The island of Reil lies at the bottom of the posterior horizontal limb of the Sylvian fissure, and is thus closely related to the frontal, parietal, and temporal lobes by which it is enveloped. A short distance under its surface, and conforming to it, is a thin plane of grey matter, the claustrum ; then follows a thin plane of white matter, called the external capsule ; and then comes the lenticular nucleus. The lenticular nucleus corresponds in extent with the island of Reil, and lies nearly inch from its surface. Its internal surface is more convex than its external (the convexity being directed inwards), and forms the outer boundary of the internal capsule. The internal capsule in horizontal sections is bent at the junction of its anterior and middle thirds, the bend or genu conforming to the most prominent part of the lenticular nucleus, while on the inner side the anterior limb is bounded by the caudate nucleus and the posterior limb by the optic thalamus. Surgically the anterior half of the posterior limb (middle third of whole and not ι inch long in horizontal section), which lies about 1 inch from the surface of the island of Reil, is of most importance, as it contains the fibres descending from the motor area of the cortex. Lying close to the genu are the fibres going to the facial nucleus, then come some going to the hypoglossal nucleus, and further back are the fibres going to the pyramidal tracts, which influence the motor cells in the anterior cornua of the cord for the supply of the limbs. Those fibres which supply the arm lie in front of those supplying the leg. The remainder of the capsule is occupied by sensory and communicating fibres. (According to Dr. Foster, fibres for the eye and head lie even anterior to the genu.)

Owing to the crowding of the fibres in the internal capsule, a lesion about the genu, even a small haemorrhage, produces an extensive result, hemiplegia or paralysis of the whole of the opposite side resulting, while a sensory disturbance generally accompanies it. The internal capsule is very rarely affected by pressure from temporo-sphenoidal abscess, but when it is, the paralysis generally affects first the leg and then, in rapid succession, the arm and face (compare with affection of motor cortex.)

The basal ganglia consist of (a) corpora striata, composed of the caudate and lenticular nuclei, of which the former projects into the lateral ventricle, while the latter is extraventricular, lying to the outer side of the caudate nucleus and of the optic thalamus; (b) optic thalami, the upper surfaces of which assist in the formation of the floor of the lateral ventricles, while parts are covered by the velum interpositum of the pia mater ; (c) claustra lying outside the lenticular nuclei ; (d) corpora quadrigemina, corpora geniculata (internal), and pineal body, which are situated below the posterior extremity of the optic thalami, and rather internal to them, forming practically the posterior boundary of the third ventricle and the roof of the commencement of the aqueduct of Sylvius, which connects the third and fourth ventricles ; (e) amygdaloid nuclei, which lie in the temporal lobes, in front and above the extremity of the descending horns of the lateral ventricles. Not only do these ganglia generally appear to be connected with the reception and transmission of impulses from and to the brain, but they also seem to exercise a semi-independent control of the more complex reflexes and co-ordination of movement. The function of the corpora striata cannot be further defined. They are intimately associated with the optic thalami and with the cerebral cortex. The optic thalamus, especially its hinder portion, together with the external geniculate body situated at its posterior extremity, and the superior corpus of the quadrigemina, is intimately associated with sight. In addition, many afferent fibres pass through the thalamus from the tegmentum of the crus to the cortex, while others, also going to the cortex through the internal capsule, originate in the thalamus (thalamic radiation). The inferior corpora of the quadrigemina and also the internal corpora geniculata are connected with hearing.

The functions of the other nuclei are indefinite or unknown.

The body of the lateral ventricle is situated internal to the caudate nucleus, which forms part of its outer boundary and floor. The c0rpus call0sum forms its roof, while it is separated from its neighbour of the opposite side by the posterior part of the septum lucidum and junction of corpus callosum with the fornix. From the body of the ventricle an anterior horn projects a short distance forwards and outwards, while posteriorly, at the splenium or posterior curled end of the corpus callosum, the posterior and descending horns diverge. The position of the divergence is represented on the surface by a point inch vertically above the external auditory meatus. The descending horn curves round the posterior extremity of the optic thalamus, and projects forwards and inwards along the temporal lobe toward the temporal pole. The posterior horn curves backwards and inwards into the occipital lobe. The bodies of the two lateral ventricles communicate anteriorly with the third ventricle, which lies beneath them, by the foramina of M0nro, and hence with one another. The third ventricle communicates with the fourth ventricle by the aqueduct 0f sylvius, whence it becomes continuous with the central canal of the cord.

The lateral ventricles communicate directly with the subarachnoid space by means of the slit-like openings at the extremity of each descending horn, and indirectly by the foramen of Magendie and lateral recesses of the fourth ventricle.

The subarachnoid space is best developed on the under surface of the medulla and the cerebellum, forming the water-bed, or cisterna magna ; but the various spaces freely communicate with one another, with the spinal subarachnoid, and, by means of the Pacchionian bodies, with the longitudinal and other sinuses. Thus the ventricular system normally communicates freely with the subarachnoid system, and an equality of pressure is preserved. In hydrocephalus the ventricles of the brain become distended, sometimes to such an extent as to leave only a narrow rim of brain matter between them and the dura. The condition is supposed to be due to blocking of the foramina of Magendie and the other communications mentioned.

Tapping of the ventricles of the brain has been occasionally advocated in the treatment of hydrocephalus. Two horizontal lines, one 2 inches above and the other inch above, and both parallel to, the zygoma represent roughly the upper and lower limits of the cornua of the ventricles, while two vertical lines, the anterior through the junction of the anterior and middle third of the zygoma, and the other 2 inches behind the tip of the mastoid process, define the anterior and posterior limits. A permanent communication may be established between the ventricle and either the subarachnoid space or the subcutaneous tissues, and the distension thereby relieved.

In health the brain pulsates markedly when the dura is opened, and it is supposed that the subarachnoid system equalizes and distributes the pressure so caused. Not merely the cardiac, but also respiratory effects, can be traced, the latter being due to the arrest of the venous return from the brain during inspiration by the mechanism explained in connection with the sigmoid sinus. An abscess or tumour of the brain increases the intracranial pressure, even a small abscess, in its rapid growth, causing considerable disturbance, whereas a tumour, growing much more slowly, may attain considerable dimensions without producing marked effects, unless it be situated at a focal point. On opening the dura in such cases absence of pulsation is frequently marked at first, then the cerebral matter is gradually extruded through the opening, the pulsation becoming apparent as the extrusion proceeds with each cardiac impulse. Superficially placed tumours and encapsulated abscesses may occasionally be extruded in this manner on opening the dura, and in most cases the pulsation assists the removal or evacuation of the pathological process. Tumours of the brain occasionally erode the cranial cavity and present externally as subcutaneous pulsating swellings.