The brain derives its blood-supply from the vertebral and internal carotid arteries, after these have anastomosed at the circle of Willis. This circle is formed posteriorly by the basilar (from junction of the two vertebráis), which gives off the two posterior cerebral arteries, which in turn supply the two posterior communicating branches of the circle, which join the internal carotids.

The internal carotids are continued up as the middle cerebral arteries, but give off anteriorly the anterior cerebral arteries, which communicate with each other through the anterior communicating artery, thus completing the circle.

The anterior, middle, and posterior cerebral vessels form a rich vascular network in the pia mater, which is a thick flocculent membrane enveloping the brain and dipping into the sulci. From it the cortical vessels run vertically into the brain, through the grey matter, to terminate in the white matter. These vessels do not anastomose, and are thus end arteries. The middle cerebral is of most importance, and runs in the fissure of Sylvius, giving off inner and outer striate branches to the lenticular and caudate nuclei and optic thalamus, all of which pass through the internal capsule. These vessels, in common with other small vessels supplying the internal nuclei, are frequently affected by arterio-sclerosis and aneurysms, which may be multiple (miliary aneurysms of Charcot), and one of the outer striate branches going to the caudate nucleus, which is larger than its neighbours, is a frequent source of cerebral haemorrhage. The middle cerebral also supplies branches to the motor area (all but a small portion of the leg area supplied by the anterior cerebral) and the centres for hearing, motor speech, and part of that for vision. This vessel is the usual source of cerebral hemorrhage, or apoplexy, generally from rupture of a small aneurysm. If small, the haemorrhage forms a localized clot in the brain substance, while if large, it may extend over a large area of the surface, giving rise to compression, or may burst into the lateral ventricle.

Rupture of one of the striate branches generally affects the internal capsule, and in these cases, although the lesion may be small, the effect is generally a hemiplegia owing to the crowding of the motor fibres. Cerebral embolism is followed by softening of the brain substance, where the smaller central arteries are affected, but is less often complete when the vessels supplying the cortical area are affected owing to the pial anastomosis. Embolism has followed the manipulation and treatment of carotid aneurysm from detachment of clots, and where one of the large vessels is blocked paralysis or death may result. Owing to the free anastomosis of the cerebral vessels, it is possible to ligature both common carotids, with an interval of a few weeks between them, the circulation being carried on by the vertebráis.

The anastomosis, together with the markedly tortuous course of the large vessels and small size of the vessels entering the brain, diminishes the pulsation affecting the brain substance. The larger cerebral vessels have no companion veins, the blood being returned by the sinuses. The small superfical veins discharge their blood into the superior longitudinal, lateral, and cavernous sinuses, while the small, deep veins from the interior of the brain empty into the straight sinus.

The mesencephalon is about ¾ inch long, and occupies the aperture in the tentorium cerebelli connecting the cerebrum above with the pons, etc., below. It presents the corpora quadrigemina dorsally and the large crura cerebri ventrally, which gradually converge to enter the pons. Each crus is encircled by the optic tract at its point of emergence from the cerebrum. Each crus consists of a tegmentum, or dorsal portion, which contains sensory fibres running to the region of the optic thalamus, and a crusta, or ventral portion, which contains the motor fibres which have descended from the cortex through the corona radiata and internal capsule to reach it. Haemorrhage into the crusta causes hemiplegia of the opposite side of the body, and, when on the inner margin, paralysis of the third nerve on the side of the lesion.

The cerebellum lies under the tentorium, which separates it from the under surface of the cerebrum, and behind the pons and medulla. It consists of two lateral hemispheres and a median portion, the vermis, and is also divided into upper and lower portions by the great horizontal fissure. It is composed of grey matter externally, and white matter internally, with several grey nuclei embedded in it. It is connected with the brain, pons, and medulla by three peduncles. The superior peduncles contain efferent fibres, which extend upwards on the dorsum of the pons to the inferior quadrigeminal bodies. They converge as they ascend, forming first the lateral boundaries and later the roof of the fourth ventricle, and they are concealed from view by the overlapping cerebellum. The middle peduncles are the largest, and contain both afferent and efferent fibres, which connect the cerebellum with the pons. The inferior peduncles also contain both afferent and efferent fibres, and consist of the restiform body of the medulla continued upwards over the pons and then turned backwards to the cerebellum. The direct cerebellar tract forms the greater portion of the afferent fibres in the inferior peduncle, which cross chiefly to the opposite side of the cerebellar cortex. The efferent fibres of the inferior peduncle form the cerebello-olivary-tract, connecting the cerebellum with the medulla. The cerebellum is not always confined to the cranial cavity, but often extends through the foramen magnum, wrapping round the medulla and even the upper extremity of the cord.

The pons is situated between the crura cerebri and the medulla, its ventral surface being composed of the transverse fibres of the middle cerebellar peduncles, which sweep across it from side to side. This ventral surface lies in contact with the dorsum sellae of the sphenoid and basilar process of the occipital bone, and presents a median groove, which lodges the basilar artery and two lateral eminences, due to the underlying masses of the pyramidal tract passing from the crura cerebri above to the medulla below. The fifth nerve emerges near its upper margin, while the sixth, seventh, and eight nerves emerge at its lower border. The dorsal aspect of the pons, together with that of the medulla, presents the lozenge-shaped fourth ventricle, which is roofed in by the thin superior and inferior medullary vela, which proceed outwards from the white centre of the cerebellum, and run respectively up and down, covering the ventricle with a peaked roof. The inferior velum is deficient at its lower border, and presents an opening, the foramen of Magendie, which permits of communication between the fourth ventricle and the subarachnoid space, similar openings occurring at the apices of the lateral recesses, which project laterally from the widest part of the space, and curve round the upper parts of the restiform bodies (foramina of Key and Retzius). The fourth nerves issue from the substance of the superior velum close to the inferior quadrigeminal bodies. The striae acousticae cross the floor of the ventricle transversely, and make the distinction between the pontine and medullary portions, and possibly connect the cochlear nucleus with the cerebellum.