The foregoing considerations have a very important bearing on the question of the de gree to which the process of nutrition is under the influence of the nervous system, a question on which, as it appears to the writer, very erro neous ideas have been commonly entertained. For it has been customary to speak of this pro cess (as well as of secretion) as dependent upon nervous agency ; or, in other words, to assert that the nervous system is not only the instru ment of the functions of animal life, but is also the primum mobile of the organic operations. Now the independent properties of the cells in vvhich all organized tissues originate, might be of itself a satisfactory proof that in animals, as in plants, the actions of nutrition are the results of the powers with which they are individually endowed ; and that whatever influence the nervous system may have upon them, they are not in any way essentially dependent upon it. Moreover there is an evident improbability in the idea " that any one of the solid textures of the living pody should have for its office to give to any other the povver of taking on any vital actions;" and the improbability becomes an impossibility, when the fact is known, that no formation of nervous matter takes place in the embryonic structure, until the processes of organic life have been for some time in active operation. The influence which the nervous system is known to have on the function of nutrition may operate in several ways. Thus, if the nerves proceeding to any set of muscles be divided, those muscles will be atrophied in consequence of the cessation of their activity, as already explained. In other instances vve
may not improbably regard the influence of the nervous system to be exercised through the medium of its controlling power over the dia meter of the bloodvessels, by which it may govern the afflux of blood. And there can be little doubt that, in some manner yet unex plained, the nervous system exerts an influence over those preliminary processes, by which the plastic element of the blood is elaborated; so that long-continued anxiety or depression of mind inay produce general atrophy, or a ten dency to tuberculous deposit. It appears to be invariably through emotional states of the mind that the nutritive process is affected ; the wi// not possessing any direct power of influ encing them. But there can be no doubt that the continual voluntary direction of the atten tion to the sensations of any part, giving rise to emotions on which the mind frequently dwells, may so far modify the nutrition of the part as to become a cause of diseased action in it. All these facts, however, point rather to the influence which the nervous system possesses over the organic functions, than to the dependence of these upon its agency ; and it may be safely asserted that no such proof of its more direct influence, as is required to counterbalance the manifest improbability which has been shown to attend it, has yet been given. Some addi tional considerations upon this important sub ject will be offered under the head of SE CRETION.
( W. B. Carpenter.)