THE CAUSE OF THE RHYTHMICAL ACTIVITY OF THE HEART 185 



10. THE CAUSE OF THE RHYTHMICAL ACTIVITY OF THE 



HEART 



In warm-blooded animals direct nerve fibers to the auricles as well as to 

 the right and left coronary plexuses, come from the two divisions of the 

 cardiac plexus, which in turn is formed by branches from the vagus and 

 sympathetic. The threads of this network are provided with numerous gan- 

 glia, and the fibers radiating to the auricle and ventricle from the network 

 are also interspersed with small ganglia. 



In the heart itself ganglion cells have been found in the following places : 

 in the auricles around the opening of the great veins, along the periphery of 

 the septum, and, though in smaller number, in the outer wall; in the atrio- 

 ventricular groove, especially in the region of the aorta and pulmonary artery 

 at the level of the semilunar valves ; and in the uppermost part of the ventricle. 



Fine nervous nets supply an abunda'nce of nerve fibers to all parts of the 

 heart. 



As appears from what has been said concerning exsected hearts, this organ 

 possesses the property of acting quite independently of the central nervous 

 system. In order to determine the cause of this peculiarity we have to inves- 

 tigate first the behavior of the separate divisions when they are isolated from 

 the whole heart. In this we shall consider chiefly the phenomena appearing 

 in the Mammalian heart, because a detailed discussion of those observed in 

 the hearts of cold-blooded animals would call for entirely too much space. 



By introdu^ng into the auricle a small instrument provided with curved 

 plates on tw r v|idt's, it is possible to sever all the nervous and muscu- 

 lar connection^ between the auricles and ventricles without producing 

 hemorrhage. After this operation the ventricles continue to pulsate without 

 interruption. 



In this experiment the line of separation can be brought close to the auricu- 

 lar boundary. Since now all the nerves which run to the heart along the 

 great arteries are afferent in function (Wooldridge), and since the results are 

 the same in case the great arteries are pinched off directly above the upper 

 edge of the semilunar valves, it follows that the isolated portion of the heart, 

 i. e., the ventricles and a very small part of the auricles, have within them- 

 selves all the conditions necessary for rhythmical activity. 



One can go still further. Porter has succeeded by means of artificial cir- 

 culation through the coronary arteries in maintaining regular rhythmical 

 contractions in isolated pieces of the ventricular wall, connected with the 

 rest of the heart only by the arterial branch. We can extend the proposition 

 stated above therefore, and say that every portion of the ventricular wall 

 possesses all the conditions necessary for rhythmical activity. 



In these experiments one meets with cases where the rhythm of the sepa- 

 rated portion is materially less than that of the whole heart or that of the 

 divisions remaining after isolation of the ventricles. But under normal cir- 

 cumstances the rate of the ventricular systole is determined by the rhythm 

 of those parts of the heart which inaugurate the systole (the venous ostia 

 of the auricles; cf. page 162). 



