THE NERVOUS SYSTEM. 



153 



1836; Val. 'Repert./ 1836, p. 156) is said to exhibit ciliary 

 motion, a statement, however, which Yirchow and I have not 

 been able to confirm, in the case of 

 an executed criminal. It lies, nor- 

 mally, immediately upon the nerve- 

 substance, but there is so frequently 

 developed beneath it, especially in 

 the fornix, the stria cornea, and 

 the septum lucidum, a filamentous 

 layer, resembling connective tissue, 

 00 1 — 005"' thick, that its occur- 

 rence at a certain age might almost be described as con- 

 stant, as in fact it is by Yirchow. The epithelium some- 

 times presents, particularly as in the third ventricle, large cells 

 of 0-008 — 0-012'", with pigment-granules and masses, together 

 with nuclei, measuring 0-003'"; in other situations, as in the 

 lateral ventricles, the cells are not more than 0-005 — 0-007" 

 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 except 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 number, 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 

 ramifications in the adipose tissue, Avhich do not demand any 

 further description. In the cranium, on the contrary, the 

 entire dura mater is vascular, 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 arterial meninges, 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 



Fig. 151. Ependyma in Man. A, from the corpus striatum; 1, from the surface: 

 2, from the side ; a, epithelial cells ; b, ucrve-fihres lying heneath : B, epithelium 

 cells from the comyni.ssura mollis ; x 350 diam. 



