CIRCULATION OF THE BLOOD. 317 



is also an element in producing the first sound. The striking of 

 the apex against the chest-wall, the so-called apex-beat, formerly 

 regarded as one of the factors of the first sound, can take no part 

 in its production, because, as has been pointed out, this action does 

 not take place. 



Every student should familiarize himself with the cardiac 

 sounds, not simply by reading about them, but by listening 

 to the human chest. A thorough knowledge of their character 

 is essential to a comprehension of the diseases of the heart. It is 

 important to remember that the impulse of the heart, the systole 

 of the ventricles, the first sound, and the closure of the mitral 

 and tricuspid valves are synchronous, and that when the second 

 sound is heard the ventricles are beginning their diastole and the 

 aortic and pulmonary valves have just closed. 



Cardiac Innervation. The cause of the beat of the heart 

 has not been definitely ascertained. The fact that it beats when 

 removed from the body shows that this action is dependent upon 

 some power within itself. The length of time that a heart so iso- 

 lated will continue to beat varies in different animals, being longer 

 in the poikilothermal than in the homoiothermal ; thus, it may con- 

 tinue for days in the former, while in the latter it may cease after 

 a few hours or even minutes. Landois states that in hearts that 

 have been excised "the last vestige of cardiac action has been 

 observed in the rabbit after 15J hours, in the mouse after 46 J 

 hours, in the dog after 96J hours, and in a three-months'-old 

 human embryo after 4 hours." 



Two theories have been advanced to explain the beat of the 

 heart: (1) That it is due to stimuli having their origin in nerve- 

 ganglia which exist in the heart ; and (2) that it is due to an in- 

 herent power of contraction residing in the cardiac muscle-cells, a 

 power independent of any nervous connection whatsoever, and that 

 this is due to the action upon the heart muscle of chemical sub- 

 stances in the blood, as calcium, sodium, and potassium salts; the 

 first being apparently essential for the chemical stimulation, while a 

 certain proportion of potassium is also necessary (Howell). The 

 effect of .the sodium chlorid is to maintain the osmotic equilibrium 

 between the muscle-cells and the surrounding liquid. 



The first of these theories is supported by the fact that there 

 are numerous ganglion-cells in the heart, and that where these are 

 most abundant, as in the auricle, there the power of contraction is 

 greater than in the part (the ventricle) where the cells are fewer ; 

 and, further, that when the apex of the heart i. e., the point of 

 the ventricle in which there are no cells is cut away, it no longer 

 beats. The second theory derives its support from the fact that a 

 piece of the apex of the ventricle of a tortoise, in which there are 

 no ganglion-cells, when suspended in a moist chamber, will beat 

 for hours. The apex of a dog's heart, in which there are no cells, 



