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ing posterior central area becomes raised up into a prominent ridge, and a similar ridge is formed immediately in front of it (Fig. 590) from the area which emits the great efferent or motor tract to control all the motor nuclei upon the other side of the brain and spinal medulla. As these ridges become raised up a depression is left between them: this is the sulcus centralis. At first it consists of two parts, a superior and an inferior (Fig. 590, r2 and r1); but as a rule these become confluent later.
The sulcus centralis in the adult takes an oblique course across the lateral convex surface of the cerebral hemisphere, and intervening between the frontal and parietal regions it forms the immediate posterior boundary of the motor area of the cortex. Its upper end cuts the supero-medial border of the hemisphere a short distance behind the mid-point between the frontal and occipital poles, whilst its inferior end terminates above the middle of the posterior ramus of the lateral fissure. Its superior extremity, as a rule, turns round the supero-medial border of the hemisphere, and is then continued backwards for a short distance on the medial surface. Although, in its general direction, the sulcus is oblique, it is very far from being straight. It takes a sinuous course across the hemisphere. This is largely due to the varying breadth of the motor areas representing the lower limb, trunk, upper limb, and head, respectively, which are placed immediately in front of it.
When the sulcus centralis is widely opened up, so that its bottom and its opposed sides may be fully inspected, it will be seen that the two bounding gyri are dovetailed into each other by a number of interlocking gyri, which do not appear on the surface (Fig. 591). Further, two of these, placed on opposite sides of the fissure, are frequently joined across the bottom of the sulcus in the form of a sunken bridge of connexion, which constitutes what is termed a deep transitional gyrus. The continuity of the sulcus is thus, to some extent, interrupted. This condition is rendered interesting when considered in connexion with the development of the sulcus. The deep interlocking gyri indicate a great exuberance of cortical growth in this situation in the early stages of the development of the sulcus; and the presence of the deep transitional gyrus is explained by the fact that the sulcus generally develops in two pieces, which run into each other to form the continuous sulcus of the adult, viz., a part corresponding to the inferior two-thirds, and a superior part, which represents the superior third and which appears at a slightly later date. In certain very rare cases the sulcus centralis is found to remain double throughout life, through a failure of its two pieces to unite. In such cases the deep transitional gyrus, which is frequently seen at the bottom of the furrow, remains on the surface. Heschl, who examined 2174 cerebral hemispheres, found this anomaly only six times; Eberstaller met with it twice in 200 brains.
FIG. 591.-SULCUS CENTRALIS FULLY OPENED UP, so as to exhibit the interlocking gyri and deep transitional gyrus within it.
Motor cortex coloured red, sensory cortex blue.
If a section is made at right angles to this sulcus in a fresh brain (Fig. 592), it will be seen that its anterior (gyrus centralis anterior) and posterior (gyrus centralis posterior) walls present a marked contrast the one to the other, and that the transition from the one type of cortex to the other takes place precisely at, or near to, the bottom of the sulcus. The anterior wall is composed of thick (3.5 to 4 mm.) motor cortex thickly laden with medullary matter arranged in the form of three or four pale bands with blurred edges and multitudes of fine pencils of fibres passing to and fro between it and the white matter of the hemisphere. The posterior wall is composed of thin (1.5 mm.) cortex containing two narrow and sharply defined white lines.
This sensory area forms little more than the posterior wall of the sulcus centralis, and barely emerges upon the surface to form the posterior lip of the sulcus (Fig. 581). Here it becomes continuous with a slightly thicker cortex with
doubled lines which are less dense than those of the sensory cortex; this area forms the crest of the gyrus centralis posterior, and then gives place to another slightly modified type of cortex which forms the anterior wall of the sulcus postcentralis. Thus the sensory cortex has two fringing bands analogous to those already noticed alongside the visual and acoustic areas.
The motor and sensory areas cross on to the medial aspect of the hemisphere, into a region known as the lobulus paracentralis. In this region a furrow (sulcus paracentralis) is sometimes found along the line of demarca-1 tion of the medial parts of the motor and sensory areas (Fig. 589).
That portion of the parietal region which intervenes between the gyrus centralis posterior and the occipital region is usually subdivided into two distinct parts (lobulus parietalis superior and lobulus parietalis inferior) by a horizontal furrow, called the sulcus interparietalis proprius. The term sulcus interparietalis is usually applied in a purely arbitrary and artificial manner to a complex of four genetically distinct and in- t dependent furrows (Fig. 593, pl, p2, p3, and p), the sulcus postcentralis inferior (p1), the sulcus postcentralis superior (p3), the sulcus inter- e parietalis proprius (ramus horizontalis) (p3), occipitalis) (p), which ends in the sulcus
Gyrus centralis anterior | Gyrus centralis posterior
White medullary substance
Boundary line between motor and sensory cortex
FIG. 592.-SECTION ACROSS THE SUPERIOR PART OF THE SULCUS CENTRALIS IN A FRESH BRAIN. and the sulcus paroccipitalis (ramus occipitalis transversus.
