722 



CONTRACTILITY. 



to its connexion with respiration, than that of 

 the greater activity of muscular action, on the 

 whole, in those animals where there is much 

 exposure of the blood to the air, and the 

 greater endurance or tenacily of life where there 

 is little. 



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 efiect 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 

 hibernating animals (as Dr. M. Ilall* 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 foetus having come to a 

 full size (implying long-continued and regular 

 action of the heart), where neither brain nor 

 spinal cord existed,f 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 ; J and 



* Philosophical Transactions, 1832 



t See Brachet's Recherr.hes sur le Systeme Ner- 

 veaux, p. 36 & seq. 



| Air. J. W. Earle, in a " New Exposition of the 

 Functions of the Nerves," has attempted to revive 

 this theory. He trusts chiefly to an experiment, 

 in which the irritability of muscles, exhausted by 

 repeated irritation, was not recovered after their 

 nerves had been cut. But this experiment is incon- 

 clusive, because the muscles had become inflamed 

 and disorganized. (See p. 70 and 71 of his work.) 

 This experiment has been lately repeated in Edin- 

 'burgh, with precautions to prevent the inflam- 

 mation of the muscles, and the result was the 

 reverse of that obtained by Mr. Earle. See Trans 

 actions of British Association, 1834. 



the only question that can remain is, whether 

 the irritation 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 " Atrirmantibus 

 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- 

 cree 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 



* Loc. cit. p. 125. 



