
finally into the subarachnoid space, where it comes in contact with the
arachnoid villi, which absorb it and return it to the venous stream in the
dural sinuses. The total amount of cerebrospinal fluid has been estimated to be
between 90 and 150 cc. in adults. If there is a block along
the route of the ventricular system, the condition of hydrocephalus results.
If such a block is located at a lateral ventricle entrance into the 3rd
ventricle, distention of one ventricle would result; if the block is at the
aqueduct of Sylvius, a distention of both
FIG. Ventriculography
Two small incisions are made
lateral and the 3rd ventricles would result; if the
obstruction is at the openings in the 4th ventricle (Magendie and Luschka),
distention of all ventricles will ensue. The type of block may be determined by
ventriculography.
FIG.A.
Encephalogram in lateral projection: (1) anterior horn, (2) body of the lateral
ventricle, (3) posterior horn, (4) third ventricle, (5) descending horn, (6)
fourth ventricle, (7) sella turcica.
FIG.B. Encephalogram in anteroposterior
projection: (1) anterior horn, (2) body of the lateral ventricle, (4) third
ventricle, (8) orbit, (9) nasal septum, (10) inferior turbinates.
SURGICAL CONSIDERATIONS
Two procedures, ventriculography and
encephalography, are valuable diagnostic aids, especially in the localization of
brain tumors and obstruction of the ventricular system ENCEPHALOGRAPHY Encephalography
consists of withdrawing cerebrospinal fluid by means of a lumbar puncture
needle and introducing air. The air slowly ascends and produces an outline of
the ventricular system which can be seen on a roentgenogram. It is important to
utilize a manometer during the procedure so that the cerebrospinal pressure is
measured. As a rule, from 20 to 25 cc. of air is introduced; the outlines of
the ventricles are seen, and any abnormality is noted. Encephalography should
not be used when there is an increase in intracranial pressure.
VENTRICULOGRAPHY Ventriculography is a more formidable procedure but is
the method of choice. It involves two incisions and two perforations of the skull.
The technic consists of making two small incisions in the scalp about
FIG.C.
Encephalogram in postero-anterior projection: (2) body of the lateral
ventricle, (3) posterior horn, (5) descending horn.
is
permitted, as well as temporary relief from intracranial pressure. Fluid is
removed and replaced by a somewhat smaller volume of air, the average amount of
air injected being from 50 to 120 cc. After this, lateral and anteroposterior
roentgenograms are taken
(Fig. A and B).
The lateral view may show deformity of the anterior or the posterior horns by
tumors situated in the frontal or occipital regions. The anteroposterior view
may reveal a deflection of the ventricles from the midline or a filling defect
of the third ventricle. During this procedure it is best to have the patient in
the sitting or semisitting position.
CISTERNAL PUNCTURE In cisternal puncture (Fig.
28) the patient may be in a sitting position or lying on one side with the head
placed somewhat forward. The first palpable cervical spinous process is located
in the midline, and a point is taken immediately above it. The needle is then
inserted in a forward and upward direction. The upward path parallels an
imaginary line that joins the external auditory meatus with the nasion; as the
needle advances it strikes the posterior occipito-atloid ligament. In
the adult this is at a depth of between 4 and
ARTERIAL
SUPPLY The arterial supply of blood is furnished to the brain by 4 vessels: 2
vertebral and 2 internal carotid arteries. The vertebral artery, a
branch of the subclavian, after ascending and perforating the dura, unites with
the same vessel of the opposite side to form the basilar. This vessel lies in the basilar groove of the pons and at its superior
margin divides into 2 terminal branches known as the posterior cerebral
arteries. The internal carotid, after penetrating the dura, reaches the
base of the brain
FIG.Cisternal puncture. An
imaginary line is constructed between the external auditory meatus and the
nasion. The needle enters above the spinous process of the 1st cervical
vertebra, parallels this line and enters the cisterna magna. The medulla is
about
CEREBRAL AND
CEREBELLAR VEINS The cerebral and the cerebellar veins are veins
of the brain proper. They do not accompany the arteries; they have no valves,
no muscle tissue around them, and their walls are extremely thin. They are
lodged for the greater part in the grooves on the surface of the brain and are
covered by arachnoid. The superior veins run upward toward the superior
sagittal sinus, turn forward and run parallel with the sinus for a short
distance before entering it. The cerebral veins are divided into external and
internal groups, depending upon whether they drain the outer surface or the
inner part of the hemisphere. The external veins are named the middle, the
superior and the inferior cerebral veins. The internal veins draining the
deeper parts of the hemisphere are the terminal ones and the great cerebral vein
of Galen.