These four furrows develop quite independently one of the other, the postcentral sulci as the posterior boundary of the sensory territory, the paroccipital sulcus as the supero-lateral boundary of the visual territory, and the more variable horizontal ramus (the sulcus interparietalis, in the strict sense of the term) as a demarcation
Posterior central gyrus
Parieto-occipital sulcus (incisura)
pl. Sulcus postcentralis inferior. p2. Sulcus postcentralis superior.
Transverse occipital sulcus
Termination of lateral fissure
FIG. 593. THE FOUR SULCI OF THE INTERPARIETAL COMPLEX OPENED UP, so as to show the deep transitional gyri intervening between them.
p3. Ramus horizontalis (sulcus interparietalis proprius).
between the two parietal lobules. The four furrows may unite to form any possible combination.
The superior parietal lobule is composed of moderately thick cortex (2.5 to 3 mm.) placed between the interparietal sulcus (ramus horizontalis) and the superior border of the hemisphere, where it becomes continuous on the medial surface with the precuneus. Each of these parts is subdivided by a transverse sulcus, the
superior lobule by the sulcus parietalis superior and the precuneus by the sulcus præcunei (Fig. 589).
The latter sulcus usually joins a small inverted U-shaped furrow (sulcus subparietalis), which encloses a cortical territory of distinctive structure-the area parasplenalis [præcunei].
The inferior parietal lobule, which from its position is the natural meeting-place for impressions coming from the visual, acoustic, and tactile territories, is naturally a region of great functional significance. It is composed of a series of areas differing in thickness and texture. The anterior region forms a convolution (gyrus supramarginalis) surrounding the upturned extremity of the lateral fissure; behind it there is a second convolution called the gyrus angularis, which surrounds a vertical sulcus angularis, often described as the extremity of the sulcus temporalis superior; but in reality it is quite independent of the latter furrow, but it often becomes confluent with it. Behind the gyrus angularis and separated from it by a transverse furrow (sulcus occipitalis anterior) there is a cortical territory (area parietooccipitalis) which may perhaps be looked upon as a specialised and outlying part of the peristriate area of the visual cortex.
THE FRONTAL REGION.
The frontal region is the biggest of the main cortical areas-the so-called "lobes." On the lateral surface of the hemisphere, it is bounded behind by the sulcus centralis and below, in part, by the lateral fissure. It presents a lateral surface, a medial surface, and an inferior or orbital surface.
On its lateral aspect the surface is broken up by a large series of furrows, which exhibit considerable variability.
The inferior precentral sulcus consists of a vertical and an oblique part. The vertical portion lies in front of the inferior part of the sulcus centralis, whilst the oblique part extends forwards and upwards (Fig. 594).
The superior precentral sulcus is a short vertical sulcus which lies at a higher level than the inferior precentral sulcus, in front of the upper part of the sulcus centralis. It is almost invariably connected with the posterior end of the superior frontal sulcus.
The anterior central gyrus is a long continuous gyrus, which is limited in front by the two precentral furrows and behind by the sulcus centralis. Inferiorly it is continuous with the area subcentralis which links it to the posterior central gyrus. The area subcentralis is limited in front and behind by the anterior and posterior subcentral sulci (not labelled in Fig. 594).
The superior frontal sulcus extends forwards in a more or less horizontal direction from the sulcus præcentralis superior.
The gyrus frontalis superior is the narrow convolution between the supero-medial border of the hemisphere and the superior frontal sulcus and the continuation of this convolution into a broad area upon the medial surface.
The inferior frontal sulcus occupies a lower level than the superior frontal sulcus. Its posterior end is placed in the angle between the vertical and horizontal parts of the inferior precentral sulcus, and is not infrequently confluent with one or other of these. It proceeds forwards towards the superciliary margin of the hemisphere and ends a short distance from this in a terminal bifurcation (Fig. 594).
The gyrus frontalis medius is the name given to the broad convolution which lies between the superior and inferior frontal sulci.
The gyrus frontalis inferior is that portion of the lateral surface of the frontal region which is placed in front of the inferior precentral sulcus and below the inferior frontal sulcus. The inferior frontal convolution includes three cortical areas (Fig. 594) differing in structure the one from the other. The sulcus diagonalis separates the intermediate of these from the posterior.
The sulcus frontalis medius begins midway between the anterior ends of the superior and inferior frontal sulci and proceeds obliquely forwards towards the frontal pole. When the furrow reaches the superciliary margin of the hemisphere it ends near a transverse furrow, called the fronto-marginal sulcus.
