348 NERVOTJS SYSTEM. 



who have often experienced diseases of the head, and are not 

 observed in any animal; so we are forced to consider them 

 as morbific productions, and not, as Pacchioni conceived, 

 glands whose excretory ducts opened into the ventricles of the 

 brain and the sinuses. 



The Pia Mater covers the upper surface of the cerebrum with 

 such uniformity as not to require a detailed description of it; 

 where it sinks into the great fissure between the hemispheres, 

 it adheres from the two sides just above the corpus callosum. 

 On the basis of the brain, it penetrates deeply into the anterior 

 fissure, or the Fissura Sylvii; is reflected over the inequalities 

 of the brain, but never in such a way as to leave them; and 

 secures the bottom of the third and of the fourth ventricle. 



The distribution of the pia mater, over the ventricles of the 

 brain, is more complicated than that over its periphery, and it 

 may be remarked, that this portion is called, by some anato- 

 mists, the Internal Pia Mater; that its texture is much more de- 

 licate, and net-like, and that it adheres more closely to the sub- 

 jacent parts. Being extended from the superior surface of the 

 cerebellum and of the Pons Varolii, it enters into the third ven- 

 tricle, under the posterior margin of the fornix, by the large 

 transverse fissure between the latter and the tubercula quadrige- 

 mina. By its course between the fornix and thalami, it consti- 

 tutes the Velum Interpositum, or the Tela Choroidea of Vicq. 

 D'Azyr. The pia mater is also introduced into the inferior cor- 

 nu of the lateral ventricles along the internal margin of the hip- 

 pocampus major, at the side of the pons varolii; and into the 

 fourth ventricle from its bottom part. 



The several plexuses of vessels found in the ventricles of the 

 brain have for their basis the pia mater; which is there arranged 

 into a great number of folds, some of them being longitudinal 

 and others crossed. Their formation, according to the new 

 views which have Seen taken of the development and growth 

 of the brain, by Tiedemann, depends upon the internal mem- 

 brane of the brain contracting itself as it finishes the deposite 

 of medullary matter called Centrum Ovale. The vessels of the 

 plexuses are the arteries, which are spent upon the surface of 

 the ventricles, and the veins derived from the same; the latter 

 are much more numerous than the first. 