VENOUS DURAL
SINUSES Venous sinuses of the dura mater are spaces between the 2 layers
of dura mater which collect blood and return it to the internal jugular vein.
Into these spaces spinal fluid is also drained from the subarachnoid space
through the arachnoid villi and granulations. Sinuses differ from other venous
structures in the body in that their walls consist of a single layer of
endothelium, as a result of which there is no tendency for them to collapse.
Seven of these sinuses are paired, and 5 are unpaired. The unpaired sinuses are
the superior sagittal,
FIG.The diploic veins. These veins
form venous plexuses between the outer and the inner tables of the skull. Four
diploic venous trunks drain these plexuses. The outer table of compact bone has
been removed to demonstrate the veins.
inferior
sagittal, straight, intercavernous and basilar. The paired sinuses are the
sphenoparietal, cavernous, superior petrosal, inferior petrosal, occipital,
transverse and sigmoid. Only those that are of practical and surgical
importance will be considered. The superior sagittal (longitudinal) sinus is
in a somewhat exposed position along the insertion of the falx cerebri. It
begins in front of the crista galli at the foramen caecum, where it
occasionally communicates with the veins of the nasal mucous membrane. It then
passes upward and backward in the upper border of the falx cerebri until it
reaches the internal occipital protuberance, where it lies a little to one side
of the median plane, usually on the right. Here it forms a dilatation known as
the confluence of sinuses (torcular Herophili), at which point the
superior sagittal, the transverse, the occipital and the straight sinuses all
meet. Here the superior sagittal bends acutely to theright, occasionally to the
left, and becomes continuous with the transverse sinus. Lateral expansions of the
sinus (lacunae lateralis) are found on each side. These lacunae receive
meningeal and diploic veins, and the superior sagittal sinus receives emissary
veins, diploic veins and those veins which drain the cerebral hemispheres. As
the superior sagittal sinus runs posteriorly, it grooves the internal aspect of
the skull, and its surface marking may be indicated by a line drawn over the
median line of the vertex from the root of the nose to the external occipital
protuberance. The inferior sagittal sinus passes backward in the lower
border of the falx cerebri. It unites with the great cerebral vein at the free
margin of the tentorium cerebelli to form the straight sinus. The straight
sinus travels backward along the attachment of the falx cerebri to the
tentorium. At the internal occipital protuberance it bends acutely to the left,
occasionally to the right, to form the tranverse sinus. It
FIG.Lateral
roentgenogram of the skull, demonstrating the arterial channels, the diploic
veins and a fracture line: (1) middle meningeal groove, (2) plexus of diploic
veins, (3) fracture line over the squamous portion of the temporal bone, (4)
coronary suture, (5) lambdoid suture, (6) sella turcica, (7) sphenoid sinus.
receives tributaries
from the posterior part of the cerebrum, the cerebellum and the falx cerebri. The transverse {lateral) sinus, which is a paired structure,
begins at the internal occipital protuberance. The right is usually
continuous with the superior sagittal sinus and the left with the straight
sinus. It receives the superior petrosal sinus and a few inferior cerebral and
cerebellar veins. It is bounded by the 2 layers of tentorium and the outer
layer of dura mater and runs horizontally at first in a lateral direction and
then forward. It lies in the transverse groove of the skull and in the attached
margin of the tentorium.
The sigmoid sinus is a continuation of the
transverse sinus and receives its name from
the
S-shaped curves which it makes. Some authors believe the term
"transverse" should be restricted to that part of the sinus that
passes between the internal occipital protuberance and the posterior inferior
angle of the parietal bone. The remaining part of the
FIG. 33. The cerebral
veins, viewed from above. The superior sagittal sinus has been opened, and the
dura mater has been reflected on the left side, exposing the subdural space.
