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Bony Landmarks of the Cranium.-At the root of the nose is the frontonasal suture (nasion); a little above it is the glabella, a slight prominence which connects the superciliary arches. About 1 in. below the posterior pole of the
FIG. 1066.-DIAGRAMMATIC REPRESENTATION OF A FRONTAL SECTION THROUGH THE SCALP, CRANIUM, MENINGES, AND CORTEX CEREBRI (modified from Cunningham).
cranium, and 2 in. above the spine of the epistropheus, is the external occipital protuberance (inion). In the child the protuberance is not developed; its position may be defined by taking a point at the junction of the upper and middle thirds of a line extending from the posterior pole of the skull to the spine of the epistropheus. About a third of the distance from the nasion to the inion is the bregma or junction of the coronal and sagittal sutures; with the head in the natural erect posture the bregma corresponds to the middle of a line carried across the vertex between the pre-auricular points of the zygomatic arches.
At birth the position of the bregma is occupied by the fonticulus frontalis, a rhom boidal membranous area which generally becomes ossified at about the eighteenth month. The size and date of closure of the fontanelle, as well as its tension and pulsation, are all points to be carefully noted in the clinical examination of children.
The lambda, or junction of the sagittal and lambdoidal sutures, situated 2 in. above the inion, can generally be felt through the scalp; a line drawn from it to the posterior border of the root of the mastoid process corresponds to the lambdoidal suture. In the adult the parieto-occipital fissure of the brain lies opposite, or a few millimetres in front of, the lambda; in the child, however, the fissure may be as much as 1 in. in front of it.
Crossing the supra-orbital margin close to its medial angle, a finger's-breadth from the medial line, are the supra-trochlear nerve and the frontal branch of the ophthalmic artery; the latter nourishes the flap in the operation of rhinoplasty. At the junction of the medial and intermediate thirds of the supra-orbital margin, 1 in. from the medial line, is the supra-orbital notch or foramen, the guide to the supra-orbital vessels and nerves. A little above the level of the lateral canthus of the eyelid is the fronto - zygomatic suture, immediately above which is the zygomatic process of the frontal bone. At the posterior end of the suture the zygomatico-temporal branch of the orbital nerve pierces the temporal fascia to reach the scalp. Half an inch above the suture is the lower margin of the cerebral hemisphere; while half an inch below the suture is a small tubercle on the posterior border
f the zygomatic bone; a line drawn from this tubercle to the lambda gives the line f the superior temporal sulcus and of the inferior cornu of the lateral ventricle.
The zygomatic arch, an important landmark, is horizontal when the head is in he natural position, and is on the same level as the inferior margin of the orbit and he inion; its superior border is at, or not infrequently a little above, the level of he lower lateral margin of the hemisphere. The superior border of the zygoma may e traced backwards immediately above the tragus and the external acoustic meatus o become continuous with the ridge formed by the supra-mastoid portion of the temporal crest. The part of the posterior root of the zygoma which lies immeliately in front of the superior end of the tragus constitutes a valuable landmark which may with advantage be termed the pre-auricular point of the zygoma, while by the term post-auricular point is understood that point upon the supra-mastoid crest which lies immediately behind, and a finger's-breadth below, the upper attachment of the auricle. The temporal vessels and the auriculo-temporal nerve cross the zygoma at the pre-auricular point, and it is there that the pulsations of the temporal artery may be felt during the administration of an anesthetic, or the vessel compressed for the purpose of checking bleeding from the temporal region of the scalp. The termination of the auriculo-temporal nerve in the neighbourhood of the parietal tuber is often the seat of a neuralgic pain in irritative conditions about the external acoustic meatus, the latter being supplied by this nerve.
Two inches vertically above the pre-auricular point is the inferior end of the central sulcus of Rolando. Two inches vertically above the middle of the zygomatic arch is the pterion (spheno-parietal suture), a point which cannot be felt, but which is nevertheless of topographical importance, as it overlies the lateral point (the point where the lateral fissure of the brain breaks up into its three branches) and the anterior branch of the middle meningeal artery.
