The question, how far the Nervous System furnishes one of the conditions necessary to the maintenance of the contractile power of muscles, has long engaged the attention of physiologists, and been the occasion of much erroneous medical theory; but in the present state of the science, need not occupy much of our attention.
The doctrine of Cullen and many other systematic writers, that the muscles derive re gular supplies of' Irritability or vital power, through the nerves, from the larger masses of' the nervous system, seems to be now pretty generally abandoned, although the terms Ner vous Influence or Energy are still suffered to retain, in the language of many medical writers, a vague and indefinite meaning, derived from that apparently erroneous theory. When we remember, that after the nerves of a muscle are cut, the muscle continues irritable under stimuli applied to itself, or to the portions of nerves below the section, as long as it retains its organization unim paired,—that section of the nerves leading to the heart has in very numerous experiments been found to produce little or no effect on its movements,—that these movements continue for hours after the head has been cut off, or even (as was first shewn by Dr.Wilson Philip) after both brain and spinal cord have been removed from the body, provided that the flow of the blood through the lungs is maintained by means of the artificial respiration,—that in hybernating anirnals (as Dr. M. Hall* has ascertained) when respiration is at a stand, the regular movements of the heart may con tinue for nine hours after the gradual but com plete destruction of the whole brain and spinal cord,—and that there are many instances on record, of the human fcetus having come to a full size (implying long-continued and regular action of the heart), where neither brain nor spinal cord existed,t—it seems impossible to maintain the purely hypothetical proposition, that the irritability of muscles is dependent on an influence or energy continually flowing to them from the brain or spinal cord; I and the only question that can remain is, whether the imitation of muscles is always effected through the medium of nerves, i. e. whether every stimulus which excites contraction in a muscle first acts on some of the nervous fibrils which enter it, and by exciting them throws the muscular fibres into action. An ex periment of Brachet* has been thought to furnish evidence of the dependence of the heart's actions on the cardiac plexus of nerves, but is so liable to fallacy, and so much op posed to the experience of others, on the effect of injuries of the cardiac nerves, that the in ference seems to have been generally dis trusted.
Without presuming to decide absolutely on a question which still divides the opinions of physiologists, and without entering on various arguments which have been stated-as furnishing probable evidence either on the one side or the other, we may observe,— 1. That the safe logical rule in such cases, is " Affirmantibus incumbit probatio ;" and therefore it does not appear philosophical to teach, that the con traction of all muscles, on stimuli being ap plied to themselves, is owing to the inter vention of nerves, until that intervention be proved. 2. That if the contraction of all muscles were excited through nerves, we might expect to find all muscles supplied with nerves, the mechanical irritation of' which, in the li ving or newly killed animal, should excite that contraction. But it has been already observed, that in the case of the involuntary muscles, physical irritation of the nerves en tering them (if strictly confined to the nerves) has very generally been found quite ineffectual for that purpose. This seems pretty clearly to indicate, that the power of exciting muscular fibres to contraction is an endowment peculiar to the nerves of the voluntary muscles, or at least enjoyed by them in a much greater de gree than by others, and designed, not to render these muscles irritable, but merely to subject their irritability to the dominion of the Will.
The observation of Fontana on this subject, made as early as 1775, and in perfect accord ance with the statements of Haller previously, and of many other physiologists subsequently, may still be quoted as more conclusive than any other which has since been brought for ward. " If you open the chest of an animal, (a cold-blooded one answers best for the ex periment) and stimulate as you please the nerves going to the heart, that muscle will neither accelerate its movements if it be moving, nor resume them if it be at rest,— even although it be prone to immediate con traction on its own fibres being touched. The nerves of the heart, therefore, are in no sense the organs of the movement of this muscle, as they are of other muscles. This experiment is certain, and the inference direct. It would be a contradiction to assert that the movements of the heart take place through the intervention of nerves, when experiment shews that nerves cannot excite these movements." IV. In regard to the laws, by which the vital powers of contractile parts may be regu lated, we have probably much to learn ; but three sets of facts have been observed, which may at present be regarded as general laws in this department of physiology.