56 NERVOUS SYSTEM. 



late, branching cells, serving, in a measure, to support the 

 nervous elements. 



The blood-vessels of the nerve-centres form an exceed- 

 ingly graceful capillary net-work with very large meshes. 

 The gray substance is much richer in capillaries than the 

 white. 



A remarkable peculiarity of the vascular arrangement in 

 the cerebro-spinal centres has already been described in con- 

 nection with the lymphatic system. The blood-vessels here 

 are surrounded by what have been called peri vascular canals, 

 first described by Robin, and afterward shown by His and 

 Robin to be radicles of the lymphatic system. 1 



Composition of the Nervous Substance. 



Our knowledge of the chemical constitution of the ner- 

 vous system is, in many regards, quite unsatisfactory ; but 

 these tissues contain certain elements that have been very 

 well determined. The chemical characters of cholesterine, 

 for example, have long been known to physiologists, as well 

 as the fact that this principle is a constant constituent of the 

 nervous substance, united in some way with the other proxi- 

 mate principles, so that it does not appear in a crystalline 

 form. Since we demonstrated, in 1862, the relations of 

 cholesterine to the process of disassimilation, this principle 

 has assumed its proper place as one of the most important 

 of the products of physiological waste of the organism. The 

 origin and function of cholesterine, with the processes for its 

 extraction from the fluids and tissues of the body, have been 

 fully considered under the head of excretion. 3 



Regarding cholesterine as an excrementitious product, 

 to be classed with principles destined simply to be elimi- 

 nated from the organism, the nerve-substance proper has 

 been found to contain the following proximate principles, 

 the chemical properties of which have been more or less 



1 See vol. ii., Absorption, p. 433. 2 See vol. iii., Excretion, p. 267, et seq. 



