THE CIRCULATION OF THE BLOOD. 269 
also a fact that in most of the cold-blooded animals the right 
vagus is more efficient than the left, owing, we think, not to the 
nerves themselves so much as to their manner of distribution 
in the heart—the greater portion of the driving part of the 
organ, 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 domi- 
nant 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. 2. This center (cardio-inhibitory) may be influ- 
enced refiexly 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 dilation, 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 
variable 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 vaginerves. That stimulation of the vagus re- 
stores regularity of rhythm and strengthens the beat of the 
