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urth ventricle may be opened up by making a somewhat larger trephine opening
To expose both hemispheres of the cerebellum a flap is turned downwards by arrying a curved incision between the bases of the mastoid processes, the centre of he incision reaching upwards to a little above the inion. The occipital arteries are
FIG. 1069.-CRANIO-CEREBRAL TOPOGRAPHY.
Guiding lines (Chiene's), deep black; sutures, fine black; meningeal arteries, red; sulci, blue.
0. External occipital protuberance (inion).
Mid-point of AB.
CD is drawn parallel to AM.
M. Mid-point between G and O.
T. Mid-point between M and O.
VW. Guide to anterior limit of transverse sinus.
S. Mid-point between T and O.
E. Zygomatic process of frontal.
P. Root of zygoma (pre-auricular point).
N. Mid-point of EP.
Site at which subarachnoid space may be opened.
R. Mid-point of PS.
divided, but the anterior extremities of the incision should, if possible, be kept behind the mastoid emissary veins. As the flap contains the suboccipital muscles. the bone itself may be removed. If more room is required, the opening in the bone may reach above the level of the transverse sinuses without wounding them as they can be displaced along with the dura. The occipital sinus is divided between two ligatures. After dividing the dura a hemisphere of the cerebellum may be displaced towards the median plane to enable the finger to be passed
between it and the posterior surface of the petrous portion of the temporal bone as far as the acoustic nerve, which occupies the angle between the cerebellum and the pons. Meningeal Arteries. When the calvaria is removed the meningeal arteries are found to adhere firmly to the dura. Of these vessels the middle meningeal artery is the only one of surgical importance. It is frequently lacerated in fractures of the skull; the blood is generally extravasated between the dura and the bone, and the bleeding point lies beneath the clot. After entering the cranial cavity through the foramen spinosum, the main trunk, which is usually about 1 in. in length, runs laterally and slightly forwards to bifurcate into anterior and posterior divisions at a point a finger's-breadth above the middle of the zygomatic arch, viz., at or close behind the point N, Fig. 1069. When the main trunk is short the bifurcation takes place opposite the middle of the zygomatic arch.
The anterior and larger division passes upwards, with a slight forward convexity, a little behind the spheno-squamosal suture and across the pterion to the sphenoid angle of the parietal bone. From that point the vessel is continued upwards and slightly backwards, behind the coronal suture; it gives off branches which ascend over the motor area. The position and general direction of the anterior branch may be said to correspond to the line MN; it follows, therefore, that the artery will be encountered in trephining over the lower and anterior par of the Rolandic area, especially over the motor centres for the tongue and face. The posterior division passes almost horizontally backwards, towards the mastoid angle of the parietal bone.
To expose the trunk of the vessel and its bifurcation, the trephine is applied immediately above the middle of the zygomatic arch. To expose the anterior division the pin of the trephine may be applied at the point A, which strikes the artery as it crosses the pterion and grooves the sphenoidal angle of the parietal bone. The inferior segment of the disc of bone removed is much thicker than the superior, as it involves the prominent ridge which passes from the tip of the great wing of the sphenoid on to the sphenoidal angle of the parietal bone. At the sphenoidal angle of the parietal bone, the artery frequently runs in a canal for a distance of half an inch. It follows, therefore, that a considerable thickness of bone has to be sawn through at the inferior segment of the circle before the dise can be removed, and during the removal bleeding may occur from the artery as it lies in the canal.
Vogt localises the anterior division at a point a thumb's-breadth behind the tubercle on the posterior border of the zygomatic bone and two fingers'-breadth above the zygoma. Krönlein trephines at a point 14 in. behind the zygomatic process of the frontal, on a line drawn from the supra-orbital margin backwards parallel to Reid's base-line. If the centre of the trephine be placed at the mid-point of the lower third of the line MA, the anterior division will be reached above the canal and the ridge at the sphenoidal angle of the parietal; if the bleeding-point is lower down, the trephine opening may be enlarged downwards along the line AN.
The course of the posterior division may be indicated upon the surface by draw ing a line backwards from the point N parallel to PR, that is to say, a finger's breadth above the zygoma and the supra-mastoid crest.
