THE ORIGIN OF THE HEART BEAT 



333 



them to assume certain automatic properties of their own and to acti- 

 vate that portion of the musculature with which they are normally 

 connected. Essentially the same explanation is given for the mode of 

 contraction of the mammalian heart, although the location of its 

 nervous elements has not been fully ascertained as yet. 



It should be stated at this time that the neurogenic theory in its 

 extreme form is untenable, and while a number of experiments could 

 be cited, tending to emphasize the importance of the nervous elements 

 as the controlling factor of the heart's action, the evidence is not suffi- 

 ciently definite to prevent us from interpreting it in a way to favor the 

 myogenic theory. The same objection, however, may be raised against 

 several of the experiments which will be mentioned later on in support 

 of the latter theory, because they permit of a two-fold interpretation, 

 thus favoring one view as much as the other. The experimental 

 evidence so far presented may be arranged as follows : 



1. If the heart of a frog is removed in its entirety, it will continue to beat for a 

 long period of time, provided, of course, that it is placed in a proper nutritive medium. 

 If it is then cut across at the sino-auricular groove, its sinus continues to contract 



aa, 



aa 



os mnc In (a 



FIG. 174. HEART OF LIMULUS FHOM DORSAL SURFACE. (Carlson.) 

 mnc, Median nerve-cord; In, lateral nerve-trunks. 



at regular intervals, while its auricles and ventricle cease beating at least for some 

 time. The latter then resume their activity, the beat seemingly originating in the 

 auricle. Their frequency of contraction, however, rarely equals the normal. If the 

 ventricle is then separated from the auricles by a cut across the auriculoventricular 

 groove, the latter continue to beat, while the former soon ceases its activity. A 

 certain time having elapsed, the ventricle again contracts but now quite independ- 

 ently of the rhythm of the other segments of this organ. 



2. Very similar results may be obtained by applying two ligatures to the heart 

 in such a way that one comes to lie in the sino-auricular groove and the other, in 

 the auriculoventricular groove. (Stannius experiment, 1852.) After the applica- 

 tion of the first, the auricles and ventricle cease beating, while the sinus continues 

 to contract. All three divisions, however, beat at regular intervals as soon as 

 the second ligature has been properly placed and tightened. As Heidenhain has 

 stated, the first ligature seems to exert a mechanical stimulus upon the inhibitor 

 ganglion, while the second serves as a stimulant for the accelerator elements. It is 

 to be noted, however, that the different segments of the heart now beat inde- 

 pendently of one another, and that the regular progression of the wave of con- 

 traction from the sinus to the apex is no longer in evidence. These experiments 

 tend to show that the different portions of the heart are imbibed with a certain 

 automatic power of their own which diminishes gradually in the direction from 

 sinus to apex. This dormant power enables the more distant ganglia to originate 

 impulses at any time after the more central elements have been destroyed or have 

 been separated from them. Since the property of automaticity seems to be 

 associated exclusively with nerve cells, the muscle cells find themselves in the 

 position of mere executors of the will of a higher controlling factor. 



