272 MANUAL OF PHYSIOLOGY. 



Other muscles, as has been seen in the pharynx, oesophagus, 

 etc., though not under the control of the will, are yet governed 

 altogether by the cerebrospinal axis; while others, of which the 

 most striking example is the heart, have nerve elements in imme- 

 diate relation to the contractile tissue, capable of exciting them 

 to contraction. 



It will materially help us in comprehending the nervous mech- 

 anism of the heart's rhythm, if we bear in mind what now seems 

 to be proved beyond doubt, namely, that the muscle tissue of the 

 heart has quite independently of any nervous influences an in- 

 herent 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 the help of any known nerve mech- 

 anism. The lower part of the frog's ventricle which is com- 

 monly admitted not to contain any nerves beats quite rhythmi- 

 cally if fed with a gentle stream of serum and weak salt solu- 

 tion, 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 almost immediately 

 after they are deprived of their blood supply ; however, sponta- 

 neously rhythmical movements can be seen in the mammalian 

 heart if removed rapidly after death. The hearts of oxen, skil- 

 fully slaughtered, commonly give a few 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 recently hanged were 



