CHAPTEE XIII 



WHEN a process of any kind takes place continuously of itself without intervention 

 from without, it is clear that, for purposes of due regulation in the living organism, 

 there must be means of modifying it in both directions ; there must be some 

 power of either increasing it or decreasing it according to necessity. Cases of 

 physiological processes of this kind are the muscular coat of the small arteries and 

 other places where we find that kind of muscular tissue known as smooth, pale, 

 or involuntary muscle. This, in its usual state, which may be regarded as 

 "resting," since it is the condition taken on when unaffected by nervous impulses, 

 is in a state of partial contraction or "tone." This tone is capable of being 

 increased by certain nerves supplying the tissue and diminished by another set of 

 nerves. In the preceding chapter we saw how the automatic movements of the 

 intestine can be stopped by the splanchnic nerve and increased by the vagus. 

 For our present purpose it is immaterial whether these movements are due to 

 periodic discharges of nerve cells in Auerbach's plexus, or inherent in the muscle 

 cells themselves, although the work of Gunn and Underbill (1914) shows that the 

 latter is the correct statement. In either case, the responsible cells can be 

 either restrained or excited. Such double effects play a fundamental part in the 

 mechanism of nerve centres, as we shall see later. Perhaps the most striking 

 instance that can be given is that of the heart. As is well known, this organ 

 continues its regular series of beats even when cut out of an animal ; but, in its 

 natural state, it can be ^topped by the vagus nerve or excited to increased rate 

 and force by the " accelerator " nerves from the sympathetic system. 



This capacity of being affected in two opposite directions is not confined to 

 living matter. Consider again our old example of an ester system in equilibrium. 

 As we have seen, if we add more water, there is increased hydrolysis ; if we remove 

 water, there is increased synthesis, or diminished hydrolysis, of ester. The effect 

 of a catalyst should also be kept in mind, as consisting merely in the acceleration 

 of the attainment of equilibrium, by addition or subtraction of water ; that is, 

 it increases both hydrolysis and synthesis ; which of these effects will be the more 

 obvious one depends on circumstances. But again, the accelerating action of a 

 catalyst can be itself increased or diminished. Pepsin hydrolyses proteins at the 

 greatest rate in the presence of a certain definite concentration of hydrogen ions ; 

 suppose that this concentration is somewhat less than the optimal one, it is plain 

 that we can increase the rate of hydrolysis by adding more acid or diminish it by 

 adding alkali. 



, The duality of phenomena illustrated in the last example reminds us further of 

 the opposition in general chemical properties between hydrogen and hydroxyl ions. 

 The existence of positive and negative electricity may also be mentioned, although 

 perhaps the final word has not been said on this question. 



In discussing the phenomena of metabolism, we saw how two processes might 

 be distinguished, the building up of a complex system or substance of high 

 potential energy, "anabolism," and the breaking down of such a system, "cata- 

 bolism," giving off energy in other forms. Such a case we saw in the secretion of 

 the salivary glands and shall meet with again in muscular contraction. The 

 tendency of much recent work, however, is to throw doubt on the universality of 

 this opposition of anabolism and catabolism as explanatory of physiological activity 



