PROPERTIES OF MUSCULAR FIBRE. 283 



nicated by the nerve is of the same character, whatever may have been its 

 source. The motor nerves cannot be properly said to terminate in the 

 muscles ; for the trunks form a kind of network in their substance, the fibres 

 which they send off returning again to themselves, by loops, or to other trunks. 

 In what manner the stimulus is conveyed and communicated, can only be at 

 present a matter of speculation. That the influence is of an electrical kind, 

 has been supposed by some ; principally on account of the similarity between 

 the muscular contractions, excited by galvanism transmitted through the 

 nerves, and those ordinarily produced by voluntary direction. But it is to be 

 remembered that other agents, both physical and chemical, may produce the 

 same effect ; and there are objections, which at present appear insuperable, to 

 the belief that nervous influence and electricity are identical, whatever may 

 be the analogy in their mode of operation. The muscles of the second class 

 appear to be, in the living body, much seldomer called into contraction through 

 their nerves, than they are by stimuli applied directly to themselves. The 

 will has no power over them ; and they would seem to be rather affected by 

 those emotional conditions of mind which volition cannot imitate. This 

 influence is continually experienced in the action of the heart, and probably 

 also affects the movements of the intestinal tube. 



380. The continual and evident influence of the Nervous System upon 

 Muscular Contractility, has led many physiologists to the belief, that the latter 

 is dependent upon the agency of the former. Two views upon this question 

 have been commonly taught, to both of which it seems necessary to devote a 

 brief consideration. The first of these is, that Muscular contractility is de- 

 rived from some influence or energy communicated from the Brain or Spinal 

 Cord. This opinion is evidently analogous to that which attributes the vital 

 properties of other parts to the Nervous System alone ; and it is open to the 

 same objection, in limine, which has been applied to the latter, the improba- 

 bility that any one of the solid textures of the living body should have for its 

 office to give to any other the pow-er of performing any vital action. More- 

 over, it is inconsistent with the fact that, in Vegetables, tissues endowed with 

 a high degree of contractility exist, and manifest their property when a stimulus 

 is directly applied to themselves ; which, nevertheless, can have no depend- 

 ence whatever upon a nervous system. In the lower classes of Animals, too, 

 there is good reason to believe that the property is much more universally 

 diffused through their tissues than nervous agency can be. Again, the 

 action of the heart may be kept up, in the highest animals, by taking care 

 that the current of the circulation be not interrupted, for a long time after 

 the removal of the brain and spinal cord ; it may even continue when com- 

 pletely separated from the body, which shows that the ganglionic system 

 cannot supply any influence necessary to it ; and there are many instances in 

 which the human foetus has come to its full size, so that its heart must have 

 regularly acted, without the existence of a brain or spinal cord. Further, the 

 irritability of muscles of the first class continues for a long time after their 

 nerves are divided, and may be called into action by stimuli directly applied 

 to the parts themselves, or to their nerves below the section, so long as their 

 nutrition is unimpaired. 



381. The loss of the irritability of Muscles, within a few weeks after the 

 section of their nerves, on which great stress has been laid by Miiller in 

 support of a modified form of the above doctrine, (it being maintained by this 

 distinguished physiologist, that, if muscular irritability is not dependent on the 

 Brain and Spinal Cord, they supply some influence essential to its exercise,) 

 is clearly due to the alteration in their nutrition, consequent upon their 

 disuse. This has been recently proved to demonstration, by the very inge- 



