324 OP THE PRIMARY TISSUES OF THE HUMAN BODY. 



life ; for the Heart will continue to contract, when the blood in its vessels is 

 entirely venous, and when the circulation in it has come to a stand. Still, the 

 dependence of its action upon a constant supply of arterial blood, is very close ; 

 and in all animals, however different the plans of their circulation, we find a 

 provision for this supply, by a special arrangement of the coronary arteries. 

 That the heart's action comes to an end much sooner, after the destruction of 

 animal life by pithing, when the coronary arteries have been tied, than when 

 they are left untouched, has been proved by the experiments of Mr. Erichsen. 1 

 In an animal that has been pithed, but whose heart has been left intact, artificial 

 respiration will easily keep up its action for an hour, or an hour and a half. 

 But when the coronary arteries were tied, a mean of six experiments gave a 

 duration, for the ventricular action, of only 23 J minutes after the ligatures were 

 applied, and 32 minutes after the pithing; and in no instance was it prolonged 

 more than 31 minutes after the application of the ligature, or 37 minutes after 

 the pithing. On the other hand, when the aorta was tied, so that the coronary 

 arteries were distended with blood, the circulation being carried on through them 

 alone, the right ventricle continued to act up to the 82d minute. 



325. It has been maintained by many Physiologists, that the Irritability of 

 Muscle is dependent upon the Nervous system ; and the loss of that irritability 

 which usually follows division of the nerves of a voluntary muscle at no distant 

 date, is continually cited as a proof of this dependence. Two views of this sub- 

 ject have been advanced, both of which demand some notice, on account of the 

 eminence of the authorities by which they have been respectively sustained. 

 The first of these is, that Muscular irritability is derived from some influence or 

 energy communicated from the Brain or Spinal Cord, or that these organs supply 

 some condition essential to its exercise ; a doctrine of which, at the present time, 

 Prof. Miiller and Dr. M. Hall may be regarded as the principal supporters. 

 The opinion that contractility cannot be an independent endowment of an organ- 

 ized structure, is at once negatived by the fact that, in Plants, we find tissues 

 endowed with a high degree of contractility, and manifesting this property, 

 without' any possible intervention of a nervous system, on the application of 

 stimuli to themselves. In the lower classes of animals, too, there is good reason 

 to believe that contractility is more widely diffused through their tissues, than 

 nervous agency can be ; and we shall see that rhythmical contractions take place 

 in the rudimentary heart, when as yet no nerves or ganglia have made their 

 appearance. 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 completely separated from the body, which shows that the great 

 centres of 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. The loss of the irritability of 

 Muscles, within a few weeks after the section of their nerves, is clearly due to 

 the alteration in their nutrition, consequent upon their disuse ( 313). This 

 has been proved to demonstration by the very ingenious experiments of Dr. J. 

 Reid. 3 "The spinal nerves were cut across, as they lie in the lower part of the 

 spinal canal, in four frogs; and both posterior extremities were thus insulated 



1 "Medical Gazette," July 8, 1842. 



8 "Edinburgh Monthly Journal of Medical Science," May, 1841 ; and "Physiological, 

 Anatomical, and Pathological Researches," p. 11. 



