THE CIRCULATION OF THE BLOOD. 257' 



orgaii,so to speak, being supplied by the right nerve ; for, when 

 even a small part of the heart is arrested, it may be overcome by 

 the action of a larger portion of the same, or a more dominant 

 region (the sinus mostly). 



Conclusions. — The inferences from the facts stated in the 

 above paragraphs are these : 1. There is in the medulla a col- 

 lection of cells (center) which can generate impulses that reach 

 the heart by the vagi nerves and influence its muscular tissue, 

 though whether directly or through the intermediation of 

 nerve-cells in its substance is uncertain. It may possibly be in 

 both ways. 3. This center (cardio-inhibitory) may be influ- 

 enced reflexly by influences ascending by a variety of nerves 

 from the periphery, including paths in the brain itself, as 

 shown by the influence of emotions or the behavior of the 

 heart. 3. The cardio-inhibitory center is the agent, in part, 

 through which the rhythm of the heart is adapted to the needs 

 of the body. 4. The arrest, on direct stimulation of the heart, 

 is owing to the effect produced on the terminal fibers of the 

 vagi, as shown by the dilatation, etc., corresponding to what 

 takes place when the trunk of the nerve or the center is stimu- 

 lated. 5. The quickening of the heart, following section of the 

 vagi, seems to show that in some animals the inhibitory center 

 exercises a constant regulative influence over the rhythm of 

 the heart. 6. The irritability and dilatability of the cardiac 

 tissue may be greatly modified during vagus inhibition. Some- 

 times this is evident before the rhythm itself is appreciably 

 altered. 7. The heart-muscle has a latent period, like other 

 kinds of muscle ; and cardiac effects, when initiated, last a vari- 

 able period. 



There are many other obvious conclusions, which the stu- 

 dent will draw for himself. 



But a question arises in regard to the significance of the 

 cardiac arrest under these circumstances, and the altered action 

 that follows. The fact that, when the heart is severed from the 

 central nervous system by section of its nerves, profound 

 changes in the minute structure of its cells ensue, points un- 

 mistakably to some nutritive influence that must have operated 

 through the vagi nerves. That stimulation of the vagus re- 

 stores regularity of rhythm and strengthens the beat of the 

 failing heart, is also very suggestive. That many disorders of 

 the heart are coincident with periods of mental anguish or 

 worry, and that in certain cases of severe mental application 

 \1 



