THE CIRCULATION OF THE BLOOD. 267 
8. In the cold-blooded animals the heart may be kept at a 
standstill by vagus stimulation till it dies, a period of hours 
(one case of six hours reported for the sea-turtle). 
9. Certain drugs (as atropine), applied directly to the heart, 
or injected into the blood, prevent the usual action of the 
vagus. : 
10. During vagus arrest the heart substance undergoes a 
change, resulting in an unusual dilatation of the organ. This 
may be witnessed whether the heart contains blood or not, 
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 vari- 
ous parts of the body, both of the surface and internal organs 
(reflex inhibition). 
13. One vagus being divided, stimulation 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. 
becomes 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 re- 
flexly; though sometimes it is very difficult to accomplish. 
But in the fish the ease with which the heart may be reflexly 
