240 



NERVOUS SYSTEM. 



[sect. 117. 



I 



Human ependyma. A. From the 

 corpus striatum; 1. from the surface; 

 2. from the side ; a. epithelial cells ; 

 b. subjacent nerve-fibres. B. Epithe- 

 lial cells from the commissura mollis. 

 Magnified 350 times. 



with the continuations of the pia mater, i.e., the floor of the 

 fourth ventricle, the aqueduct of Sylvius, the floor and lateral 

 walls of the third ventricle, the fifth ventricle, the roof, the 

 anterior and posterior horn, and a large part of the descending 

 horn of the lateral ventricles, and the hollow within the olfactory 

 bulbs in the embryo, possess a special investment, the so-called 

 Fi 3 . 98. ependyma ventriculorum (fig. 98J. This 



is a simple layer of epithelium, which 

 is seated normally immediately upon 

 the nervous substance; still there 

 is so frequently developed under it a 

 striated layer of connective tissue of 

 o'oi'" to 0'05'" in thickness, that the 

 occurrence of the latter may almost 

 be regarded as constant at a certain 

 a°e. In man it is ciliated, at least in 

 certain places, and sometimes, perhaps, 

 it is so all over. 



The vessels of the above-described membranes present very 

 various conditions. The dura mater of the cord is poor in vessels 

 like the fasciae. That of the brain, on the other hand, has many 

 vessels, especially its outer layer, corresponding to a periosteum, 

 which supports the meningeal arteries, partly for its own use, 

 partly for the cranial bones which receive numerous branches; 

 and which conducts by means of its veins,- a part of the blood of the 

 bones. The arachnoidea is non-vascular, whilst the pia mater 

 both of the cord of the. brain, not only supports the very dense 

 ramifications of the vessels of the nervous substance, but also 

 independently of the plexus chorioidei contains a considerable 

 number of vessels of its own. Fohmann and Arnold assume the 

 existence of lymphatic vessels in the pia mater of the surface of 

 the cerebrum and cerebellum as also in the plexus chorioidei, but 

 it is doubtful whether they occur here. 



The membranes of the central nervous system possess, in part 

 at least, nerves also. In the dura mater of the brain, some of 

 the nerves run in the periosteal lamella of the membrane, following 

 pretty closely the course of the meningeal arteries, and are 

 especially distinct upon the middle meningeal artery, coming from 

 the sympathetic and the trigeminus. They occur, moreover, upon 

 the anterior and posterior meningeal arteries, and there is a 

 nervus tentorii, from the fifth (Arnold), which also goes to 

 the larger sinuses of the dura mater. Nerves are not known to 



