The arteries in the plexus choroides, which are peculiarly minute and tortuous, probably have an important office, which we cannot even conjecture. The veins are large which arise from them, and some are peculiarly conspicuous. The great central vein of the brain is styled, from Galen, who first described it, vena Galeni. From nearly the centre of the brain, this vein collecting all the blood, enters the fourth sinus, where it receives the blood from the inferior longitudinal sinus. But to be more distinct, we shall begin, with the latter.

The great longitudinal sinus commences near the root of the nose, and runs backward close to the skull.

over the head, in the direction of the sagittal suture. The vena Galeni, running backward, forms the chord of this arc. At nearly two thirds from the front, to the occipital bone in this chord, the inferior longitudinal sinus descends to meet it, forming the fourth sinus. It descends in a curved line, not concentric with, or parallel to, the great longitudinal sinus, but nearly in the same direction; and in the prolongation of this chord, where it meets the longitudinal sinus near the occipital bone, the left and right lateral sinuses branch off, terminating in their respective jugulars, after a horizontal course of some length.

Nearly at the fore part of the base of the skull, below where the vena Galeni assumes a distinguished bulk, there are some other sinuses, irregular in their shape, and tortuous in their course, which terminate also in the jugulars.

The great sinus is formed by the splitting of the laminae of the dura mater, and as the separation is more distant above, the cavity of the sinus is triangular; the inferior lesser sinus runs along the edge of the falx, without any peculiar form; but the fourth sinus, which is continued along the tentorium, by the joint action of this membrane and the falx, is also drawn into a triangular shape. The other sinuses are apparently enlarged tortuous veins, which we need not particularly mention.

It is obvious from this description, that nature anxiously retains a portion of blood in the head, even at the risk, as we have said, of accidental accumulation. But she has not incurred this danger, without attempts to lessen or relieve it. Though the longitudinal and the occipital sinuses are bound by the dura mater and tentorium, the rest are free, and admit of dilatation. At the point of meeting the angle of the occipital bone, they are so closely tied down, that the ancients supposed the blood in this part subjected to a peculiar pressure, and the point of union was styled the torcular Herophili, from its discoverer. The inferior sinuses, however, have no such restrictions; and they communicate by veins, styled the emmissariae of Santorinus, which communicate also with the external veins. Vicq. dazyr informs us, that those sinuses which lie near the orbits and ethmoidal bone, the orbitar and carvenous, anastomose by a number of small veins with the hinder veins of the nose; so that the advantages of critical haemorrhages from the nose in relieving acute fevers where the head is affected, will be sufficiently obvious. But, though authors have described the emissaries of Santorinus with particular care as the source of relief in apoplexies, and similar disorders arising from accumulation, their bulk is by no means equal to this office; and they seem chiefly useful in preventing partial compression, from the too great fulness of any particular sinus.

The uses of the sinuses have been differently explained by former authors; and these cavities have been supposed reservoirs, to prevent any sudden interruption in the blood through the vena cava to the heart from pressing too strongly on the tender medullary organ. If this were, however, the case, they are adapted very imperfectly for the purpose, since they are closely tied down where. dilatation might be safely allowed; and without any restriction in the base of the skull where their compression must necessarily be injurious. If it be true that the nervous fibres require an active circulation for the support of their excitement, it must be an object of the highest importance that the arteries of the brain should not be suddenly emptied, or frequently liable to the accidents that may occasion it. This purpose the sinuses answer very completely; and, as we shall find, that during inspiration the course of the blood in the upper vena cava is obstructed, this function will appear to be an assistant in the same office.

See Winslow's Anatomy; Haller's Physiology; Willis' Anatome Cerebri; Vicq. dazyr on Brain, and his Memoirs in the Academy of Sciences for 1781; Mala-carne Encephalotomia Nuova; Monro on the Nervous System.

Cerebrum elongatum. See Medulla spinalis.