When the frontal branch of the anterior division is injured, the clot is in the fronto-temporal region, and involves more especially the motor area for the face, and, on the left side, Broca's convolution; when the anterior division is wounded, the clot, which is larger, involves the parietotemporal region, and the motor symptoms are due to pressure upon the centres for the arm and face; in injuries to the posterior division the clot overlies the parieto-occipital region, and the localising symptoms are sensory (Krönlein). In more extensive meningeal hæmorrhage the clot may cover the greater part of the hemisphere.
The superior sagittal sinus, which enlarges as it extends backwards, occupies the median plane of the vertex from the glabella to the internal occipital protuberance where it opens into the confluens sinuum, and becomes continuous usually with the right transverse sinus. Opening into the sinus, especially in the posterior part of the parietal region, are the para-sinoidal sinuses, into which arachnoideal granulation project. In opening the skull over the posterior part of the vertex, the edge of the trephine should be kept at least three-quarters of an inch from the median plane.
The transverse sinus may be mapped out on the surface by drawing a line, lightly convex upwards, through a point a little above the inion to the asterion (14 in. behind and 1 in. above the centre of the external acoustic meatus) at, or a little in front of, the point R, Fig. 1069, and thence in a downward and forward direction to a point in. inferior and posterior to the centre of the external acoustic meatus, where it finally curves medially and forwards to open into the jugular bulb, which occupies the jugular foramen. According to Moorhead the highest part of the sinus lies a finger's-breadth above the middle of a line extending from the inion to the middle of the external acoustic meatus. The anterior border of the descending or mastoid portion of the sinus may be mapped out by drawing a line VW from a point a finger's-breadth behind the post-auricular point of the temporal crest to the anterior border of the tip of the mastoid process. In wounds of the sinus the hæmorrhage is very free, owing to the inability of its walls to collapse, but the bleeding is easily controlled by plugging.
Of the cerebral arteries, the middle supplies almost the whole of the motor area, and one of its lenticulo-striate branches, which enters the brain at the anterior perforated substance, is called "the artery of cerebral hemorrhage" from the frequency of its rupture in apoplexy. The extravasated blood involves the motor part of the internal capsule. The postero-medial central branches of the posterior cerebral artery, which enter the brain at the posterior perforated substance, supply the thalamus and walls of the third ventricle; hæmorrhage from one of these branches is apt to rupture into the ventricle. The postero-lateral central branches of the posterior cerebral artery supply the thalamus, and when one of these vessels ruptures the hæmorrhage is apt to invade the posterior or sensory part of the internal capsule.
Semilunar Ganglion. The topography of the semilunar ganglion is of importance in relation to its surgical extirpation for trigeminal neuralgia. The ganglion is situated in the dura at the apex of the petrous portion of the temporal bone, at the medial part of the middle fossa of the base of the skull. The surgeon reaches it by an extra-dural route through an opening in the anterior and lower part of the temporal fossa immediately above the zygomatic arch. The bone is removed down to or, even better, beyond the level of the infra-temporal crest, which forms the boundary line between the lateral and basal portions of the cranium. By temporarily resecting and depressing the zygomatic arch a portion of the floor of the middle fossa, medial to the infra-temporal crest, can be removed. The dura is separated from the fossa so as to admit of the ligature of the middle meningeal artery immediately after its entrance into the cranium through the foramen spinosum. By separating the dura still further in a medial and forward direction, the mandibular division of the trigeminal nerve is exposed as it enters the foramen ovale, and, after it, the smaller maxillary division, as it passes in a forward and slightly downward direction to enter the foramen rotundum. To expose the ganglion itself and the trunk of the nerve the dura is then carefully separated in a backward and medial direction; in doing this care must be taken not to wound the cavernous sinus and the trochlear and abducent nerves which lie in its lateral wall. The oculo-motor nerve and the carotid artery are less likely to be injured. The ganglion has a grayish-red colour and a felted surface, while the portio major or trunk of the trigeminal nerve is almost white, and striated longitudinally. After dividing the mandibular and maxillary divisions of the nerve close to their foramina of exit, the ganglion is seized with forceps and removed by twisting it away from its trunk and the first division.
