CHAP, v.] NUTRITION. 829 



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 within the downward, katabolic, phase and 

 have a downward character; thus when a muscle contracts, the 

 result is a conversion of more complex bodies into simpler bodies ; 

 and the same as far as we can see is true of most other cases. But 

 it is open for us to suppose that nervous impulses might affect the 

 upward, anabolic, phase and have a constructive influence. There 

 are no reasons for regarding such an action as impossible; and 

 indeed some phenomena, such as those of inhibitory nerves and the 

 antagonism between these and augmentor nerves, pointedly suggest 

 some such view. Thus we may suppose that an inhibitory impulse 

 produces such changes in the cardiac muscular substance that the 

 upward constructive processes are assisted and the downward 

 disruptive processes checked, whereby the setting free of energy is 

 checked and so the beats hindered or stopped, the immediate 

 inhibitory effect being followed by a period of rebound in which 

 the savings of the inhibited period are spent in increased action. 

 Conversely we may suppose that an augmentor impulse hinders the 

 anabolic and assists the katabolic changes, and conversely also 

 when it has done its work leaves the tissue with diminished 

 capital manifested by feebler beats or by the absence of the power 

 to beat. And similarly in the case of the respiratory centre and of 

 other tissues. When we have to study the origination of visual 

 impulses in the retina we shall come upon a view that a wave of 

 light may affect what we shall call a visual substance either 

 by promoting anabolic constructive changes or by increasing 

 katabolic destructive changes according to its wave length. There 

 is then evidence to a certain extent for the view on which we are 

 dwelling ; but, without discussing the matter any further, we may 

 say that the conception though suggestive has not yet been 

 demonstrated and so far can only be spoken of as probable. 



549. One value perhaps of such a view lies in the fact that 

 it warns us against assuming that a nervous impulse can only 

 produce disruptive katabolic changes such as are seen in muscular 

 contraction or in secretion. The effects of stimulating a nerve 

 going to a muscle or a salivary gland are striking and obvious, and 