The intact dura on the right side reveals the relationship of the extradural
space and the middle meningeal vessels. A small flap of arachnoid has been
reflected to show the position of the cerebral veins.
FIG.Venous sinuses at the base of the skull.
The right internal carotid artery is shown surrounded by the cavernous sinus.
sinus (to the
jungular foramen) is known as the sigmoid sinus, which curves downward, leaves
the tentorium, passes between the two layers of dura and ends at the jugular
foramen, where it becomes the internal jugular vein. The continuation of the
sinus as the internal jugular explains the propagation of a transverse sinus
thrombosis. This justifies the ligation of the internal jugular to prevent the
spread of septic emboli to the heart. The superior petrosal sinus joins it at
its first bend, and the inferior petrosal at its termination. It forms an
important posterior relation of the tympanic antrum. In suppurative conditions
of the tympanic antrum or cavity this sinus may become the site of a septic
process that travels through the cerebellar tributaries and forms a cerebellar
abscess. It is separated from the mastoid cells by only a very thin plate of
bone; hence, diseases from the middle ear into the mastoid cells can form a
suppurative process that involves the sinus (sinus thrombosis). Its
communications are numerous and important; it is connected by the mastoid
emissary vein with the occipital vein, by the occipital sinus with the
transverse sinus and by the posterior condylar emissary vein with the
suboccipital plexus. The cavernous sinus is situated to either side of
the body of the sphenoid bone and is continuous with the ophthalmic veins in
front. Posteriorly, it divides into the superior and the inferior petrosal
sinuses. It surrounds the internal carotid artery. Injury in this area may
result in the formation of an arteriovenous aneurysm which produces stasis in
the superior ophthalmic vein. This may bring about a pulsating exophthalmos due
to pulsations of the cartoid artery transmitted to the engorged venous spaces.
Cavernous sinus thrombosis may follow inflam- matory lesions of the face and
the upper lip, the extension taking place through the facial, the nasal and the
ophthalmic veins. This sinus is intimately related to the gasserian ganglion
and may be injured during operations on the latter. Infections involving the
cavernous sinus are frequently accompanied by basilar meningitis. The circular
sinus consists of the two transverse venous connections between the
cavernous sinuses. The sphenoparietal sinus runs along the lesser wing
of the sphenoid to the superior sagittal sinus. The occipital sinus is
extremely variable in size and lies in the attached border of the falx
cerebelli. The basilar sinus is a wide trabeculated space behind the
dorsum sellae which unites the cavernous and the inferior petrosal sinuses of
opposite sides. It also communicates below with the spinal veins.
TEMPORAL AND INFRATEMPORAL REGIONS MUSCLES OF
MASTICATION
The muscles of mastication include the temporal,
the masseter, the external and the internal pterygoids, the mylohyoid and the
anterior belly of the digastric. All of these are supplied by the trigeminal
nerve (mandibular branch). Since they do not appear over the same region of the
face, only those concerned with the temporal and the infratemporal regions will
be discussed here. Temporal Fascia. This
strong aponeurotic layer covers the upper aspect of the temporal muscle,
attaches above to the superior temporal line and splits below into two layers
attached to the lateral and the medial margins of the upper border of the
zygoma. The space thus formed is occupied by fatty tissue and some small blood
vessels. Temporal Muscle. This muscle
arises from the whole of the temporal fossa (frontal, parietal, squamous
portion of the temporal and greater wing of the sphenoid bones), and its fibers
converge on a gap that exists between the zygoma and the side of the skull. The
fibers pass downward deep to the zygomatic arch, then beneath the masseter, and
become inserted into the margins of the deep surface of the coronoid process.
