424 VETERINARY PHYSIOLOGY 



IV. The Maintenance and Control of the Cardiac 



Rhythm. 



That there is an intrinsic mechanism in the heart for 

 the maintenance and control of its action is shown by the 

 fact that the excised heart continues to beat in cold-blooded 

 animals for a considerable time without any supply of blood 

 and in warm-blooded animals if oxygenated blood is supplied 

 at a suitable temperature. 



In considering the nature of this mechanism, it must be 

 borne in mind : first, that two distinct questions have to be 

 investigated : — 



\st. How the rhythmic contractions are initiated and 

 maintained ; 



2nd. How they are propagated over the heart ; 

 and second, that nerve structures as well as muscular fibres exist 

 in the heart, so that either one or other or both of these 

 may be involved in the starting and conduction of con- 

 tractions. 



1. The Initiation and Maintenance of Rhythmic Contraction. — 

 (1) In the embryo, the heart begins to beat before any 

 nervous structures can be shown to have migrated into it. 



(2) Hooker finds that, after removal at a very early stage 

 of development of the anterior part of the neural canal from 

 which the neurons to the heart come, the formation of the 

 heart still goes on while it beats in a normal manner. 



(3) A little piece of the heart of an embryo kept under 

 aseptic precautions in the animal's blood plasma will grow, 

 and will manifest typical rhythmic contractions. 



(4) Even in the adult amphibian it is possible to start 

 rhythmic contractions in the apex of the ventricle — a part 

 in which nerve cells have not been observed — either by 

 repeated rhythmic stimulation or by distending it with 

 Ringer's solution perfused through a tube. The conclusion 

 thus seems inevitable that rhythmic contraction is primarily 

 a function of the muscle. 



But there is evidence that, when nerve structures have 



