EXCITATION AND INHIBITION 421 



result in a total abolition of the dilator effect, it does not disprove the hypothesis 

 that the excitatory and inhibitory nerves do actually play on the same muscle cell 

 or nerve cell, although not in the way of interference of wave motion. It will be 

 found, in fact, that experimental results necessitate this view. We must remember 

 that, at the time of von Frey's experiments, it was not known that the activity 

 of the gland cells gives rise to certain products, " metabolites," possibly of an acid 

 nature, which diffuse to the arterioles and, acting there as chemical agents, cause 

 dilatation. This, naturally, continues its action after the actual stimulation of 

 the nerve has ceased. Attention has been directed to it chiefly by the work of 

 Barcroft(1907). 



In cases where a process is a spontaneous one, it will be clear that it will 

 resume its activity on cessation of the inhibitory influence. 



There is more evidence in favour of a second group of theories, founded on 

 the nutrition of cells. The foundation of this view was laid by Hering in his 

 papers on sensations of light (1878) and worked out in more detail in a celebrated 

 paper in "Lotos " (1889). It rests on the idea of the opposition between assimila- 

 tion and dissimilation in Bering's words, or anabolism and catabolism in those of 

 Gaskell (1886, p. 46). Verworn has adopted it and elaborated it in connection 

 with his biogen theory. 



There are, however, many objections to be brought against the theory. In 

 the first place, we may consider the main principle. When a cell is actually 

 increasing in the substance of its protoplasmic machinery, there is no doubt of 

 the fact that it is engaged in building up complex systems out of simpler food 

 materials ; this is assimilation or anabolism ; it may be considered as analogous 

 to the manufacture of a petrol motor. Again, there are certain cell processes 

 about which there can be no doubt that they consist in disintegration or breaking 

 down, with the giving off of energy ; the oxidation of glucose to carbon dioxide 

 and water is such a case. This is dissimilation or catabolism. Now the theory 

 of Hering is based on the idea that the two phenomena are mutually exclusive, 

 and, no doubt, this might be the case if the molecules in question were of the 

 same kind, as is assumed in the theory of biogens. So that if, as is pretty clear, 

 the phenomena known especially as vital, or obvious manifestations of activity, 

 are associated with the breaking down of molecules and giving off of energy, 

 catabolic action will be synonymous with excitation. Further, if we accept the 

 view that building up of new material is inconsistent with catabolic activity, we 

 are justified in regarding the opposite process, or anabolism, as being associated 

 with inhibition. 



The theory also supposes that both these processes can be accelerated or 

 started by nervous influences. As to the catabolic processes, there is no dispute, 

 but we have already seen (page 288) that there is no satisfactory evidence of the 

 direct influence of nerves on growth of protoplasm ("trophic nerves"). 



We have, moreover, also found evidence in various directions that material 

 used for energy purposes does not become an integral part of the protoplasmic 

 molecules, but is used by the protoplasmic machinery in a way analogous to that 

 in which fuel is burnt in an internal combustion engine. The building up of a 

 material of high potential energy, for the purpose of giving off this energy in an 

 available form on its breakdown, appears to be effected by the concurrence of 

 another reaction in which energy is set free by oxidation, as in the case of 

 secretion, and we shall find a further striking instance in muscular contraction. 



Again, it is very difficult to form an idea of how the increase of anabolism can 

 result in a decrease of catabolism. Take the illustration used by Forbes (1912, 1, 

 p. 152). The cell is compared to a water tank, provided with an inlet pipe and an 

 outlet, both supplied with adjustable stopcocks. The outlet is supposed to be 

 partly opened, and the stream of water represents the outgoing energy, catabolism. 

 The inlet is connected with a supply at a somewhat higher level, and is opened to 

 such an extent that the level in the tank is kept constant. This inflow, anabolism, 

 is thus equal to the outflow. Now the theory under discussion implies that, if we 

 increase anabolism by opening the inlet wider, we shall diminish the rate of 

 outflow. In point of fact, of course, by increasing the inflow, we raise the level of 



