CENTRAL NERVOUS SYSTEM. 289 



opening into the cranial cavity as a result of mental effort or emotion. In 

 the normal adult this bulging cannot, of course, occurto anything like such an 

 extent, and the space for the arterial blood must he gained both by driving 

 out the blood from the cerebral veins within the cranium and through the 

 expulsion of the subdural fluid. 



Influence of Glands. — In the growth of the nervous system it is not only 

 the quantity, but the peculiar qualities of the blood that are important, and 

 among the various glands the activity of which is so necessary for the growth 

 of the nervous, as well as the other systems, and is also needed for its full 

 maintenance, the thyroid appears as very important. In sporadic cretinism, 

 associated as it is with atrophy of the thyroid, the feeding of sheep's thyroids 

 has produced remarkable growth-changes in all parts of the body — the nerv- 

 ous system included. 



At the same time, experimental extirpation of the thyroid is followed by 

 destructive changes in the central system, caused by disturbances in its nutri- 

 tion. The future will doubtless reveal other forms of internal secretion which 

 also have a significance for the activity of the central system. 



Starvation. — In starving animals the nervous system loses but very little 

 in weight. 1 This small loss is most striking in view of the fact that exten- 

 sive histological changes occur in the cell-bodies. However, if we consider 

 the cell-bodies as the part mainly affected during starvation, then the small 

 mass of the cell-bodies would go far toward explaining the result, but it does 

 not explain why the myeline is so resistant. 



Fatigue. — The histological basis of fatigue as expressed by the changes in 

 the individual cells, has already been discussed. The fatigue of the system as 

 a whole is but the expression of fatigue in large numbers of its elements, but 

 the manner in which the changes show themselves is somewhat complicated. 



When the attempt is made to raise a weight by the voluntary contractions 

 of the muscles of the index finger at regular intervals, say once a second, it 

 is found that if the weight be heavy the power of the finger decreases, and 

 the weight soon ceases to be lifted as high as at first. Finally a point is 

 reached when the voluntary effort produces little or no elevation of the weight. 

 if, however, despite this failure, the effort is still made at regular intervals, 

 it happens, in some persons, that this power returns gradually, and a few sec- 

 onds later the contractions are very nearly as high as at the beginning of the 

 experiment (Mosso). This phenomenon may repeat itself many times, giving 

 a record formed by groups of contractions most extensive near the centre of 

 each group, these latter being separated by portions of the curve in which 

 the contractions are very small or wanting (see Kig. l'JO). (Sec ( Jeneral 

 Physiology of Nerve and Muscle, p. 135.) 



Daily Rhythms. — Within the cycle of the astronomical day the progress 

 of events leading to fatigue is not a steady one. Lombard'-' found that if the 

 capacity for voluntary effort was measured by the amount of work which 



1 Voit: Zeitechrift fiir Biologie, 1894, lid. xx.\. 



2 Journal of Physiology, 1892, vol. xiii. 

 Vol.. tr. 19 



