The supenor cerebellar arteries pass bac wards round the crus cerebri, pamllel to tl fourth nerve, and divide into numerous branch on the upper surface of the cerebellum, som of which anastomose with bmnches of the inf I rior cerebellar artery over the posterior margi of the cerebellum. Some branches of the arteries are distributed to the velum interpo situnn.
The posterior cerebral arteries are the largest branches of the basilar. They diverge and pass upwards and backwards round the crus cerebri, and reach the inferior surface of the posterior lobe, anastomosing in the median fissure with ramifications of the anterior cerebral, and on the outside with branches of the middle cere bral arteries. Numerous small vessels pass from these arteries at their origin, and penetrate the interpeduncular space, and one or two are dis tributed to the velum. Shortly after its origin each of these arteries receives the posterior com municating branch from the carotid.
A remarkable freedom of anastomosis exists between the arteries of the brain. This takes place not only between the smaller ramifica tions, but likewise between the primary trunks. The former is evident all over the surface of the cerebrum and cerebellum. The latter con stitutes the well-known circle of Willis. This anastomosis encloses a space, somewhat of an oval figure, within which are found the optic nerves, the tuber cinereum, the infundibulum, the corpora mamillaria, and the interpedun cular space. The anterior communicating ar tery, between the anterior cerebral arteries, com pletes the circle in front. The lateral portion of the circle is formed by the posterior com municating artery, and it is completed behind by the biffircation of the basilar into the two posterior cerebral arteries. Thus, a stoppage in either carotid, or in either vertebral, would speedily be remedied. The coalescence of the vertebrals to form the basilar affords conside rable security to the brain against an impedi ment in one vertebral ; and, should the basilar be the seat of obstacle, the anastomoses of the inferior cerebellar arteries with the superior ones would ensure a sufficient supply of blood to that organ. If either or both carotids be stopped up, the posterior communicating arte ries will supply a considerable quantity of blood to the intracranial portions of them ; or, if one carotid be interrupted, the anterior com municating branch will be called into requisi tion to supply blood from the opposite side.
Interruption to the circulation in both cain tids and both vertebrals is productive of a com plete cessation of cerebral action, and death immediately ensues, unless the circulation can be quickly restored. This was proved clearly by Sir A. Cooper's experiments on rabbits. The circulation may, however, be interrupted in both carotids, or in both vertebrals, without permanent bad effect ; or in one carotid or one vertebral, provided the condition of the remain ing vessels be such as not to impede the circu lation in them. In cases where the neighbour ing anastomotic branches are not sufficient to restore the circulation to a part from which it has been cut off by the obliteration of its proper vessel, the cerebral substance of that region is apt to experience a peculiar form of softening* or wasting, which is distinguished by the ab sence of any discoloration by the effusion of blood, or of any new matter.
The four great channels of sanguineous sup ply to the brain are continued up straight from the aorta itself, or from an early stage of the subclavian. The columns of blood contained in them are propelled very directly towards the base of the brain, through wide canals. Were such columns to strike directly upon the base of the brain, there can be no doubt it would suffer materially. Considerable protection, how ever, is afforded to the brain ; first, by the blood ascending against gravity, during at least a great portion of life ; secondly, by a tortuous arrangement of both carotids and vertebrals before they enter the cranial cavity; the carotid being curved like the letter S in and above the carotid canal, and the vertebral being slightly bent between the atlas and axis, then taking a horizontal sweep above the atlas, and after it has pierced the occipito-atlautal ligament, in clining obliquely upwards and in wards ; thirdly, by the breaking up of the carotids into three branches ; by the inclined position of the ver tebrals, and by their junction into a single vessel, which takes a course obliquely upwards, and afterwards subdivides into smaller branches. Such arrangements most effectually break the force of the two columns, and, as it were, scat ter it in different directions.