42 THE ACTIVE FORCES OF LIVING ORGANISMS 



a practice to keep cattle which they wish to fatten 

 in dark stalls, and whatever drawbacks it may have 

 from a sanitary point of view, there is little doubt 

 that by diminishing the supply of light, and thereby 

 limiting the degree of oxidation, a more rapid gain in 

 weight may be obtained. Although the rapidity of 

 growth in children and young animals is doubtless 

 due to several factors, one of the most important is 

 undoubtedly the amount of sleep which they get. In 

 the first few weeks of its life an infant will often sleep 

 eighteen or twenty hours out of the twenty-four, and 

 the longer it sleeps the better it will thrive. It is 

 said that during sleep the brain is ansemic, and the 

 constriction of the pupil of the eye suggests contrac- 

 tion of the cerebral bloodvessels. However this may 

 be, the diminution of respiratory movements, the 

 slowing of the pulse, the lessened production of urea, 

 and the lowered temperature, all indicate decreased 

 metabolism ; and yet such metabolism as still takes 

 place is, we may suppose, anabolic in its character 

 rather than katabolic. 

 The con- During expiration, as already remarked, the brain 



tractility n i ■ 



of nerve- as a wholc riscs and the blood-pressure sinks ; whilst 

 during inspiration the reverse happens, the brain 

 sinking and the arterial blood-pressure rising. These 

 variations, it was said, cannot but exercise a certain 

 influence on the nerve-cells, and we may expect them 

 to be accompanied by appreciable changes in the state 

 of oxidation and of contraction. The question of the 

 contractility of the nerve-cell is, however, one of such 



