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The remainder of the true temporal region is composed of an extensive district below the superior temporal sulcus. It is composed of thicker cortex than the superior temporal area, ranging from 3 mm. just below the superior temporal sulcus to 2.5 mm. at the inferior border of the hemisphere. It is composed of three bands of different texture, the middle temporal gyrus, the inferior temporal gyrus, and the pararhinal gyrus, which fringes the area piriformis on the tentorial surface. Upon the lateral aspect of the temporal region a series of irregular furrows are situated along the line of demarcation between the gyrus temporalis medius and the gyrus temporalis inferior; they are considered to represent a sulcus
temporalis medius, but they are subject to much irregularity, especially in highly developed brains. The sulcus temporalis inferior, which forms the line of demarcation between the gyrus temporalis inferior and the gyrus Sulcus orbitalis fusiformis, is placed upon the inferior aspect of the temporal region.
The great extent of the middle and inferior temporal gyri constitutes one of the outstanding features distinctive of the Pararhinal gyrus human brain. Flechsig
has shown that the fibres passing to and from these two gyri are the last to become medullated, later even than the important Sulcus sagittalis parietal and frontal areas.
Inferior frontal gyrus
Area temporalis polaris
temporal gyrus Inferior temporal sulcus (anterior part)
Inferior temporal sulcus (posterior part)
Sulcus collateralis trans
Sulcus calcarinus lateralis
Area peristriata THE VISUAL AREAS
We have already seen (Figs. 578 and 584) that each optic tract ends in the lateral geniculate body, the pulvinar of the thalamus, and the superior colliculus. From the lateral geniculate body (and according admitted by all) a tract
to most writers the thalamus also, though this is not arises which conveys visual impulses back to the occipital pole of the hemisphere. This radiatio thalamo-occipitalis (Gratiolet's optic radiation) is seen from various points of view in the figures mentioned, but it is possible (see Fig. 587) to expose it in a section which will display it in its relationship to the rest of the visual path (Fig. 586).
From this it will be seen that the fibres of the optic radiation, after emerging from the lateral geniculate body, bend backwards in the lateral wall of the ventricle and proceed to an extensive district of thin cortex (1.5 mm. or less in thickness), occupying an area of about 3000 sq. mm. of the medial surface and pole of the occipital area. The cortex in this area is distinguished by the presence of
FIG. 585.-CORTICAL AREAS on the tentorial and orbital aspects of the cerebral hemispheres.
a very distinct white line or stria, which was first noticed by Gennari in the year 1776.
If this visual receptive area striata of the occipital cortex is excised and spread out in one plane, it will be found to present an elongated ovoid form and a superficial extent of about 3000 sq. mm. (varying in different brains from about 2700 to 4000). The narrow extremity of the oval is placed a short distance behind and below the splenium of the corpus callosum; and from this point the area extends horizontally backwards to the occipital pole, or even beyond it on to the lateral aspect of the hemisphere. In the course of development this area striata becomes folded along its axis during the sixth month, and the furrow thus formed is called the sulcus calcarinus. This name was applied to the furrow by Huxley because its deep anterior part indents the whole thickness of the medial
wall of the hemisphere, and the swelling so produced in the posterior cornu of the lateral ventricle was supposed by the older anatomists to resemble a cock's spur, and was hence called calcar avis (see Fig. 566, p. 637).
The anterior part of this furrow is much deeper, more constant in form and position, more precocious in development, and phylogenetically much older than the posterior part. As it is the part of the sulcus which gives rise to the calcar avis, it is the true calcarine; while the newer, shallower posterior part is wholly on the caudal side of the calcar avis, and is called sulcus calcarinus posterior. If the area striata is prolonged on to the lateral surface, it also may become folded in the line of its axis, and so give rise to a sulcus calcarinus lateralis.
There is a fundamental distinction between the calcarine sulcus and the posterior calcarine in their relations to the area striata. For the stria of Gennari is found only in the inferior wall of the sulcus calcarinus, which is therefore a sulcus
limitans; whereas the stria extends throughout both walls of the posterior calcarine sulcus, and in most cases beyond its lips on to the surface of the cuneus and the gyrus lingualis (Figs. 588 and 589), i.e. the exposed cortical areas placed upon the superior and inferior aspects respectively of the sulcus calcarinus posterior.
Along the superior and inferior boundary lines of this area shallow limiting sulci usually develop (Fig. 589), and these furrows often pass backwards into little arched sulci polares, which are furrows of the operculated variety (see p. 646), called into existence by the broadening out of the area striata (not an actual broadening, but an unfolding) as it passes round the edge of the hemisphere.
At the point of transition from the deep sulcus calcarinus into the shallower sulcus calcarinus posterior (Fig. 588) a submerged ridge is usually found-the gyrus cuneolingualis anterior; and other similar ridges, which may be exposed on the
FIG. 588.-THE PARIETO-OCCIPITAL AND THE CALCARINE SULCI FULLY OPENED UP, so as to show the deep transitional gyri marking off the several elements of the <-shaped system.
Area striata, uniform blue; area parastriata, large blue spots; area peristriata, fine blue dots.
surface or may be submerged, are often found interrupting the posterior and lateral calcarine sulci themselves (Fig. 588).
The posterior and lateral calcarine sulci are subject to a very wide range of variation in form, but they are always axial foldings of the area striata.
