THE CENTRAL NER VE SYSTEM 



823 



Besides the neuroglia, the central nerve system contains as supporting tissues numerous fine 

 and coarse septa or trabeculse derived from the investing pia, or from the sheaths of bloodvessels. 



Chemical Composition. The amount of water in nerve tissue varies with the situation. 

 Thus, in the gray substance of the cerebrum it constitutes about 83 per cent., in the white sub- 

 stance from the same region about 70 per cent., while in 

 the peripheral nerves, such as the sciatic, it may fall to 

 60 per cent. 



The solids consist of neuroalbumins, neuroglobulins, 

 nucleoproteins, neurokeratin (in the gray substance 

 proteins constitute about one-third of the total solids), 

 lecithins, cerebrosides (chiefly phrenosin), cholesterin, 

 unidentified organic sulphocompounds, aminofatty sub- 

 stances, nitrogenous extractives, and inorganic salts 

 with some collagen, fat, etc., in the adherent connective 

 tissue (W. J. Gies). 



THE CENTRAL NERVE SYSTEM. 



The central nerve system, as it is convention- 

 ally distinguished from the sympathetic system, 

 is composed of a central axial aggregation of 

 o-anp-lia forming the brain and spinal cord. 



which are connected with 

 the body bv 43 pairs of 



the other tissues of 

 nerves, of which 12 



pairs are attached to the brain and 31 pairs to 

 the spinal cord. The functional relations of 

 the central mechanisms with the periphery are 

 maintained by the essential cell elements of the 

 nerve tissues, the neurones. The chief task in 

 the study and analysis of the structure of the 

 nerve system lies in the dovetailing of features 

 visible to the naked eye with those visible only 

 under high magnifying powers. By the com- 

 bination of macroscopic with microscopic fea- 

 tures the attentive student is enabled to resolve 

 or reconstruct in the three dimensions of space, 

 and see with his mental eye the opaque interior 

 transparently resolved into intricate yet well- 

 defined projecting and associating mechanisms. 

 Assistance in such study may be derived from 

 illustrations depicting hidden structures in ac- 

 cordance with this principle. 



Preliminary Considerations. White Substance 

 and Gray Substance. The central axis of the 

 nerve system contains two categories of sub- 

 stance, their difference to the eye being one of 

 color. They are conventionally designated the 

 white and gray substance. The white substance 

 (alba), which forms about two-thirds of the neural 

 axis, is the conducting substance, and its charac- 

 teristic appearance is due to the myelin sheaths 

 which invest the axones in it. The gray substance 

 (cinerea; grisea) is the sentient and reacting mass containing the cell bodies of neu- 

 rones. Its color is due to its translucency, its greater vascularity, and to a certain 

 amount of pigment material in the cell elements. The white and the gray substance 

 is not sharply demarcated everywhere, for although the white substance is exclu- 



FIG. 594. Neuroglia cells of brain 

 shown by Golgi's method. A. Cell with 

 branched processes. B. Spider-cell with 

 unbranched processes. (After Andriezen.) 

 (From Schiifer's Essentials of Histology.) 



