278 MANUAL OF PHYSIOLOGY. 



inherent tendency to rhythmical contraction. This is shown by 

 the following facts. The heart cannot continue contracted like a 

 skeletal muscle (in tetanus) under any circumstances, or like an 

 unstriated muscle (in tonus) except when the tissue is spoiled by 

 deficient nutrition, etc. The hearts of many of the lower animals 

 contract rhythmically without any nerve elements being found 

 by the most careful microscopic examination. A strip cut from 

 the ventricle of the tortoise can, by judicious excitations, be 

 taught to beat rhythmically without tlie help of any known nerve 

 mechanism. The lower part of the frog's ventricle which is 

 commonly admitted not to contain any nerves beats quite 

 rhythmically if fed with a gentle stream of serum and weak salt 

 solution, and there is no reason to assume that there is any greater 

 difficulty in conceding to muscle tissue, than to nerve cells, the 

 property of acting with a regular rhythm. 



In cold-blooded animals, such as a frog or tortoise, the heart 

 will beat even for days after its removal from the animal, if it be 

 protected from injury and prevented from drying. In warm- 

 blooded animals the tissues lose their vitality shortly after they 

 are deprived of their blood supply: however, spontaneously 

 rhythmical movements can be seen in the mammalian ventricles 

 if removed rapidly after death. The hearts of oxen, skillfully 

 slaughtered, commonly give several beats after their removal 

 from the thorax. If a blood current be kept up through the 

 vessels of the heart tissue, this spontaneous contraction will go on 

 for some time, or even will recommence after having ceased. 



The hearts of two criminals who were hanged were found to 

 continue to beat for four and seven minutes respectively after 

 the spinal cord and the medulla had been separated. 



These facts prove conclusively that the stimulus which causes 

 the heart to beat rhythmically arises in the muscle tissue of the 

 organ or in close relation to it. Upon physiological grounds 

 alone we might conclude that in the heart tissue of the vertebrata 

 there exist the nerve elements with which we are familiar ana- 

 tomically. These nerve cells only require their nutrition to be 

 kept up by a continued blood supply in order to develop the 

 energy necessary for their function. 