Some anterior fibers descend beyond the coronoid to reach the anterior border
of the ramus of the mandible. A true view of the muscle can be obtained only if
the temporal fascia is removed and if the zygoma is divided and turned downward
together with the masseter muscle. The muscle is a powerful elevator of the
mandible, and its posterior fibers act as a retractor of the same bone. Nowhere
else in the body is a group of muscles opposed by so weak a group of opponents
as in this region. The temporal, the masseter and the
internal pterygoid muscles produce the great biting and grinding power, but
their opponents which depress the mandible (external
pterygoid, digastric, mylohyoid and geniohyoid) are able to afford only weak
resistance. Therefore, when a state of spasm is produced, the stronger group
prevails. Should this spasm be clonic, a chattering of the teeth occurs, but if
the spasm is tonic, the mouth is rigidly closed, and the condition known as
trismus or lockjaw results. This locking of the jaw is a frequent symptom of
tetanus but is also found in any condition that might produce an irritation of
the mandibular branch of the trigeminal nerve, as is sometimes seen in caries
of the lower teeth or during the cutting of a lower wisdom tooth. Masseter Muscle. This muscle is held firmly by
the masseter fascia, which binds it to the margins of the ramus and the body of
the mandible; the muscle covers nearly the entire lateral surface of the ramus
of the mandible. An expansion of the fascia overlies the fat pad of the cheek
and holds it against the buccinator muscle. The parotid duct lies within the
fascia and is protected by it. The muscle arises by two closely associated
heads: a superficial and a deep, which arise from the surface of the zygomatic
arch. It is inserted into the lateral surface of the ramus and the coronoid
process of the mandible. The muscle raises and protrudes the mandible, and its
fibers may be felt well if the jaws are clenched firmly. The transverse facial
vessels, the parotid duct and branches of the facial nerve all lie on its
lateral surface, and the parotid gland overlaps it posteriorly. The muscle does
not cover the head and the neck of the mandible. Pterygoid
Muscles. These muscles lie on a deeper plane and are almost
completely hidden by the ramus of the mandible. Only a small part of the
external pterygoid can be seen through the mandibular notch. Many authors
prefer to consider the internal {medial) pterygoid muscle as being
associated with the masseter, and these two muscles
have been likened to the two bellies of the digastric. The fibers of the
internal pterygoid originate from the medial surface of the lateral pterygoid
lamina and by a small slip from the tuberosity of the maxilla. They pass
downward and backward and insert into the medial surface of the ramus of the
mandible from the mandibular foramen to the angle. The muscle elevates the
mandible, protrudes it and pulls it to the opposite side. Superficial to this
muscle is the ramus of the mandible, the external pterygoid, the lingual and
the inferior dental nerves, the maxillary and the inferior dental vessels and
the sphenomandibular ligament. Deep to it are the tensor palati and the levator
palati, the superior constrictor of the pharynx and the eustachian (auditory)
tube. The external (lateral) pterygoid muscle originates by two heads:
the lower from the lateral surface of the lateral pterygoid lamina, and the
upper from the undersurface of the great wing of the sphenoid. The fibers are
directed backward and become inserted in the digital fossa on the front of the
neck of the mandible and to the capsule and the disk of the mandibular joint.
In this way its contraction opens the mouth by sliding the condyle forward and
protruding the jaw. One muscle acting alone pulls the chin over the opposite
side. The internal maxillary artery crosses the lower head of the muscle
obliquely and, as a rule, runs superficial to it. Buccinator.
This forms the muscle layer of the cheek and is discussed here for the sake
of completion. It arises from the outer alveolar margins of both the upper and
the lower jaws in the region of the molar teeth and passes to the angle of the
mouth, where it blends with the orbicularis oris. The middle fibers decussate
at the angle of the mouth, so that the uppermost fibers pass to the lower lip and
vice versa. It is supplied by the buccal branches of the facial nerve. By its
action of retracting the angle of the mouth and flattening the cheek, it
compresses the cheek so that during mastication food is pushed beneath the
molar teeth. Compression of the cheek against the gums prevents masticated food
from becoming lodged there. In paralysis of the facial nerve it becomes
necessary for the patient constantly to dislodge the food with his finger. This
muscle also aids in the act of blowing and whistling. Its superficial surface
is covered with buccal pharyngeal fascia, and its deep surface is lined with
the mucous membrane of the cheek. Posteriorly, it is covered by the buccal fat
pad, which separates it from the masseter and the temporal muscles, anteriorly,
by the superficial fascia, which contains the facial artery, the anterior
facial vein, the buccal nerve and artery and the buccal branches of the facial
nerve. The muscle is pierced by the parotid duct and twigs from the buccal
nerve.
