To make as clear as possible the points of practical importance which have, of late years, been put on a definite basis, and which the surgeon may have to recall and act upon at very short notice
, cranio-cerebral topography will be spoken of under the following headings:
- Relation of the brain as a whole to the skull.
- Relation of the chief sulci and gyri to the skull.
- Localisation of the chief sulci and gyri.
Before alluding to the above, it is necessary to say distinctly that the following surface-markings and points of guidance are only approximately reliable, for the following reasons:
- In two individuals of the same age and sex the sulci and convolutions are never precisely alike.
- The relations of the convolutions and sulci to the surface vary in different individuals.
- That as the surface area of the scalp and outer aspect of the skull are greater than the surface area of the brain, and as the convexities do not tally, lines drawn on the scalp or skull cannot always correspond precisely to cerebral convolutions or sulci.
It results from the above that when a definite area of the surface is said to correspond accurately in any individual to a definite area of the brain surface, this result has been correlated from many examinations; and that as surface-markings, shape, and processes of skull and arrangement of surface are all liable to variations in different individuals, the surgeon must allow for these variations by removing more than that definite area of skull which is said to correspond exactly to that part of the brain which he desires to expose.
The Outline op the Brain and its Fissures in Relation to the Sutures of theSkull. (Cunningham.)
S.M. Supraciliary margin of the cerebrum. — I.L.M., Infero-lateral margin of the cerebrum. — l.S. Position of highest part of the arch of the transverse (lateral) sinus. — R., Central sulcus (Fissure of Rolando). — s1 . Anterior horizontal limb of lateral (Sylvian) fissure. — s2 . Anterior ascending limb of lateral (Sylvian) fissure. — s3 . Posterior horizontal limb of lateral (Sylvian) fissure. — P.B. Opercular portion (Pars basilaris) of the inferior frontal convolution. — P.T. Triangular portion of the inferior frontal convolution. — P.O. Orbital portion of the inferior frontal convolution.
Relation of the brain as a whole to the skull
To trace the lower level of each cerebral hemisphere on the skull, the chalk would start from the lower part of the glabella; thence the line pursues a course, slightly curved upwards, about a third of an inch above the supraorbital margin; next, crossing the temporal crest about half an inch above the external angular process, it passes not quite horizontally but descending slightly to a point in the temporal fossa just below the tip of the great wing of the sphenoid (pterion), 25 cm. (1 in.) behind the external angular process. From this point the line of the level of the brain, now convex forwards and corresponding to the anterior extremity of the temporal lobe, would dip down, still within the great wing of the sphenoid, to about the centre of the zygoma. Thence the line would travel along the upper border of this process about 6 mm. (0.25 in.) above the roof of the external auditory meatus, and thence just above the base of the mastoid and the posterior inferior angle of the parietal, and so along the superior curved line, and corresponding to that of the tentorium and horizontal part of the transverse (lateral) sinus, to the external occipital protuberance.
The upper margin of each hemisphere would be represented by a line drawn from just below the glabella, sufficiently to one side of the middle line to allow for the falx and superior sagittal sinus, to one immediately above the superior external occipital protuberance and inion.
Relation of the chief fissures and convolutions to the skull. Localisation of the chief sulci and gyri. These headings will be taken together.
It will be well to first indicate the position of the chief sutures which mark off the parietal bone, under which lies that part of the brain which is most important to the surgeon — the motor area. The upper limit of the bone will be indicated by the line already spoken of as giving the upper margin of the hemisphere — the sagittal line, or suture. The anterior limit of the parietal bone, formed by the coronal suture, may be traced thus: The point where it leaves the sagittal suture (the bregma) will be found by drawing a line from a point just in front of the external auditory meatus (the pre-auricular point) straight upwards on to the vertex; from this point a line drawn downwards and forwards to the middle of the zygomatic arch would indicate that of the coronal suture. Under this suture lie the posterior extremities of the three lateral frontal convolutions; for the frontal lobe lies not only under the frontal bone, but extends backwards under the anterior part of the parietal, the central sulcus (fissure of Rolando), which separates the frontal from the parietal lobe, lying from 3.7 to 5 cm. (1.5 to 2 in.) behind the coronal suture at its upper extremity and about 2.5 cm. ( 1 in.) at its lower.