The frontal tuber (better marked in the child) overlies the middle frontal convolution. The parietal tuber, which varies considerably in the definiteness with which it can be recognised, overlies the termination of the posterior horizontal limb of the lateral fissure of the brain, and therefore also the supra-marginal convolution, which is named by Turner the convolution of the parietal tuber. The part of the temporal crest which intervenes between the zygomatic process of the frontal bone and the coronal suture lies a little above the level of the inferior frontal sulcus. The highest part of the temporal crest crosses the anterior central gyrus at the junction of its middle and lower thirds, that is to say, at the junction of the motor areas for the arm and face. In the child the temporal muscle, which is relatively much smaller than in the adult, reaches only a short distance above the squamous suture, and, therefore, only as far as the level of the inferior end of the central sulcus of Rolando.
The thickness of the skull-cap varies at different parts and in different individuals. The inner table is only half the thickness of the outer table, but both possess the same degree of elasticity. When the vault is fractured from direct violence, the inner table is more extensively fissured than the outer table, because the elements of the latter are compressed, while those of the former are stretched apart. The weak areas at the base of the skull through which fractures are liable to extend are in the anterior cranial fossa, the orbital parts of the frontal bone, and the cribriform plate of the ethmoid; in the middle cranial fossa, the region of the glenoid cavity of the temporal bone, and of the foramen ovale of the sphenoid; in the posterior fossa, the fossæ of the occipital bone. The strong petrous part of the temporal is weakened by the tympanic cavity and by the deep jugular fossa.
Cranio-Cerebral Topography. Of the many methods which have been devised for mapping out the relations of the cranial contents to the scalp, that introduced by Professor Chiene is, probably, the most useful from a clinical point of view; no figures or angles have to be remembered, and the primary surface lines are drawn from bony points which are not variable, whilst the secondary lines are drawn, for the most part, between mid-points of the primary lines. The method is as follows (Figs. 1067 and 1068):
"The head being shaved, find in the median line of the skull between the glabella (G) and the external occipital protuberance (O) the following points:"First, the mid-point (M); second, the three-quarter point (T); third, the seveneighth point (S).
Find also the zygomatic process (E), and the root of the zygoma (preauricular point) (P), immediately above and in front of the external acoustic meatus. Having found these five points, join EP, PS, and ET. Bisect EP and PS at N and R. Join MN and MR. Bisect also AB at C, and draw CD parallel to AM."
The line MA corresponds to the superior and inferior precentral sulci, and may therefore be termed the pre-central line. The origins of the superior and inferior frontal sulci may be indicated by the points of union of the upper and middle and the middle and lower thirds of the line MA, the lower point being at the level of the temporal crest.
The line ET, termed the oblique or lateral line, intersects the pre-central line at the point A, which overlies the pterion, and corresponds therefore to the lateral point of the lateral cerebral fissure and to the anterior division of the middle meningeal artery. AC overlies the posterior horizontal limb of the lateral fissure of the brain, which terminates at the level of the temporal crest, in the inferior part of the triangle HCB. This triangle contains the parietal tuber, and may, therefore, be termed the supra-marginal triangle. The termination of the lateral line, at the three-quarter sagittal point T, overlies the parieto-occipital fissure.
By joining TR, RO, a triangle is mapped out which delimits the surface of the occipital lobe; the line TR corresponds to the lambdoidal suture, while RO corresponds to, or lies a little above, the tentorium and the upper border of the transverse sinus. CD, the post-central line, corresponds to the superior post-central sulcus, and lies a little behind the inferior post-central sulcus.
The parallelogram AMDC overlies the Rolandic area, i.e. the anterior central gyrus and the posterior central gyrus, separated by the central sulcus.
The pentagon ABRPN maps out the temporal lobe, with the exception of its apex, which is directed downwards, forwards, and inwards, a finger's-breadth in front of the point N.
A finger's-breadth below AB is the superior temporal sulcus, the posterior extremity of which turns upwards to terminate at B, the point which indicates, therefore, the position of the angular gyrus.
The central sulcus of Rolando may be mapped out upon the scalp by drawing a line downwards and forwards for a distance of 33 in. from a point half an inch behind the mid-sagittal point M at an angle of 67° to the sagittal line (Hare). This angle may readily be found by Chiene's plan of folding a sheet of paper first to half a right angle and again to a quarter of a right angle (45° + 22·5° = 67.5°). According to Cunningham, the average angle which the fissure makes with the sagittal line is 70.
