178 THE CIRCULATION OF THE BLOOD 



when physiologically isolated from the parts of the heart which contain the 

 ganglia, the behavior of isolated strips of the heart, especially of the ventricle, 

 and the rhythm of the embryonic heart are all held to be in favor of the myo- 

 genic theory. 



Automaticity of the Heart. Whether one adopts the neurogenic 

 or myogenic theory of the heart's beat, he has still to explain the origin of 

 the heart's rhythm. In the former case one must look to the nervous 

 apparatus for the origin of the rhythm; in the latter case, the muscular ap- 

 paratus, a fact to which Brown-Sequard long ago called attention. In the 

 former view the problem is to explain not only the periodic origin of the 

 nerve discharges from local cardiac ganglia, but also to explain the orderly 

 discharge of nerve impulses which maintains the proper sequence between 

 sinus, auricle, and ventricle. 



To perhaps the majority of physiologists the facts are best explained 

 by the myogenic theory. The origin of the rhythm is here supposed to be 

 due to the automatic property of the muscle itself. The sequence is ex- 

 plained on the observed facts, first, that muscular contraction in cardiac 

 muscle is conducted throughout the continuity of the mass, and second, 

 the most highly rhythmic part of the muscular tissue of the heart, the sinus, 

 sets the rhythm for the entire heart. 



The function of the nervous system, by this view, is not to originate the 

 rhythm, but to regulate it, the detail of which will be discussed below. 



Relation of Rhythm to Nutrition. The whole heart, like the 

 muscular parts of which it is composed, responds delicately to its condition 

 of nutrition. In the frog's and turtle's hearts the muscular fibers are brought 

 in intimate contact with the blood contained within its cavities. In the 

 mammalian heart, on the other hand, a distinct system of vessels, the coronary 

 vessels and the vessels of Thebesius, supply blood to the organ. If the heart 

 is supplied with nutrient fluid similar to its normal blood, and with proper 

 aeration to insure plenty of oxygen, it contracts with a strong rhythm for 

 many hours. This rhythm, however, responds quickly to changes in the 

 composition of the nutrient fluid. An abundant supply of oxygen is absolutely 

 necessary to the maintenance of rhythm in the mammalian heart, though the 

 heart, especially a cold-blooded heart, will contract for a time in an atmos- 

 phere of hydrogen. No doubt the organic constituents of blood are very 

 essential to the prolonged maintenance of rhythm in the heart, but the heart 

 is not dependent on these ingredients for its immediate reactions. The in- 

 organic salts seem to be peculiarly closely related to the development and 

 character of the cardiac rhythm, figures 172 and 173. Both the cold- 

 blooded heart and the mammalian heart respond very quickly to the influ- 

 ence of these salts. The details of this influence have been discussed on 

 page 173. It is somewhat surprising, however, that the highly organized 

 mammalian heart will contract rhythmically for hours on purely inorganic 