The squamoso-parietal suture, which marks the lower border of the anterior two-thirds of the parietal bone, is not so easy to define, owing to the irregularity and variations of its curve. Its highest point is usually 4.3 cm. (1.75 in.) above the zygoma.
The lambdoid suture, which forms the posterior boundary of the parietal bone, will be marked out by a line which starts from a point (lambda) about 6.2 cm. (2.5 in.) above the external occipital protuberance, and runs downwards and forwards to a point on a level ivith the zygoma, 3.7 cm. (1.5 in.) behind and 1.2 cm. (0.5 in.) above the centre of the meatus.
The position of the chief sulci will now be given: — Lateral (Sylvian) fissure.— The point of appearance of this, on the outer side of the brain, practically corresponds to the pterion — a point which lies in the temporal fossa, about 3.7 cm. (1.5 in.) behind the external angular process and about the same distance above the zygoma. From this point the lateral (Sylvian) fissure, which here separates the frontal and parietal from the temporal lobe, runs backwards and upwards, ascending gently, at first in the line of the squamo-parietal suture, then crossing this suture about its center, and thence, ascending more rapidly, it climbs up to the temporal ridge, to end 1.8 cm. (0.75 in.) below the parietal eminence. Its termination is surrounded by the supramarginal convolution, to which the parietal eminence corresponds with sufficient accuracy. Such being the surface-marking of the chief or posterior horizontal limb of the lateral fissure, it remains to indicate briefly the two shorter limbs which bound the inferior frontal convolution, which, on the left side, contains the centre for speech (Broca's convolution), and corresponds to a point lying three fingers'-breadth vertically above the centre of the zygomatic arch. (Stiles.) Of these, the anterior horizontal runs forwards across the termination of the coronal, just above the line of the spheno-parietal suture. The ascending limb runs upwards for about 2.5 cm. (1 in.) just behind the termination of the coronal suture, or 5 cm. (2 in.) behind the external angular process. To indicate the lateral fissure (fissure of Sylvius), a line should be drawn from a point corresponding to the pterion backwards, at first gently and then more abruptly upwards, 1.8 cm. (0.75 in.) below the parietal eminence. The ascending limb (point of bifurcation) will start from a point 5 cm. (2 in.) behind and slightly above the external angular process, and run obliquely forwards and upwards for about 2.5 cm. (1 in.).
The central sulcus (fissure of Rolando). — This most important fissure, around which the motor area is grouped, runs downwards and forwards, separating the frontal and parietal lobes between the following points: The upper end of the fissure, the upper Rolandic point, will be found about 1.2 cm. (0.5 in.) behind the centre of the sagittal line (p. 1270) — a line drawn from just below the glabella to the external occipital protuberance. On the skull this upper Rolandic point will be nearly 5 cm. (2 in.) behind the bregma. The lower extremity of the fissure, the inferior Rolandic point, would be about 1.2 cm. (0.5 in.) behind and a little above the bifurcation of the lateral fissure, i. e. 5 cm. (2 in.) behind and a little above the external angular process (fig. 933). Owing to the obliquity of the central sulcus (fissure of Rolando), this lower point will be nearer the coronal suture than the upper, being distant from it about 2.5 cm. (1 in.). A line about 9.3 cm. (3.75 in.) long, drawn between the upper and lower Rolandic points (the Rolandic line), will give the direction of the central sulcus with sufficient accuracy, when the two genua, the upper concave and the lower convex forwards (fig. 931), are allowed for. The Rolandic line forms with the sagittal line an angle anteriorly of 65° to 70°.
To find the central sulcus (fissure of Rolando) the following methods have been employed. In a given case it is well to employ them all, so that any error in one may be checked by another measurement:
I. The upper and lower Rolandic points are found as just directed, and the Rolandic line is drawn between them.
Drawing op a Cast of the Head of an Adult Male. (Prepared by Professor Cunningham to illustrate cranio-cerebral topography.)
II. The upper Rolandic point is taken as before. The position of the lower end of the fissure is thus determined: From the very end of the external orbital process, where this rises up to join the temporal crest, draw a horizontal line 5.6 cm. (2.25 in.) long, and from the extreme end of this draw a vertical line of a little over 2.5 cm. (1 in.). Between these upper and lower Rolandic points, passing rather obliquely forwards, lies the central fissure. (Lucas Championnière.)