Tympanic (Mastoid)
Antrum. The tympanic
antrum is a large recess situated in the posterior part of the petrous
portion of the temporal bone. It is about the size of a small pea and is really a large
mastoid air cell. The aditus (aditus ad antrum), an oval slit with its
long axis nearly vertical and measuring about a YA inch, connects the
epitympanic recess with the antrum. Any obstruction of this narrow aperture
favors stasis and retention of inflammatory exudates that may find their way to
the mastoid cells. Both the aditus and the antrum lie just below the tegmen
tympani. Since the aditus opens close to the roof of the antrum, it is not in
an efficient place to drain that cavity. The tympanic antrum is relatively
larger and more superficial in the child than in the adult. Superiorly,
it has a roof that is the backward continuation of the tegmen tympani and is,
therefore, in close relationship to the middle cranial fossa and the temporal
lobe of the brain. Involvement of this wall may cause a subtemporal abscess.
Its anterior wall has in its upper part an opening which communicates
with the epitympanic recess; this opening has been referred to above as the
aditus. Its posterior wall opens into mastoid air cells and separates
the antrum from the sigmoid (transverse) sinus and the cerebellar hemisphere.
The lateral wall is formed by the squamous part of the temporal bone, is
about Vz inch thick in the adult, is the wall of surgical approach and
projects laterally in that part of the temporal bone that is covered by the auricle.
Above the promontory and even above the fenestra vestibuli is found the canal
for the facial nerve (aqueduct of Fallopius). This canal contains the
facial nerve as it travels in its intrapetrous portion. It also forms a ridge,
the wall of which is so extremely thin that the nerve may be seen through it.
Above the fenestra ovalis the facial canal, together with the external
semicircular canal, form the inner boundary of the aditus. This is an important
relationship and should be kept in mind in any operative procedure in this
region. At the medial wall of the aditus the facial canal curves downward and
opens on the inferior surface of the temporal bone at the stylomastoid foramen.
The chorda tympani nerve passes forward etween the handle of the malleus
and the long process of the incus, reaching a small opening in front of the
upper part of the tympanic ring. The mastoid process does not exist at
birth but begins its development at the end of the first year. As it grows, its
diploe is gradually replaced by air cells. The mastoid cells usually
occupy the whole of the mastoid process, which has a very thin coating of
compact bone. In the upper part the cells communicate with the antrum; at the
middle of the mastoid process they increase in size. Since the mastoid cells
are developed as outgrowths from the mucous membrane of the middle ear and the
antrum, they are lined by the membrane and are filled with air from these
cavities. The cells near the apex of the mastoid are smaller and do not
communicate with those above; the lowermost cells contain marrow and not air
and represent the unaltered diploe of the cranial bones. Infection from the
tympanic cavity may invade these cells, spread down the mastoid process and
invade the deepest-lying cells. If the formation of air cells (pneumatisation)
is complete, the entire mastoid process is composed of these large air spaces;
this is known as the pneumatic type of mastoid. However, if pneumatisation is
interfered with so that the cells do not develop, the diploic type of mastoid
process results, in which the structure resembles the other cranial bones
(outer and nner tables with diploe between). When this occurs, the antrum is
the only cell present. The sclerotic type of mastoid process is one in which
the process is composed of very dense compact bone and is usually the result of
a chronic infection that has interfered with the absorption of the diploe and
the pneumatisation process. It results in an acellular mastoid that is
extremely hard.
FIG. Simple mastoid
operation (antrotomy). The two uppermost figures reveal the surgical
anatomy in the region to be explored. Note the level at which the section
is taken m the figure on the right and then shown in cross section in the
figure on the left. Figures A to E depict the steps in
the operation.
MASTOIDITIS, SIMPLE AND RADICAL MASTOID SURGERY In
mastoiditis, suppurating mastoid cells can involve the lateral sinus. This
involvement may be the result of contamination of the small veins that reach
the sinus through the bone or by direct infection from a perisinus abscess.
From the lateral sinus, extension can take place to the internal jugular vein
or even to the other side of the skull by way of the confluence of sinuses.
Mastoid disease may extend upward through the roof of the antrum, involve the
brain and the meninges and result in meningitis, extradural or brain abscesses.
