The temporo-sphenoidal lobe presents on its lateral surface again three transverse convolutions, superior, middle, and inferior (c, c', c", Fig. 322); the superior, bounding the fissure of Sylvius and also called Infra-marginal, is continuous with the angular gyrus. The remaining two sets on the basal surface have been already mentioned as forming, with those of the occipital lobe, the inferior occipito-temporal convolutions.

On examining the Internal aspect of the cerebral hemisphere (Fig. 323) certain convolutions are to be distinguished. The superior frontal convolution and the ascending frontal and parietal convolutions are here partly visible. More distinctly on the internal surface we distinguish in front the Marginal convolution (marg. c.) which is continuous with the superior frontal and lies along the superior longitudinal fissure. Immediately behind this convolution we come to the superior parietal lobule which, on its mesial surface, is called the Precuneus or Quadrate lobe (see figure). The parieto-occipital fissure is here very marked, and it is joined at an angle by the calcarine fissure (calc. f.) in such a way as to demarcate a triangular surface, the Cuneus (cuneate I.). On this aspect also appear the inferior occipitotemporal convolutions (i. oc.-temp. c). Within this external ring of convolutions we have now a deeper layer. Immediately bordering on the corpus callosum and following the fornix we trace from before backwards the Gyrus fornicatus. Having skirted the corpus callosum from before backwards it turns round at its posterior extremity, and passes downwards into the Gyrus hippocampi. This gyrus is also continuous with the gyrus cunei and the median occipito-temporal. The gyrus hippocampi passes forward towards the anterior extremity of the temporo-sphenoidal lobe, where it terminates in a hook-like curve, the Gyrus uncinatus (unc. c).

Fig. 323. - Internal aspect of right cerebral hemisphere. Explanation in text. (Quain).

In the cornu Ammonis of the lateral ventricle there lies a convolution whose surface is turned inwards and forms an elongated rounded projection in the cornu; this is the Gyrus dentatus (d. c).

Functions Of The Convolutions

The localization of function in the cortex of the brain is of so much practical importance that it is well, when opportunity offers, to familiarize the mind with the topography of the convolutions in relation to function.

The Cerebellum

There are only a few points which require notice here as to the general arrangements of the parts in the cerebellum. Like the cerebrum it is divisible into two lateral hemispheres, the right and left lobes. These are united by a central piece, which is most marked on the under surface, called the vermiform process. The cerebellum is divided by many fissures which run horizontally and leave narrow convolutions called the Folia. One of these fissures, deeper than the rest, and called the great horizontal fissure, divides the cerebellum into an upper and a lower portion. In its internal structure the cerebellum presents white matter which runs outwards from the peduncles diverging towards the folia and forming a tree-like expansion, the Arbor vitse. In the midst of the white substance in each hemisphere there is a small grey nucleus, not unlike the olivary body, called the Corpus dent a turn.

Arteries Of The Brain

With a view to the identification of the numerous lesions of the arteries in the brain, it will be proper here to refer briefly to the distribution of these vessels. The Circle of Willis (see Fig. 327, p. 661) gives off at the base three main arteries to the brain, the posterior, middle, and anterior cerebral. The Posterior cerebral artery, besides giving certain central branches to be afterwards referred to, is distributed on the surface of the brain, supplying the greater part of the occipital and temporo-sphenoidal lobes with the exception of the upper temporo-sphenoidal convolution. The Middle cerebral artery, or the artery of the fissure of Sylvius, is of great importance as being much more frequently the seat of lesions than the others. Besides its central branches, afterwards considered, it supplies the middle district of the brain all round the fissure of Sylvius, including the parietal lobe, the posterior parts of the frontal lobe, and the superior convolution of the temporo-sphenoidal lobe. The Anterior cerebral artery is distributed to the anterior parts of the frontal lobe.

These arteries, with the exception of their central branches, run in the sulci of the convolutions, and divide into successive orders of branches which lie in the pia mater. The larger branches anastomose sparely, while the finer twigs are end-arteries (Duret). All the arteries hitherto considered run on the surface of the brain in the soft membranes. The actual nutrient arteries are branches of these, and penetrate from them into the substance of the brain. We may thus distinguish the larger arteries of the surface and the smaller or nutrient arteries.

The Nutrient arteries again are divisible into two groups, which may be designated the cortical and central systems.

The Nutrient arteries of the cortex pass off not merely from the finer twigs of the larger arteries, but also from the larger branches (see Fig. 324). They are fine thread-like vessels which pass down perpendicularly into the brain substance. Some of them are short and supply the grey substance of the convolutions, others are longer and reach the white substance, their territory here extending to the boundaries of that of the central arteries. In stripping the membranes from the surface of the brain these nutrient arteries are seen like fine hairs emerging from the brain substance. If a portion of the soft membranes be now floated in water these fine vessels will be seen like bristles passing from all the branches. All the nutrient arteries are end-arteries.

The Central nutrient arteries have a somewhat different arrangement from those of the cortex. They are long vessels of larger calibre than the cortical ones, and they pass off from the main arteries very near their origin. The most important are those which come off from the middle cerebral just outside its origin and pass through the anterior perforated space to the basal ganglia. Of these, three sets of branches have been distinguished: (1) the lenticular branches, short twigs lying internally which pass to the internal parts of the lenticular nucleus; (2) the lenticulo-Hlriate branch, a longer and larger vessel which lies outside the first and supplies the outer parts of the lenticular nucleus and adjoining parts of the internal capsule and caudate nucleus; this branch is most frequently the seat of rupture with consequent haemorrhage; (3) the lenticulo-optic branch is posterior to the former and supplies the posterior part of the lenticular nucleus and the anterior part of the optic thalamus. The remaining central branches are supplied by the anterior and posterior cerebral arteries. The former sends branches which supply the anterior part of the caudate nucleus, and the latter sends two sets, an internal and an external, to the optic thalamus. All these central arteries are end-arteries.

Fig. 324. - Arteries of the brain showing cortical distribution. A, a tertiary branch of main artery; B, primary twig; C, C, C, secondary twigs. The nutrient arteries are seen to pass off from all these branches as fine hair-like offsets. (Kuss after Duret).

The arteries or the pons and medulla oblongata are like the cortical arteries in their small size and like the central ones in respect that they come off from large stems, and pass directly into the substance of the part.