868 INFLUENCE OF NERVES ON NUTRITION. [BOOK n. 



Within the limits and subject to the conditions however 

 thus fixed by race and personality, general influences produce 

 general variations in nutrition. The rate of nutrition of a tissue 

 for instance is dependent on the food, on the amount and nature 

 of the food material brought to the tissue by the blood. We 

 have seen that proteid food, in contrast to carbon food, markedly 

 increases the metabolism of the body. Since this increase tells 

 not only on the nitrogenous but also on the carbon metabolism 

 ( 523), it cannot be the result of a mere lux us consumption of 

 the proteid food itself; and unless we suppose that the presence 

 of the excess of proteid material either in the alimentary canal, 

 or while passing through the capillaries of some organ such as 

 the liver, acts as a stimulus to some reflex nervous machinery 

 through whose action the metabolism of certain or of all the 

 tissues is hurried on, we must conclude that it is the direct access 

 of proteid material to the tissues themselves which stirs them 

 up to increased metabolic activity. That proteid food should do 

 this and not carbohydrate or fat, seems to be connected with 

 the fact just dwelt on that proteid material is the pivot of 

 metabolism. 



548. In the preceding chapters of this work we have had 

 abundant evidence that the metabolism of the tissues is subject to 

 the government of the central nervous system ; the contraction of 

 a muscle, the secretory activity of a gland, the increased or 

 diminished production of heat all afford instances of nervous 

 impulses affecting metabolism. In most of these instances the 

 changes induced fall for the most part within the downward, 

 katabolic, phase and have a downward character; thus when a 

 muscle contracts, the main result is a conversion of more complex 

 bodies into simpler bodies ; and the same so far as we can see is 

 true of most other cases. But when a secreting gland, such as a 

 salivary gland, is stimulated to activity by nervous impulses the 

 katabolic events leading to secretion, though dominant, are accom- 

 panied as we have seen by anabolic events, by reconstruction of 

 the secreting cell substance for instance ; and these latter we 

 must also attribute to the nervous impulses. Something of the 

 same kind takes place in muscle during its contraction and in 

 other tissues during their activity. So that we are led to con- 

 ceive of nervous impulses as affecting katabolic changes on the 

 one hand and anabolic changes on the other. And we may 

 further conclude that the total result of the action of nervous 

 impulses on the same tissue will differ according as those im- 

 pulses provoke chiefly katabolic or chiefly anabolic changes. We 

 seem to see an instance of this in the antagonistic effects on the 

 heart of augmentor and inhibitory nerves. As we have seen 

 ( 161) stimulation of the cardiac inhibitory fibres seems to pro- 

 duce such changes in the cardiac muscular substance that the 

 upward constructive processes are assisted and the downward 



