THE NERVOUS SYSTEM. 



397 



Fig. 151. 



motion, a statement, however, -which Virchow and I have not been able 

 to confirm in the case of an executed criminal. 

 It lies, normally, immediately upon the nerve- 

 substance, but there is so frequently developed 

 beneath it, especially in i\\e fornix, the stria 

 cornea, and the septum lucidum, a filamentous 

 layer, resembling connective tissue, 0-01-0-05 

 of a line thick, that its occurrence at a certain 

 age might almost be described as constant, as 

 in fact it is by Virchow. The epithelium 

 sometimes presents, particularly as in the third ventricle, large cells of 

 0'008-0 g 012 of a line, with pigment-granules and masses, together with 

 nuclei, measuring 0-003 of a line; in other situations, as in the lateral 

 ventricles, the cells are not more than 0-005-0-007 of a line in size, but 

 almost as thick as wide, with roundish nuclei and a good many yellowish 

 granules, which are generally crowded in the interior.* 



The vessels of the membranes just described present very various 

 conditions. In the first place, in the dura mater of the cord, if we ex- 

 cept those on the external surface and the numerous arteries and veins 

 of the cord by which it is perforated the vessels are very few in num- 

 ber, and in this respect the membrane presents more of the conditions 

 of a muscular fascia or tendinous expansion. On the other hand, there 

 exist in this situation, between the dura mater and periosteum of the 

 vertebral canal, the well-known venous plexuses, as well as finer ramifi- 

 cations in the adipose tissue, which do not demand any further descrip- 

 tion. In the cranium, on the contrary, the entire dura mater is vascu- 

 lar, but especially in its external periosteal layer, which, partly for its 

 own supply, partly for that of the cranial bones, to which it gives off 

 numerous branches, supports the arterice meningece, and also conveys 

 through its veins a portion of the blood returned from the bones. 

 Besides this, in the cerebral dura mater are lodged the venous sinuses, 



FiG. 151. Ependyma in man. ./#, from the corpus striatum 1, from the surface: 2, from 

 the side; a, epithelial cells; b, nerve-fibres lying beneath ; , epithelium cells from the com- 

 missura mollis. Magnified 350 diameters. 



* [According to the recent researches of Virchow, the spinal ependyma differs some- 

 what from that lining the ventricles of the brain. It is more gelatinous and resistant, 

 and contains cells which are much larger than those of the cerebral ependyma. It lies in 

 the middle of the gray matter, in the exact situation where the spinal cord exists in the 

 foetus, and forms a continuous filament, extending to the/J/i/w? terminate. 



The ependyma is prolonged without distinct boundaries, between the nervous elements. 

 Virchow has found a continuous layer of a similar substance in the interior of the higher 

 nerves of sense. He has quite recently also discovered in its deeper layers peculiar 

 granules, with the chemical reaction of cellulose. (Vide 118, infra.) DaC.] 