III. Here (1) the sagittal line is taken; (2) and (3) at a right angle to this two lines are drawn vertically over the side of the skull, — the one starting from the pre-auricular point, the other at the level of the posterior border of the mastoid process, — and meeting the sagittal line about 5 cm. (2 in.) behind the second; (4) from the junction of the lines 1 and 3, one is drawn diagonally downwards, reaching 2 about 5 cm. (2 in.) above and a little in front of the external auditory meatus. (Reid and Godlee.)
IV. The central sulcus may be found by the measurements given by Makins and Anderson. The chief cranial landmarks employed here are : (1) the glabellar point; (2) the inial point; (3) the mid-sagittal point, midway between these; (4) the pre-auricular point, in front of the tragus, on a level with the upper border of the external auditory meatus; (5) the angular point, over the external angular process, on a level with the upper border of the orbit; (6) the squamosal point, at the junction of the middle and lower thirds of the line, between the pre-auricular and mid-sagittal points. By the aid of these points, three lines may be drawn upon the shaven scalp definitely related to the principal fissures of the outer surface of the brain: — (1) The sagittal line, from the glabella to the inion. (2) The squamosal line, from the angular to the squamosal point, and about two inches beyond; in this line and its continuation lies the horizontal part of the fissure of Sylvius. (3) The frontal line, from the pre-auricular to the mid-sagittal point. The fissure of Rolando crosses this line at an acute angle. Its upper end, carried to the midline, lies three-eighths of an inch behind the mid-sagittal point ; its lower end, in the squamosal line, three-eighths of an inch beyond the squamosal point.
Cranio-cerebral Topography. (Anderson and Makins.)
Some further points in the surgical anatomy of the cranium must be referred to: — The middle meningeal artery. This vessel, entering the middle fossa by the foramen spinosum, grooves the great wing of the sphenoid and divides into two branches. The anterior grooves the anterior inferior angle of the parietal bone, and is then continued upwards and slightly backwards between the coronal suture and central sulcus, almost to the vertex; the posterior branch takes a lower level, running backwards under the squamous bone to supply the parietal and anterior part of the occipital bones. If a skull, bisected antero-posteriorfy, be held up to the light, it will be seen how thin are the bones over the chief branches of this vessel, thus accounting for the slight violence sometimes sufficient to rupture it. The groove it occupies in the parietal is sometimes converted into a canal. A wounded artery retracting here may be very difficult to secure. The veins which accompany the artery are thin-walled and sinus-like before they open into the sphenoparietal sinus, another explanation of the obstinacy of this haemorrhage. According to the point of rupture, three haematomata should be remembered (Krönlein), anterior or fronto-temporal; middle, or temporo-parietal; and posterior, or parietooccipital. The first two are much the most frequent, and exposure of the pterion, with free removal of the adjacent bone, will suffice for dealing with them.The figure and its accompanying letter-press show the different sites for trephining for those common complications of otitis media, viz., temporo-sphenoidal abscess, thrombosis of the lateral sinus, and cerebellar abscess.
Drainage of the lateral ventricle. — Where the anterior fontanelle is closed, Poirier and Keen have opened the descending cornu through the middle temporal convolution, the pin of the trephine Deing placed 3.1 cm. (1.25 in.) behind the external auditory meatus, and about the same distance above Reid's base-line. This point corresponds with sufficient accuracy to d, in the figure. The needle should here be directed to a point about 5 cm. (2 in.) above the opposite ear.
