THE CIRCULATION OF THE BLOOD. 973 
the vagus itself; and, finally, through fibers with similar action 
in the sympathetic system, usually so called. 
The share taken by these factors is certainly variable in dif- 
ferent species of animals, and it is likely that this is true of the 
same animals on different occasions. It is also conceivable, 
and indeed probable, that they act together at times, the inhibi- 
tory action being diminished and the augmentor influence in- 
creased. 
Human Physiology.—Of the three cardiac nerves—superior, 
middle, and inferior—the strongest, which is the middle one, 
passes from the inferior cervical ganglion to the middle, from 
which it proceeds to the heart, and the inferior, may be re- 
garded as the chief augmentor cardiac nerves. 
That man’s pneumogastric contains inhibitory fibers is evi- 
dent from the experiment of Czermak, who, by pressing a bony 
tumor in his neck against his vagus nerve, could arrest his 
heart. Another individual could arrest his heart-beat at will, 
and if not through the vagus, how ? 
We are probably all aware of alterations in the rhythm of 
the heart from emotions. During a period of intense, brief, 
sympathetic anxiety, as in watching two competitors during a 
severe struggle for supremacy, a change in the rhythm of the 
heart, amounting, it may be, to momentary arrest, may be 
observed. 
Enough has been said, we trust, to show that the nerves of 
the heart can no longer be regarded merely as the reins for 
bridling the cardiac steed; but that all the phenomena of accel- 
eration, slowing, or other changes of rhythm, are only the out- 
ward evidences of profound vital changes accompanied by cor- 
responding chemical and electrical effects. If these views be 
correct, nervous influence must play no small part in the causa- 
tion and modification of disordered conditions; and we would 
extend such a view to all the organs of the body, and especially 
in the case of man. The heart’s rhythm can, however, be 
modified in other ways than we have as yet described. 
Though an isolated heart, fed by serum or some artificial 
nutritive fluid, may beat well for a time, it is liable to peri- 
odic interruptions, which are probably owing to its imperfect 
nutrition. 
Many drugs greatly modify the heart-beat; but, in attempt- 
ing to explain how the result is accomplished, the difficulty is 
in unraveling the part each anatomical element plays in the 
total result. Does the drug act on the muscular tissue, the 
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