Krönlein's scheme for projecting the more important cerebral areas on the surface of the cranium is as follows:-A base line, the same as that advocated by Reid, is drawn from the infra-orbital margin backwards through the upper border of the external acoustic meatus to the occipital region, which it strikes a little below the inion, Fig. 1068. A second line is drawn backwards parallel to it from the supraorbital margin. Three vertical lines are now projected between these two parallels: the anterior from the centre of the zygomatic arch (C), the middle from the preauricular point (D), the posterior from the posterior border of the base of the mastoid process (E). The latter is prolonged upwards to the sagittal line on the cranial vault, and the direction of the central sulcus of Rolando is obtained by drawing a line from this point obliquely downwards and forwards to the point where the anterior vertical meets the superior horizontal line (Sylvian point). The inferior extremity of the sulcus corresponds to the point where the middle vertical line, prolonged upwards, meets the Rolandic line. The lateral fissure line is obtained by bisecting the angle formed by the Rolandic line and the superior horizontal line.
The topographical distribution of function in the cerebral cortex is shown in Fig. 1068, in which the areas worked out by Grünbaum and Sherrington in the anthropoid apes have been transferred to the human brain. The above observers have shown that, while the motor area occupies the whole length of the anterior central convolution and of the central sulcus of Rolando (with the exception of its very extremities), it nowhere extends on to the exposed surface of the posterior central convolution; nor does it extend as far down on the medial surface of the hemisphere as the sulcus cinguli. Extirpation of the hand area, for example, is
ollowed by severe paralysis of the hand, but the use and power of the hand is egained in a few weeks; ablations, on the other hand, of even large portions of he posterior central gyrus do not give rise even to transient paralysis.
În some of the animals experimented on, the motor area was found to extend o the deeper part of the posterior wall of the central sulcus of Rolando. Anteriorly t extended into the pre-central sulci as well as into the occasional sulci which cross he anterior central gyrus; indeed the hidden part of the motor area fully equals in
FIG. 1067.-CRANIO-CEREBRAL TOPOGRAPHY.
Shows relations of the motor and sensory areas to the gyri, and to Chiene's lines.
0. External occipital protuberance (inion).
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.
extent that contributing to the free surface of the hemisphere. The motor areas extend a little in front of the superior and inferior pre-central sulci, which cannot therefore be regarded as physiological boundaries.
Reference to Fig. 1068 shows that, of the main areas, that for the lower extremity occupies the upper third of the motor region, that for the upper extremity the middle third, while the face occupies the inferior third. The relative topography of the chief subdivisions of these main areas is shown in Fig. 1069. It must be remembered, however, that there exists much overlapping of the adjacent areas.
The body of the lateral ventricle, which is equal in length to the posterior
horizontal ramus of the lateral cerebral fissure, occupies a level midway between it and the temporal line. The anterior horn of the ventricle opposite the lower part of the coronal suture while the posterior horn is opposite the posterior part of the temporal line. The inferior horn corresponds to the second temporal gyrus.
The lateral ventricle may be tapped or drained from above, by traversing brain tissue for a depth of 4 to 5 cm. through the superior frontal sulcus, 1 in. (two fingers'-breadth) in front of the point X2, Fig. 1067, the instrument being directed downwards and backwards (Kocher).
FIG. 1068. SCHEME SHOWING RELATIVE TOPOGRAPHY OF THE CHIEF SUBDIVISIONS OF THE MOTOR AREA (adapted from Grünbaum and Sherrington). Guiding lines, red; sulci, blue.
Keen drains the ventricles through an opening 14 in. behind the external acoustic meatus and the same distance above Reid's base-line (a line drawn backwards from the inferior margin of the orbit through the centre of the external acoustic meatus, X3, Fig. 1067), the instrument being passed into the brain towards the summit of the opposite auricle. If the ventricle is not distended it will be reached at a depth of two inches from the surface.
To open the subarachnoid space, the pin of a small trephine is placed over the mid-point of the line EA, Fig. 1067; the dura is incised as it crosses the stem of the lateral fissure of the brain from the frontal lobe to the anterior extremity of the temporal lobe. Care must be taken to keep in front of the middle meningeal artery.
The cisterna cerebello-medullaris, situated between the under surface of the cerebellum and the medulla oblongata, may be reached by turning down a flap of soft parts, and removing a circle of bone immediately above the foramen magnum.