THE NERVOUS SYSTEM. 151 



of these spaces between the cerebellum and medulla oblongata, 

 and under the pons Varolii, the crura cerebri, the fossa Sylvii, 

 &c, open directly into the subarachnoid space of the spinal 

 cord, whilst the smaller, corresponding to the sulci, and over 

 which the membrane composed of connective tissue is stretched, 

 are perhaps partially in communication with each other, but, 

 at all events most of them, not with the larger spaces just 

 mentioned. The arachnoid, as has been correctly stated by 

 Henle, is nowhere in connection with the lining membranes of 

 the cerebral ventricles. The structure of the membrane is the 

 same as in the spinal cord, except that the anastomosing 

 fasciculi and spiral elastic fibres are for the most part thicker, 

 measuring as much as 0-01'" or even 0-02'"; and the former 

 frequently present, as it were, special and more homogeneous 

 sheaths of connective tissue, beneath which, fat- aud pigment- 

 granules are often deposited. The outer surface is covered 

 with an epithelium in all respects like that of the dura mater. 



The pia mater cerebri is more vascular but more delicate 

 than that of the spinal cord, and covers all the elevations and 

 depressions on the surface of the brain, if not very closely yet 

 quite exactly, with the single exception of the floor of the fourth 

 ventricle, above which it is stretched across from the calamus 

 scriptorius, as far as the nodulus, the free border of the vela 

 medullaria inferiora and the flocculi, forming the tela chorioidea 

 inferior, from Avhich points it proceeds to invest the under surface 

 of the inferior vermiform process and of the tonsillce. The pia, 

 mater penetrates into the interior of the brain only at one point, 

 viz. at the transverse fissure of the cerebrum, where it passes 

 beneath the splenium corporis callosi, investing the vena magna 

 Galeni as well as the pineal gland, forming the tela chorioidea 

 superior, with the plexus chorioideus ventriculi tertii ; and pass- 

 ing beneath the fornix, also constitutes the vascular plexuses of 

 the lateral ventricles, which are continuous with the pia mater 

 at the base of the brain, between the crus cerebri and the 

 inferior lobe. "With respect to its intimate structure, the 

 cerebral pia mater contains so many vessels, that in parts the 

 connective tissue which forms the matrix appears as a sub- 

 ordinate constituent. It is rarely, as in the spinal cord, dis- 

 tinctly fibrous, for the most part more homogeneous, approaching 

 in character f Reichert's membranes/ or immature connective 



