Pia muter of the brain.—In tracing the pia mater of the spinal cord upwards, it will be found gradually to become much thinner and more delicate as it passes from the medulla ob longata to the hemispheres of the cerebellum and cerebrum. In connexion with these latter parts it becomes of extreme tenuity, and owes its physical tenacity chiefly to the intimate con nexion of the visceral layer of the arachnoid membrane with it. The cerebral pia mater is al most exclusively composed of numerous ramifi cations of minute vessels which are accompanied by white fibrous tissue in small quantity. These vessels divide and subdivide to the last degree of minuteness, and are admirable objects for examining the structure of capillary vessels. The pia mater adheres closely to the whole surface of the brain cerebellum, and connect ing, parts, and numlerless vessels pass from it into the nervous substance in contact with it. On the surface of the bmin it dips down into the sulci or furrows between the convolutions, and adheres to the superficial grey matter. Wher ever there is a depression or fissure of the brain, the pia mater is found dipping into it. It likewise sinks into the fissures between the laminm of the cerebellum.
We shall obtain, however, a very inadequate notion of the extent of the pia mater, if we confine our examination of it to the exterior of the brain and cerebellum. At certain situations this membrane is continued into the divities or ventricles of these organs where it doubtless fulfils some office connected with the support and nutrition of certain parts of them. These situations are four in number, as follow : on each side, the fissure between the crus cerebri and the middle lobe of the brain, behind, the transverse fissure between the cerebellum and cerebrum and, lastly, the inferior extremity of the fourth' ventricle.
Choroid plexuses of the lateral ventricles.— These are apparently folded processes of the pia mater which enter the inferior part of the lateral ventricles on each side, and are continued upwards and forwards to their middle portions, where they become continuous with each other in the foramen commune anterius, and with a middle process, the velum. Each dioroid plexus forms a somewhat cylindrical process, which, when traced from below upwards and from behind forwards, will be found to follow. the direction of the lateral ventricle as far for wards as the apex of the horizontal portion of the fornix, gradually diminishing in thickness, and assuming the character of a simple mem branous expansion. It projects freely into the cavity of the ventricle, having no connection with the walls of that cavity excepting along the margins of the fissure, at which it enters, where the membrane of the ventricle adheres. to it, being probably reflected upon it.
Very numerous and tortuous bloodvessels are contained in these processes, forming a plexus which has given name to the folds themselves.
The surface of each choroid plexus presents rnany slight projections or folds resembling in which are contained loops and plexi form anastomoses of minute vessels, very si milar to the arrangement of the vessels of the villous processes of the chorion of the ovum, or those of the tufts of the placenta. These vessels are surrounded by an epithelium which has much the appearance of that of serous membranes. From the great number of these vessels and from the delicate nature of the epithelial covering which surrounds them, it is plain that the choroid plexuses are well suited either for the purpose of pouring out fluid or of absorbing it.
with the choroid plexuses may be readily de monstrated. At its anterior extreinity it(..orre sponds to the forainen commune anterius. The velum interpositum is best exposed in the dis section from above downwards by removing carefully in succession the corpus callosum and the fornix. In raising the velum itself, in order to disclose the cavity of the third ventricle, it is necessary to be very careful, as from the in timate connexion which the pineal body has with it towards its base, that body may be readily disturbed from its position.
Choroid plexuses of the fourth ventricle.— The choroid plexuses of the fourth ventricle are two small processes of pia mater united along the median line, presenting the same villous character as those of the lateral ventri cles. These folds seem as if they had been pushed up into the fourth ventricle by the lower laminm of the inferior vermiform process. Their position may be best seen by opening the fourth ventricle from above, where they will be found lying on each side of that portion of the median lobe of the cerebellum which stops up the inferior extremity of the fourth ventricle. These plexuses are in every respect similar, as far as regards structure, to the larger ones which are found in the lateral ventricles, and, like them, exhibit a delicate epithelium upon their surface. Upon the centre of each epithe lium cell Valentin states that a pigment cor puscule is deposited. ( Fig. 365.) The epithelium may be best seen by examin ing the edge of a fold. It becomes very distinct when acted upon by acetic acid. As its particles are very delicate and consist only of a single layer, they are easily detached. The cells of epithelium are most of them six-sided, and contain a clear nucleus, or several minute gra nules. Valentin states that cilia may be seen playing upon this surface, especially in the embryo. I have obsenrecl the peculiar punc tiforrn or spiniform formations to which he alludes, which look like the remains of forrner vibratile cilia.