CENTRAL NERVOUS SYSTEM. 



737 



normal adult this bulging cannot of course occur to anything like such an 

 extent, and the space for the arterial blood must be gained in the first instance 

 by driving out the blood from the venous sinuses within the cranium and 

 through the removal 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 necessary for the growth 

 of the nervous, as well as the other systems, and 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 nervous 

 system included. 



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

 structive changes in the central system, caused by disturbances in its nutrition. 



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

 in weight. 1 This small loss is most striking, and would seem to be best 

 explained on the assumption that the other tissues are used to keep up the 

 central system, which, when even slightly reduced in weight, ceases to act. 



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 



lihlll I I I 



Illllll III 



FIG. 208. A record of the extent of the flexions of the forefinger lifting a weight at regular intervals. 

 The light lines are those for the voluntary contractions ; the heavy lines, those for contractions follow- 

 ing the direct stimulation of the flexor muscles by electricity. In the former there are periods, in the 

 latter none. The arrow shows the direction in which the record is to be read (Lombard). 



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 

 occurs in some persons that this power returns gradually, and a few seconds 

 later the contractions are very nearly as high as at the beginning of the ex- 

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

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



1 Voit : Zeitschrift fur Biologie, Bd. xxx., 1894. 

 47 



