The fifth ventricle must be viewed as ori ginally part of the third, which has been closed off by the full developement of the septum lucidum and fomix, and the union of their lateral halves along the median plane.
All these cavities are lined by a delicate membrane nearly allied to, if not identical with, serous membranes. It is covered by an epithelium, ciliated according to Purkinje and Valentin, beneath which are delicate fibres of areolar tissue exactly of the same kind as those found in connection with serous membmnes. I have never seen any basement membrane. This membrane is reflected around the pro cesses of pia mater which are found in the ventricles, and in this respect presents ark ad ditional point of analogy to the serous mem branes, the portion which lines the walls of the ventricles corresponding to the parietal layer, and that which adheres to the pia mater resembling the visceral layer of those mem branes. It is the reflection of this membrane from the walls to the enclosed pia mater which serves to shut off the ventricular cavity from the sub-arachnoid space, at the anterior part of the horizontal fissure, and at the inferior extremity of the fourth ventricle. If any communication take place between the intra-ventricular and sub-arachnoid fluid, it must be, as already remarked, by transudation through this mem brane.
Of the circulation in the brain.—Haller cal culates that the human brain receives rather more than one-fifth of the whole blood of the body. Whether this calculation be correct or no, it is certain that an organ of such great size, of such high vital endowments, so active, and which exerts so considerable an influence upon all other parts of the body, must necessarily require a large supply of the vital fluid. Four large arteries carry blood to the brain, namely, the two internal carotids and the two vertebral:. Each carotid penetrates the cranium at the fommen on the side of the sella Turcica, and almost immediately divides into three branches, the anterior and the middle cerebral arteries and the posterior communicating artery.
The anterior cerebral arteries supply the inner sides of the anterior lobes of the brain : they ascend through the great longitudinal fissure, and pass along the upper surface of the corpus callosum, giving off branches to the inner con volutions of both hemispheres of the bmin.
These arteries anastomose with each other just beneath the anterior margin of the corpus cal losum by a transverse branch, called anterior communicating artery. The middle cerebral arteries, the largest branches of the carotid, pass outwards in the fissures of Sylvius, and supply the outer convolutions of the anterior lobes, and the principal portion of the middle lobes. At the inner extremity of each fissure of Sylvius numerous small branches of these arteries pene trate, to be distributed to the corpus striatum. The choroid arteries which supply the choroid plexus sometimes arise from these arteries, but also occasionally come from the carotid itself. The posterior communicating artery is an ana stomotic vessel, which passes backwards along the inner margin of the middle lobe on the base of the brain, and communicates with the poste rior cerebral artery, a branch of the basilar.
The vertebral arteries, having passed through , the canals in the tmnsverse processes of the cervical vertebne, enter the cranium through the occipital foramen towards its anterior part. In their ascent they incline towards each other in front of the medulla oblongata, and at the posterior margin of the pons they coalesce to form a single vessel, the basilar, which extends the whole length of the pons.
The vertebral arteries fumish the anterior , and posterior spinal arteries, and the inferior cerebellar arteries. These last vessels ari from the vertebrals very near their coalescenc they pass round the medulla oblong-ata to rea the inferior surface of the cerebellum, to whic they are principally distributed.
From the basilar artery numerous small v sels penetrate the pons. At its anterior extr mity kt divides into four arteries, two on ea side. These are, the two superior cerebella and the two posterior cerebral arteries.