THE CIRCULATION OF THE BLOOD. 265 



11. The heart may be arrested by direct stimulation, espe- 

 cially of the sinus, and at the points at which the electrodes are 

 applied there is apparently a temporary paralysis. The same 

 alteration in the beat may be noticed as when the main trunk 

 of the vagus is stimulated. 



12. The heart may be inhibited through stimulation of Tari- 

 ous parts of the body, both of the surface and internal organs 

 (reflex inhibition). 



13. One vagus being divided, stimidation of its upper end 

 may cause arrest of the heart. 



14. Stimulation of a small part of the medulla oblongata 

 will produce the same result, provided one or both vagi be 

 intact. 



15. Section of both vagi in some animals (the dog notably) 

 increases the rate of the cardiac beat. The result of section of 

 one pneumogastric nerve is variable. The heart's rhythm is 

 usually to some extent quickened. 



16. During vagus inhibition from any cause in mammals 

 and many other animals, the heart responds to a single stimu- 

 lus, as the prick of a needle, by at least one beat. An observer 

 studying for himself the behavior of the heart in several groups 

 of animals with an open mind, for the purpose of observing all 

 he can rather than proving or disproving some one point, be- 

 comes strongly impressed with the variety in unity that runs 

 through cardiac physiology, including the influence of nerve- 

 cells (centers) through nerves ; for it will not be forgotten that 

 normally nerves originate nothing, being conductors only, so 

 that when the vagus is stimulated by us we are at the most but 

 imitating in a rough way the work of central nerve-cells. We 

 can only mention a few points to illustrate this. 



In the frog a succession of light taps, or a single sharp one 

 (" Klopfversuch " of Goltz), will usually arrest the heart reflexly ; 

 though sometimes it is very difficult to accomplish. But in the 

 fish the ease with which the heart may be reflexly inhibited by 

 gentle stimulation of almost any portion of the animal is won- 

 derful. Again, in some animals the vagus arrests the heart for 

 only a brief period, when it breaks away into its usual (but in- 

 creased) action. 



In the fish, menobranchus, and probably other animals, the 

 irritability of some subdivision of the heart is lost during the 

 vagus inhibition — i. e., it does not respond to a mechanical 

 stimulus. 