When the area striata crosses on to the lateral surface of the hemisphere a small semilunar furrow develops a short distance in front of its anterior edge. This is the sulcus lunatus. The larger the lateral extension the closer does the edge of the area striata approximate to the caudal lip of the sulcus, which under such circumstances assumes a definitely operculated form. Such cases occur most often in the left hemisphere and in the brains of primitive people; and they represent a perfect realisation of a furrow once supposed not to occur in the human brain, but to be distinctive of the ape. Hence it used to be called the "Affenspalte" or sulcus simialis.
The area striata is surrounded by two peripheral concentric bands an inner, which may be called area parastriata, and an outer, the area peristriata. Sulci develop along the boundary lines of each of these areas; and those which indicate the superior and inferior limits of the peripheral band (i.e. peristriate area)
make their appearance relatively early in development and become very deep furrows.
The inferior of these is placed upon the tentorial surface, and is known as the sulcus collateralis; the superior limiting furrow of the visual territory (its peristriate part) is upon the superior surface of the hemisphere, and is usually regarded as the ramus occipitalis of the sulcus interparietalis. But it is genetically independent of the latter furrow, and may be distinguished as the sulcus paroccipitalis.
Near the supero-medial margin of the hemisphere there is a furrow, which indicates the line of demarcation between the para- and the peristriate areas-the sulcus occipitalis paramedialis. It may be situated upon either the medial or the superior surface of the hemisphere. In some cases it belongs to the category of limiting sulci, in others to the group of operculated sulci (see p. 646).
FIG. 589.-THE CORTICAL AREAS ON THE MEDIAL ASPECT OF THE CEREBRAL HEMISPHERE.
Sulcus polaris inferior
Sulcus calcarinus lateralis
Passing horizontally forwards upon the lateral surface of the hemisphere there is a constant furrow formed by the axial folding of part of the peristriate area, approximately in line with the axial folding of the striate area (sulcus calcarinus lateralis); it is the sulcus occipitalis lateralis. When there is a fully developed sulcus lunatus the lateral occipital sulcus joins it near its midpoint (Fig. 581, p. 654).
The sulcus (or fossa) parieto-occipitalis (Fig. 554) is usually a deep furrow upon the medial aspect of the hemisphere which passes vertically downwards from the supero-medial border and appears to join the calcarine sulcus near its union with the posterior calcarine, forming upon the surface a Y-shaped pattern, the stem of which is calcarine, the limbs posterior calcarine and parieto-occipital respectively, and the wedge-shaped area between the limbs the cuneus (Fig. 588; compare with the pattern shown in Fig. 589, where the parieto-occipital sulcus is not labelled).
If, however, the lips of these three furrows are divaricated (Fig. 588), the parietooccipital depression will be found to be separated from the calcarine by a prominent
submerged cortical ridge, the gyrus cunei; and the parieto-occipital will be found to be something more than a mere sulcus. It is, in fact, a great fossa in which are submerged the anterior parts of the area parastriata and area peristriata, and the posterior part of the parietal area known as the præcuneus, as well as the sulci which separate these territories one from the other. It is a great trough formed by the splenium of the corpus callosum as in the course of its development it thrusts itself backwards and crumples up the cortex. When the corpus callosum fails to develop, no parieto-occipital fossa makes its appearance. The part of the sulcus that notches the supero-medial border (Figs. 589 and 593) forms a distinct element, which Retzius has called the incisura parieto-occipitalis.
Sulcus Collateralis.-The collateral sulcus is a strongly marked furrow on the tentorial face of the cerebral hemisphere. It begins near the occipital pole and extends forwards towards the posterior end of the rhinal fissure, with which it sometimes becomes confluent. In its posterior part it is placed below, and parallel to, the calcarine fissure, from which it is separated by the lingual gyrus. From the posterior extremity a sulcus proceeds forwards and then laterally across the inferior surface of the occipital region, forming a V-shaped pattern with the collateral sulcus (Fig. 585). As it is serially homologous with the latter, being, like it, an inferior boundary of the area peristriata, it may be called the sulcus collateralis transversus. The lingual gyrus is sometimes subdivided by a furrow (sulcus sagittalis gyri lingualis) midway between the collateral sulcus and the inferior margin of the area striata. It is the line of demarcation between the parastriate and peristriate areas, and when deep is often mistaken for the collateral sulcus.
leads into the temporal region and The facts of clinical medicine show that large areas in these two regions beyond the limits of the cortex in which the acoustic and optic radiations end are concerned with the functions of hearing and vision. A large part of the parietal area is interposed between these temporal and occipital territories, and its integrity and normal functioning is a necessary condition for the proper performance of many acts, such as reading written or printed documents, in the appreciation of which both hearing and vision have played some part. But the parietal region also includes the cortical area in which a part, at least, of the chief thalamo-cerebral tract ends the bundle of fibres that represents the third stage of the great sensory pathway, the first stage of which is formed by the spinal and cerebral sensory nerves and their central prolongations, and the second stage by the spino-thalamic, bulbo
thalamic, and ponto-thalamic fasciculi, which pass upwards in the medial lemniscus and end in the ventral nucleus of the thalamus (Figs. 579 and 580).
The sensory area in question forms part of the gyrus centralis posterior, which intervenes between two oblique furrows, the sulcus centralis and the sulcus postcentralis, which extend across the whole breadth of the hemisphere above the sulcus lateralis (Fig. 581).
Sulcus Centralis. During the sixth and seventh months of foetal life the expand
We have seen that the acoustic pathway the visual pathway into the occipital region.
THE PARIETAL REGION.
FIG. 590.-LEFT CEREBRAL HEMISPHERE, from a fœtus in
Inferior part of central sulcus.
0. Orbital wall.