TISSUES, ORGANS, AND SYSTEMS. 



Ill 



special investments of connective tissue, the so-called neurilemma. The 

 gray substance contains a great preponderance of nerve-cells, besides 

 which, in certain localities, there is a finely-granular matrix and free 

 nuclei; but it is rarely found quite unmixed, being usually mingled more 



Fig. 39. 



or less with nerve-fibres. This is more especially the case in most gan- 

 glia, in the gray substance of the spinal cord, and in the so-called gan- 

 glia of the cerebrum ; while, on the other hand, in the gray cortex of 

 the cerebrum and cerebellum, it is found in some localities almost with- 

 out nervous fibres. This substance possesses vessels even in much 

 greater abundance than the white ; and in the peripheral ganglia there 

 are also different forms of connective tissue, which serve to invest their 

 separate parts. 



The chemical composition of the nervous substance has hitherto, by 

 no means, been sufficiently investigated. In the white substance, the 

 central bands of the nerve-tubules consist of a protein compound very 

 similar to the fibrin of the muscles ; the medullary sheath, chiefly of fats 

 of different kinds, and the membrane, of a substance similar to the sar- 

 colemma. The gray substance contains a preponderance of albuminous 

 matter, besides a considerable quantity of fat. 



The physiological importance of the nervous tissue consists, in the 

 first place, in its subserving movement and sensation ; secondly, in its 



FIG. 39. Nerve-cells of the substantia ferruginea from the floor of the fourth ventricle in 

 man ; magnified 350 diameters. 



