PERCENTAGE OF WATER 127 



in the brain and cord of rats according to age and to the normal 

 body weight, we pass to the question of the extent to which the 

 percentage of water may be modified experimentally under 

 special conditions. The amount of modification which has been 

 experimentally induced is thus far extremely slight, nevertheless 

 some deviation seems to occur. The evidence is as follows: 



(a) Some conditions which increase the percentage of water in the 

 brain and cord. Dr. Watson ('05) when working on the effects 

 of the bearing of young on the weight of the central nervous 

 system in the albino rat, noted that the mated animals had both 

 heavier brains and heavier cords. He noted also that the mated 

 rats, as compared with the unmated of like age, had the follow- 

 ing percentages of water: 



NO. OF PERCENTAGE OP WATER 



cases Brain Cord 



Female mated (8) 77 . 47 68 . 51 



Female unmated (10) 77 . 37 68 . 29 



This shows that the mated rats had in the brain 0.10 per cent 

 more water than the unmated, and in the spinal cord 0.22 per 

 cent more. Thus, even though the brain and cord in the mated 

 series were absolutely heavier, yet if the higher percentage of 

 water be taken as an index of a lesser maturity, the central ner- 

 vous system of the mated rats must be regarded as physiologi- 

 cally younger. Such slight differences would, of course, not be 

 worthy of remark if they had been obtained merely by the averag- 

 ing of widely varying data, but in this case comparisons were 

 made by Watson in five different groups for the brain, and five 

 for the cord, and in only one (in the cord) out of the ten com- 

 parisons, did the mated rats show a smaller percentage of water. 

 Thus though the difference is small, it was found to occur in the 

 same sense in nine cases out of the ten. This seems to justify 

 the conclusion which Watson drew that mated female rats had a 

 slightly higher percentage of water in the brain and spinal cord 

 than the unmated females belonging to the same litters. 



Hatai ('07) also has made observations on the modifications 

 of body growth as the result of which the percentage of water in 

 the central nervous system was slightly increased. 