Ear. The skin covering the lateral surface of the auricle is tightly bound down to the perichondrium, hence inflammations of it are attended with little swelling but much pain. The posterior auricular artery, which ascends along the groove at the posterior attachment of the auricle, is immediately anterior to the incision for opening the tympanic antrum.
The external acoustic canal, the general direction of which is medially, forwards, and downwards, possesses various curves of practical importance. The highest part of the upward convexity, which is also the narrowest part of the canal, is situated at the centre of its osseous portion; beyond this the floor sinks to form a recess in which foreign bodies are liable to be imprisoned. Of the two horizonta'
curves the lateral is convex forwards, the medial concave forwards. The skin of the osseous portion of the canal is thin and fused with the periosteum, hence when chronically inflamed it is liable to give rise to secondary periostitis and osseous narrowing of the canal.
The relations of the osseous walls of the canal are of importance to the The whole of the upper wall and the upper half of the posterior wall, developed from the squamous portion of the temporal bone, consist of two layers of
FIG. 1070.-VIEW OF THE LATERAL WALL OF THE MIDDLE EAR.
Section through the left temporal bone of a child, to show the relations of the tympanum and tympanic antrum to the middle and posterior fosse of the skull.
compact bone, a superior and an inferior, which are continuous, the former with the inner table, the latter with the outer table of the skull. The superior plate passes medially to the petro-squamosal suture, where it becomes continuous with the lateral edge of the tegmen tympani, which roofs over the epitympanic recess and the tympanic antrum; the lower plate bends downwards and medially at its deepest part to form the lower and lateral wall of the recess and the anterior part of the lateral wall of the antrum (Trautmann). It follows, therefore, that when the tympanic antrum is abnormally small, due to sclerosis of the bone, or when it is encroached upon by a far-forward transverse sinus, it, along with the epitympanic recess, can be opened by perforating the junction of the upper and posterior walls of the osseous canal, the instrument being directed medially and slightly upwards. Upon the upper and posterior segment of the external acoustic margin is the suprameatal spine; this small but important process, developed from the squamous portion,
can usually be distinctly Posterior tympano- made out in the living sub
ject by pressing upwards and backwards with the forefinger placed in the external acoustic meatus. The lower half of the posterior wall of the osseous canal (posterior part of the ob tympanic plate) is fused with the anterior part of the mastoid process, and closes the lower and anterior set of mastoid cells (border cells). related respectively
FIG. 1071.-LEFT TYMPANIC MEMBRANE (as viewed from the external acoustic meatus). x3. (From Howden.)
Anteriorly and inferiorly the osseous canal is
mandibular articulation and the parotid gland; hence it follows that blows upon
chin may fracture the tympanic plate as well as the base of the skull, that pain on mastication is usually complained of in acute inflammatory affections of the meatus and middle ear, and that in young children, in whom the tympanic plate is incompletely ossified, suppurative inflammation is liable to extend from the ear to the parotid region.
Clinically, to obtain a view of the membrana tympani a speculum and a reflecting mirror are employed; the auricle is pulled upwards, backwards, and laterally in order to
FIG. 1072.-VIEW OF THE LABYRINTHINE WALL OF THE MIDDLE EAR.
Section through the left temporal bone of a child, to show the relations of the tympanum and tympanic autrum to the middle and posterior fosse of the skull.
straighten the cartilaginous part of the canal. The healthy membrane is pearly gray, semiopaque, slightly concave, and obliquely placed, being inclined laterally, especially above and behind.
The handle and lateral process of the malleus, both embedded in the membrana tympani, are the only objects distinctly seen when the healthy ear is examined with the speculum.
FIG. 1073.-SECTION THROUGH LEFT TEMPORAL BONE, SHOWING TYMPANIC WALL OF TYMPANIC CAVITY, ETC.
The lateral process of the malleus projects laterally, and presents itself, therefore, as a distinct knob-like projection at the superior part of the membrane; passing forwards and backwards from this process are the anterior and posterior malleolar folds of the membrana; they form the lower limit of the pars flaccida of the membrane, and correspond to the line of the chorda tympani nerve. The handle of the malleus, situated at the junction of the upper quadrants, is seen passing downwards and backwards to the point of maximum