WATER CONTENT — MAMMALIAN NERVOUS SYSTEM 445 



According to this table the (gray) cortex has lost 2 points and 

 the (white) callosum 17.6 points in the process of maturing. 



It is never possible at maturity to obtain the cortex or any 

 other gray mass without some admixture of myelinated fibers 

 and I have therefore, provisionally, credited one point of the 

 loss, noted by de Regibus in the water content of the cortex, to 

 the presence of myelin. This implies but a small proportion of 

 myelin since if we assume that myelin has 48 per cent of water 

 the reduction of 1 per cent would mean that about one thirty- 

 ninth of the mass was represented by myelin. According to 

 this assumption the mature gray substance (cortex) — when the 

 myelin is excluded — contains 87 per cent of water, and in the 

 computations which follow the neurons without myelin are 

 assumed to have 87 per cent of water, except at ten days, when 

 they are credited with 88 per cent. 



The fact that the fibers without myelin have at birth a high 

 percentage of water (88%) while at maturity, after myolination, 

 they have lost 17.6 points is the sort of evidence which fur- 

 nishes the basis for the current, but unsupported view, that the 

 loss of water is to be associated with the formation of the myelin. 

 It is our object to present more precise information bearing on 

 this point. 



To obtain a notion of the apjiroxunato distribution of the water 

 between the myelin and neiu'ons proper, it is necessary to have 

 data on the relative abundance of these two constituents of the 

 brain. 



In 1913 W. Koch and ]\I. L. Koch made a study of the chemical 

 composition of the brain of the albino rat at six ages between 

 birth and maturity, and of the spinal cord at one age. The 

 data thus obtained are those which will be utilized here. The 

 authors determined seven fractions: Proteins, organic extractives 

 and inorganic constituents, which three taken together, we shall 

 designate protein (or non-lipoid), and phosphatides, cerebro- 

 sides, sulphatides and cholesterol, which four taken together, we 

 shall designate lipoid. 



These data give us at each age, therefore, the protein and the 

 lipoid present ui the brain, or to be a little more exact, we should 