On the medial aspect of the frontal lobe there are two convolutions, the larger peripheral area which forms part of the gyrus frontalis superior and a smaller inner part encircling the corpus callosum, which is called the gyrus cinguli. These gyri are separated by the sulcus cinguli (Fig. 589).
The posterior part of the sulcus cinguli is genetically distinct from the anterior part and it circumscribes a broader area, the lobulus paracentralis, which is continuous with the gyri centrales of the lateral surface of the hemisphere.
FIG. 594.-AREAS ON LATERAL ASPECT OF LEFT CEREBRAL HEMISPHERE.
The superior, middle, and inferior frontal sulci are not labelled. The middle is in the midst of the green area, the superior and inferior respectively at its superior and inferior boundaries.
On the orbital aspect of the frontal region there are two sulci, viz., the olfactory and the orbital.
The olfactory sulcus is a straight furrow which runs parallel to the medial orbital border of the hemisphere. It is occupied by the olfactory tract and bulb, and it cuts off a narrow strip of the orbital surface close to the medial border, which receives the name of gyrus rectus.
The orbital sulcus is a composite furrow which assumes many different forms. It consists essentially of a U-shaped furrow, the convexity of which is directed anteriorly (Fig. 585), and one or two variable branches passing forwards from it.
The conventional manner of subdividing the cortical territory anterior to the sulcus centralis into gyri, which has just been sketched, is apt to convey a misleading idea of the distribution of the anatomical areas of differentiated cortex.
The gyrus centralis anterior together with the major portion of the paracentral lobule and the posterior part of the middle and superior frontal gyri form a natural subdivision of the cortex, which Brodmann calls the regio præcentralis. It is composed of a series of areas of different structure, which may be grouped as the area præcentralis posterior (the true motor area), the area præcentralis intermedius, and
the area præcentralis anterior (Fig. 594). Most of the motor area is hidden in the sulcus centralis, but towards the supero-medial margin of the hemisphere a considerable area emerges upon the surface of both the gyrus centralis anterior and the paracentral lobule.
Brodmann calls the rest of the frontal territory the regio frontalis; but in the colour scheme adopted in Figs. 585, 589, and 594 the inferior frontal gyrus and the orbital area posterior to the orbital sulcus have been associated with the "precentral" rather than the "frontal" regions.
WEIGHT OF THE BRAIN.
The average weight of the adult male brain may be said to be about 1360 grammes. The female brain weighs rather less, but this is to be expected from the smaller bulk of the female body. Probably the relative weight of the brain in the two sexes is very much the same. The variations met with in brain-weight are very great, but it is doubtful if normal intellectual functions could be carried on in a brain which weighs less than 960 grammes. In microcephalic idiots brains of extremely small size are met with.
THE MENINGES OF THE ENCEPHALON AND
The brain and spinal medulla are enclosed within three membranes, which are termed the meninges or meningeal membranes. From without inwards these are: (1) the dura mater, (2) the arachnoid, and (3) the pia mater. The space between the dura mater and the arachnoid receives the name of subdural space, while the much more roomy interval between the arachnoid and the pia mater is called the subarachnoid space.
The dura mater is a dense and thick fibrous membrane which possesses a very considerable degree of strength. Its arrangement within the cranial cavity is so different from that within the vertebral canal that it is customary to speak of it as consisting of two parts, viz., a cranial and a vertebral, although in adopting this subdivision it must be clearly understood that both portions are continuous with each other at the foramen magnum.
Dura Mater Encephali.-The cranial dura mater is adherent to the inner surface of the cranial wall, and performs a double office. It serves as an internal periosteum for the bones which it lines and it constitutes an envelope for the brain. Its inner surface, which bounds the subdural space, is smooth and glistening, and is covered with a layer of endothelial cells. The outer surface when separated from the cranial wall, is rough, this being due to numerous fine fibrous processes and blood-vessels which pass between it and the bones. Its degree of adhesion to the cranial wall differs considerably in different regions. To the vault of the cranium, except along the lines of the sutures, the connexion is by no means strong, and in the intervals between the fibrous processes which pass into the bone there are small lymph spaces (epidural spaces) where the outer surface of the membrane is covered by endothelial cells. So long as the sutures are open the dura mater is connected with the periosteum on the exterior of the skull, along the sutural lines, by a thin layer of fibrous tissue which intervenes between the bony margins. Around the foramen magnum, and on the floor of the cranium, the dura mater is very firmly adherent to the bone. This is more particularly marked in the case of the projecting parts of the cranial floor, as, for example, the petrous portions of the temporal bones, the clinoid processes, and so on. This firm adhesion in these regions is still further strengthened because the nerves, as they leave the cranium through the various foramina, are followed by sheaths of the fibrous dura