Up to this point the outside of the cranium has been mainly considered; it remains to draw attention to some of the chief points in the surgical anatomy of the interior, especially of the base. The three fossae are of paramount importance in fracture. In the anterior fossa the delicacy of parts of the floor, the connection of this with the nose and orbit, and the exact adaptation of its irregular surface to that of the frontal lobes, no 'water-bed' intervening, are the chief points. Thus the slightness of a fatal fissure, the frequent presence of bruising after a blow perhaps on the occiput, which has been considered to have caused only concussion, the characteristic palpebral haemorrhage, and the infection of a fracture here are all explained, together with the possibility and gravity of a fracture here from a severe blow on the nose. In the middle fossa the frequency of fractures is explained by the facts that while here, as in the other fossae, a fracture often radiates down from the vertex, the overlying vault being a region often struck, the base is weakened by numerous foramina and fissures. Further, the resisting power of the petrous bone must be lessened by the cavities for the internal ear, the carotid, and, to a less degree, by the jugular fossa. For fluids to escape through the external meatus, the dura, the prolongation of the arachnoid into the internal meatus, the membrani tympani, and probably the internal ear, must all be injured. The presence of the middle meningeal artery and the cavernous sinus in this fossa must also be remembered, especially in such operations as that on the Gasserian ganglion. Posterior fossa : It is not sufficiently recognised that fractures here are, owing to the anatomy of the parts, in some respects the most important of all. It is here that a small fissure-fracture, ultimately fatal, with severe occipital and frontal bruising and some intradural haemorrhage, has been so often overlooked, especially in the drunken. This is explained by the supposed strength of the bone, this being really very thin in places, by the thickness of the soft parts, and the abundance of hair. Further, there is no very apparent escape of cerebral contents as in the anterior and middle fossae. Blood, etc., may trickle into the pharynx far back, or a deep-seated ecchymosis coming up after two days, under the muscles about the mastoid process, may call attention to the damage within.
Showing the Relations of the Lateral Sinus to the Outer Wall op the Skull, and the Position of the Trephine-opening, a, for Exploring it.
Reid's base-line is shown passing through the middle of the external auditory meatus and touching the lower margin of the orbit, x x indicate the site of the tentorium as far as it is in relation to the outer wall of the skull. The anterior x shows the point where the tentorium leaves the skull and is attached to the upper border of the petrous bone, a, Trephine-opening to expose transverse sinus, its centre being 1 in. behind and 0.75 in. above the centre of the meatus. This opening can easily be enlarged upwards, backwards, downwards, and forwards (see the dotted lines) by suitable angular forceps. It is always well to extend it forwards so as to open up the mastoid antrum, c. b, Trephine-opening to explore the anterior surface of the petrous bone, the roof of the tympanum, and the petro-sauamous fissure, its centre being situated a short inch above the centre of the meatus. At the lower margin of this trephine-opening a probe can be insinuated between the dura and the bone, and made to search the whole of the anterior surface of the petrous, c, Trephine-opening for exposing antrum, 0.25 inch above and behind the centre of the meatus, d, Trephine-opening for temporo-sphenoidal abscess (Barker), 1.75 inches behind and above centre of meatus. The needle should be directed at first inwards, and a little downwards and forwards, e, Trephine-opening for cerebellar abscess, 1.5 inches behind and one inch below the meatus. The anterior border of the trephine should be just under cover of the posterior border of the mastoid process. Such an opening is well removed from the transverse sinus, and a needle, if directed forwards, inwards, and upwards, would enter an abscess occupying the anterior portion of the lateral lobe of the cerebellum, the usual site of an abscess in this part of the brain. (Barker.)
Dura mater. — This, described as acting as an endosteum, while bringing blood-vessels to the bone, lacks the chief power of periosteum elsewhere in that it has very little bone-producing power; the influence of its partitions and its damping effect on vibrations are great in blows on the head. Its varying adhesions, according to site and age, must be remembered. Thus while it is intimately connected over the base with its adhesions to the different foramina, it is more loosely connected with the vault, as is shown in middle meningeal hemorrhage. In early and later life the closeness of its connection with the bones is also more marked. Its junction with the inter-sutural membrane has been referred to. Its relation to tne arachnoid beneath, and then the origin of a pachymeningitis, sometimes salutary by shutting off the arachnoid, and a leptomeningitis, almost invariably fatal, must be remembered.
Finally, the existence of the cerebro-spinal fluid with its power of lessening the evil of vibrations and its aid in regulating intra-cranial pressure, must be borne in mind. The chief collections, in which the subarachnoid meshwork is almost absent, are met with in front and behind the medulla. That in front, also lying under the pons, Mr. Hilton's 'water-bed' sends a prolongation forwards to the optic chiasma, but does not extend under the frontal lobes. The collection behind lies between the medulla and under surface of the cerebellum. Here, by the foramen of Magendie, this collection and, also, the intraventricular communicate with the subarachnoid space of the spinal cord.