WATER CONTENT — MAMMALIAN NERVOUS SYSTEM 449 



If we compare the drawings of Watson ('03), showing the in- 

 crease of the visible m^-eUn in the cerebral hemispheres and in 

 the spinal cord of the rat, with the chemical results here used, 

 we see that the histological pictures show a more gradual ap- 

 pearance of myelin than the chemical results, or the water 

 determinations, would suggest. This probably depends on the 

 fact that it is only a fraction of the lipoids forming the sheaths, 

 which takes the haematoxylin stain, and -this stainable fraction 

 forms at first a smaller, but later a larger portion of the entire 

 sheath (Koch and Koch '13; Smith and Mair '08). 



There is still one more modification in the formation of myelin. 

 Tribot ('05) has contrasted in terms of diy substance the relati^'e 

 amounts of albuminoides and the fats in the nerv-e tissue of the 

 guinea-pig at tlifferent ages. The percentage value of the fats 

 increases from 11 days (his first observation) up to 120 days — 

 after which it begins to fall. The fats of Tribot are the lipoids 

 of Koch's analysis and it is of interest to note that the 120 day 

 record for the brain in table 2, column (4), also shows the highest 

 proportion of corrected lipoids. The observations of Dunn ('12) 

 which show in the myelinated fibers of the second cer\'ical nerve 

 of the rat the highest relati\e areas for the myelin sheaths at 

 75 days and 132 daj's — seem to fit with these other observations 

 and to suggest that the fonnation of lipoids with advancing age 

 fluctuates in such a way as to show a maximum about the end 

 of the active growing period of the central nervous system. 



This discussion of possible factors modifying these determi- 

 nations has been introduced to clear the way for further work on 

 the main question, but so far as one can foresee the effect of 

 taking them into consideration, it would tiMul to make more 

 uniform the values thus far obtained. 



CONCLUSIONS 



We conclude from these results that there is no evidence that 

 the cell bodies and their unsheathed axons suffer more than a 

 slight loss of water between birth and maturity, and that the 

 progressive diminution in the water content of the entire brain 

 and spinal cord is mainly due to the accumulation of myelin. 



