58 VETERINARY PHYSIOLOGY 



about the same amount as in the grey matter, but 

 cholesterol and lipoids, other than lecithin, constitute more 

 than 15 or 16 per cent. 



From the fatty material of the white sheaths various 

 mixtures of lipoid substances have been isolated. These 

 have been named Cerebrosides and have been classified into — 



1. Galactosides yielding a sugar — galactose, nitrogen, but 

 no phosphorus. 



2. Phosphatides, of which lecithin is the most important 

 (p. 20). 



3. Cholesterol, a monohydric alcohol, belonging to the 

 group of terpenes. 



IV. Physiology of Neurons. 



The neurons form a most intricate labyrinth throughout 

 all parts of the body, and more especially throughout the 

 central nervous system. Each is brought into relationship 

 with many others by its dendritic terminations, and there is 

 a continued interaction between them, the activity of any 

 one influencing the activity of many others. In this way 

 the constant activity of the nervous system, which goes on 

 from birth to death, during consciousness and in the absence 

 of consciousness, is kept up. 



It is unnecessary and gratuitous to invoke the conception 

 of automatic action on the part of any portion of the nervous 

 system. Throughout life these neurons are constantly being 

 acted upon from without ; and activity, once started by any 

 stimulus, sets up a stream of action which may be 

 coexistent with life. 



A. SINGLE NEURONS, 



OR Neurons lying side by side in Nerves. 



It has been shown that the great purpose of neurons is to 

 enable external changes to produce appropriate reaction 

 (p. 52). The changes set up in the receptors at the surface 

 must be conducted to the stations in the central nervous 

 system, and again conducted out to the muscles. Conduction 

 is thus the great property of nerve. 