Mastoiditis usually follows an acute disease of the tympanic cavity because of
the mucous membrane continuity. Once the mastoid cells have become involved,
the infection may spread in one of many ways, traveling either to the
transverse sinus or to the meninges and the brain; the facial nerve may become
involved, or the cortex of the mastoid process itself might be perforated. In
performing a simple mastoid operation (mastoidectomy or antrotomy), it
is possible to injure vital structures. If the opening in the antrum extends
too far upward, the middle cranial fossa may be opened; if too far backward,
the transverse sinus is entered, and if the opening is placed too deep, the
facial nerve may be injured. In doing this operation, an incision is made about
1/2 inch posterior to and parallel with the insertion of the auricle;
separation of the soft parts from the bone subperiosteally is carried anteriorly
to the posterior margin of the auditory canal, superiorly and anteriorly to the
suprameatal spine and posteriorly far enough to expose the mastoid process.
Chiseling is begun in an angle formed by the temporal line above and the
posterior bony wall of the canal in front. The chisel should always chip in a
direction parallel with the auditory meatus. After the antrum is opened, it is
explored, and the opening is enlarged as desired. The diseased cells and
carious bone are removed; the wound is irrigated, dried and packed. Closure
with drainage follows. The radical mastoid operation converts the
mastoid antrum, the cells and the middle ear into a single cavity, and all the
ossicles except the stapes are removed. Auditory
(Pharyngotympanic, Eustachian) Tube. The pharyngotympanic
tube is an osseocartilaginous tube about 1 and 1/2 inches long which
connects the tympanic cavity with the nasopharynx. The posterior third is bony,
and the anterior two thirds partly cartilaginous and partly fibrous. The mucous
membrane lining the tube is continuous with that of the middle ear and the
pharynx. Through this tube the air pressure on both sides of the ear drum is
equalized; should the tube become obstructed by edema, etc., air cannot enter,
and a negative pressure results in the tympanic cavity. With the atmospheric
pressure on the outer side of the drum, the membrane retracts into the cavity
and a sensation of fullness in the ear results. The course of the tube is
downward, medially and forward from the tympanic cavity, its narrowest part,
the isthmus, lying at the junction of the cartilaginous and bony parts. The
cartilaginous portion opens from the lateral wall of the nasal fossa close to
the pharyngeal opening and presents medial and lateral walls which lie so close together that only a slitlike cavity results.
The bony part of the tube is in relation superiorly to the canal of the tensor
tympani muscle, anterolaterally to the petrotympanic fissure, and
posteromedially to the carotid canal and its contents. Normally, the pharyngeal
orifice is closed. During swallowing and yawning it opens by means of the
action of the tensor veli palatine muscle. The tympanic orifice is located in
the anterior wall of the tympanic cavity below the canal for the tensor tympani
muscle. Inflation of the middle ear may be accomplished by Valsalva's
method. The patient closes the mouth and the nose and forcibly blows out
the cheeks. This drives air through the auditory tube, a sense of fullness is
felt in the ears and hearing is diminished because of the resulting distention
of the tympanic membrane. The same may be accomplished by the Politzer
method, where the nozzle of a Politzer bag is inserted into the nostril,
and the nose is closed. The patient then swallows, and the bag is compressed,
forcing air into the tympanum.
Cranial Nerves (table)
RECOMMENDED LITERATURE:
1. Mark
W. Wolcott. Ambulatory Surgery End The Basic Of Emergency Surgical
Care.-Philadelphia:J.B.Lippincott Company,2001.-752p.
2. Michael
F. Mulroy.Regional Anesthesia /The
3. Richard
M. Stilman,M.D.,E.A.C.S. General Surgery /Review And
Assessment/
4. Kent M.
Van De Graff, Stuart Ira Fox, Karen M. Lafleur. Synopsis of
Human Anatomy and Physiology /WCB McGraw-Hill/, 2004.-675p.
5. John
J. Jacobs. Shearer’s Manual Of Human Dissection
/McGraw-Hill Information Services Company, 1998.-300p.
6.
7. Philip
Thorek. Anatomy In Surgery /J.B.Lippincott Company/,1996.-935p.