ON NUTRITION IN GENERAL. 511 



inentary 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 pro- 

 teid 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. 



461. 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 in- 

 stances 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 conver- 

 sion 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 con- 

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

 pulses 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 accord- 

 ing 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 fur- 

 ther, we may say that the conception, though suggestive, has not yet been 

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



462. 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 kata- 

 bolic 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 strik- 

 ing and obvious, and the behavior of a muscle or a gland as far as contraction 

 and secretion are concerned is, within certain limits, under experimental con- 

 trol. But there are certain phenomena, seen chiefly in the course of disease, 

 and lying, to a very small extent only, within the control of experiment, 

 which seem to show that the central nervous system governs the metabolic 

 changes, the nutrition, not only of muscle and gland, but of various other 

 tissues, in a deeper and more general way than that of simply promoting (or 

 hindering) contraction or secretion. Thus, as we have seen ( 81), when the 

 connection between a muscle and the central nervous system is severed, the 

 muscle eventually wastes and loses its vitality ; when all the nerves going to 

 the submaxillary gland are severed, the gland, instead of being, as in the 

 normal condition, intermittingly active and quiescent, pours forth a continu- 

 ous "paralytic" secretion and eventually degenerates and wastes. When in 



