190 CIRCULATION OF THE BLOOD 



effects depend upon their condition at the time of stimulation. A definite 

 decision of the matter is not possible at this time. 



The view has often been expressed that the vagus influence on the heart 

 is of a nutritive or trophic nature. The following facts might be construed 

 in favor of such a view : the strength and working power of the heart, as well 

 as the ability of the heart muscle to propagate a stimulus, increases after 

 vagus stimulation; the heart's activity, if it is weak, is materially raised by 

 vagus stimulation; and in the asphyxiated animal the heart beats longer if 

 the vagi are left intact, than if they be cut, etc. But these phenomena might 

 be explained also by the longer resting period after each systole. 



Conclusive proof of the correctness of this view would be afforded, if 

 degenerative changes could be demonstrated on a heart whose vagi had been 

 cut. Such have in fact often been mentioned, and it has even been asserted 

 that they are confined to different parts of the ventricles, according as ^he 

 right or left vagus is cut. But we have the researches of Pawlow and FriecP- 

 enthal to the contrary. They find that the heart of dogs which had survived 

 bilateral vagotomy for several months presented no anatomical changes what- 

 soever. The long time during which the animals remained alive in these 

 researches, as well as in those of Nikolai'des and Ocafia, itself goes to show 

 at least that the vagus cannot be exclusively a trophic nerve for the heart. 



That the inhibitory process is, nevertheless, accompanied by demonstrable 

 molecular changes, and that the stoppage is not therefore a kind of paralysis, 

 appears from the electrical variations in the heart muscle which accompany 

 vagus stimulation. In the turtle's heart it is possible to separate the auricles 

 from the venous sinus without injuring the nervous fibers of the former. The 

 auricles stop for a time. If now the apex be killed by immersion in hot 

 water, and both base and apex be then led off to a galvanometer, the usual 

 demarcation current is observed with the injured spot, i. e., the apex, negative 

 toward the base. If the vagus is stimulated the auricles remain at rest; but 

 the galvanometer shows a positive variation (Gaskell). This variation of the 

 animal current is evidently opposite in sign to that which takes place in 

 the work of the heart muscle (cf. page 179). Fano obtained quite similar 

 results when he stimulated the vagus of an active turtle heart so feebly that 

 it was not stopped but only retarded. The positive phase of the variation 

 was increased, but the negative was diminished or abolished altogether. 



Since now the negative variation is quite certainly the expression of a 

 dissimilatory process, one would be forced by the appearance of a positive 

 variation on stimulation of the vagus to the conclusion that this nerve calls 

 out processes of a synthetic nature. If, however, this is true, it follows from 

 the above observations on vagotomy that these synthetic processes are not of 

 critical importance for the maintenance of the normal structure of the heart. 



In discussing the intracardial innervation of the heart (page 186), the 

 significance of the ganglia was left an open question. It will be appropriate 

 to revert to the subject here, because some observations on the vagus should 

 be able to give the desired answer. Langley has shown that nicotine puts an 

 end to the transmission of an impulse through the sympathetic ganglion cells 

 with which the nerve fibers (preganglionic) coming from the central nervous 



