3. The third doctrine, of which Dr. Alison has been one of the most philosophical and consistent advocates, is to the effect that the whole organic or vegetative life of' animals,— e. every thing which goes on in them with out the intervention of any sensation or other mental act, including the functions of nutrition and secretion, — may go on without the in tervention of the nervous system, and stands in no relation of dependence to any changes in nervous matter ; but that these changes exert a powerful controlling and modifying influence on the organic functions, increasing or dimi nishing their activity, or even altering their character ; just as, to use the appropriate illus tration of Dr. John Reid, the movements of a horse are influenced by the hand and heel of the rider, although they are in themselves in dependent of him, being executed in virtue of the power inherent in the animal.
Now, in support of this last view of the subject, it may be urged, in the first place, that in one great division of the organised world, namely, in the vegetable kingdom, the functions of nutrition and secretion are per formed, not only independently of, but with out any kind of influence from, a nervous systern ; each act being the result of the pro perties inherent in the several parts of the structure itself, called into play by the appro priate stimuli. We should have a right to expect, therefore, that the corresponding func tions in animals should be adequately per formed by a similar mechanism ; and it is fair, therefore, to throw the burthen of proof upon those who maintain the contrary'. If we fol low out in this case the great general principle of Cuvier, which every day's experience only shows to be more strictly correct and more widely applicable, — that the different classes of animals may be considered as so many ex periments ready prepared for us by nature, who adds to or takes from their several organs, just as we might wish to do in our laboratories, showing us at the same time the various results of these combinations,—we see that a comparison of different organisrns affords us a much better ground for the de termination of this question, than can be ob tained from the results of such experiments as have been already cited ; it not being pos sible to make such experiments, without such injury to the organism as is of itself a serious disturbing cause. We notice, on looking at the highest animal, that the organic functions are brought into very close relation with the animal powers, and are liable to be consider ably modified by the exercise of the latter. But, as we descend the scale, we find the nervous system constituting a less and less predominant part of the organism, and the apparatus of organic life becoiming more and more disconnected from it ; until, in zoo phytes, we are scarcely able to distinguish a nervous system at all, whilst all the operations of growth, nutrition, and secretion take place very much as in plants, in which no nervous system exists. Thus we find that " the ner
vous system lives and grows within an ani mal, somewhat as a parasitic plant does in a vegetable," deriving its nutriment from the structure in the midst of which it is deve loped, and capable of exercising a certain ac tion upon it, but being strictly a superaddcd part, and having rather an adaptive than an essential connection with that structure.
Now this view has derived from late dis coveries in minute anatomy, as complete a confirmation as any such facts are capable of affording. For it has been shown, not merely that the functions of nutrition and secretion are common to animals and plants, but that the component elements of the organs by which they are performed are in both instances essentially the same. We have seen that the act of secretion is effected, even in the most complex gland, by the agency of aggregations of cells, each of which lives for and by itself, and appears to be dependent upon no other external conditions, than those which are re quired for the growth of the simplest cellular plant, namely, food and warmth. And it is difficult to conceive how, over that most es sential part of the secreting process — the de velopement of the secreting cells — the ner vous system can exert any direct influence.
Another natural experiment, whose imme diate bearing is rather upon the physiology of nutrition than upon that of secretion, but which is really as conclusive in regard to the latter as the former, is exhibited to us in the early growth and developement of the em bryonic structure ; hich makes considerable progress, especially in invertebrated animals, before any trace of the nervous system can be detected. And in the hufnan species the case is not unfrequent, of the fcetus coming to its full size with the usual variety- of textures in its composition, but without either brain or spinal cord. It has been said, however, that in such instances the ganglia of the sympa thetic system probably exist, and supply the influence supposed to be needed; but there are cases on record, in which these would seem to have been carefully looked for and not de tected.* And moreover, even if their uni form presence were to be admitted, and the power of sustaining the operations of nutri tion and secretion be supposed to reside in them, how are we to explain the effects of in juries of the brain and spinal cord. the gangli onic centres being left intact ? We can see no other consistent account of these phenomena, than that which is presented by the last of the three hypotheses enufnerated ; the functions of nutrition and secretion (like the contrac tility of muscular fibre) not being regarded as dependent for their ordinary exercise upon any power supplied by the nervous system, but being considered to be modified by causes operating through it.