TISSUES, ORGANS, AND SYSTEMS. 



97 



neurilemma. The grey 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 or less with nerve- 

 Fig. 39. 



fibres. This is more especially the case in most ganglia, in the 

 grey substance of the spinal cord, and in the so-called ganglia 

 of the cerebrum ; while on the other hand, in the grey cortex 

 of the cerebrum and cerebellum, it is found in some localities 

 almost without 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 sarcolemma. The grey 



Fig. 39. Nerve-cells of the substantia ferruginea from the floor of the fourth ven- 

 tricle in man, x 350. 



i. 7 



