CHAPTER XXIX. 



THE CAUSE OF THE HEART BEAT— PROPERTIES OF 

 THE HEART MUSCLE. 



General Statement. — The cause of the heart beat has naturally 

 constituted one of the fundamental objects of physiological inquiry. 

 The various views that have been proposed in different centuries 

 reflect more or less accurately the advancement of the science. 

 With each new discovery of general significance a new point of view 

 is obtained and the theories of the heart beat, like those of the other 

 great problems of physiology, shift their standpoint from generation 

 to generation. The general modern conception of this problem 

 is referred usually to Haller (1757), who first taught that the 

 activity of the heart is not dependent on its connections with the 

 central nervous system. As we.shall see, the heart beat is controlled 

 and influenced constantly by the central nervous system, but never- 

 theless the important point has been established beyond question 

 that the heart continues to beat when all these nervous connections 

 are severed. The central nervous system regulates the activity of 

 the heart, but has nothing to do with the cause of its rhythmical 

 contractions. The heart, in other words, is an automatic organ. 

 When in 1848 Remak discovered that nerve cells are contained in 

 the frog's heart it was natural that the causation of the beat should 

 be attributed to this tissue. Subsequent histological work has 

 demonstrated the existence of numerous nerve cells in the substance 

 of the heart tissue of all vertebrates, and the view that the au- 

 tomaticity of the heart is due in reality to the properties of the 

 contained nerve cells was the prevalent view throughout the 

 middle and latter part of the nineteenth century. In the latter part 

 of the century an opposite view arose, — namely, that the muscular 

 tissue of the heart itself possesses the property of automatic 

 rhythmical contractility. Both these points of view persist to day. 

 The theory that refers the automaticity of the heart beat to the 

 contained nerve cells is designated as the neurogenic theory of the 

 heart beat; the one that refers this property to the muscle tissue 

 itself is known as the myogenic theory. Beyond this question lies 

 the still deeper problem of the explanation of the automaticity 

 itself, the cause or causes of the rhythmical excitation, whether 

 occurring primarily in the muscle cells or in the nerve cells. 



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