THE CARDIAC NERVES. 577 



heart may be maintained in this condition varies in different speciea 

 and indeed to some extent in different individuals.* In some ani- 

 mals — cats, for example — the strongest stimulation of the nerve 

 serves frequently only to slow the heart instead of causing complete 

 standstill. In dogs the heart is stopped by relatively weak stimu- 

 lation, although if the stimulation is maintained the heart, as a 

 rule, escapes from the inhibition. In some dogs the heart may 

 be held inhibited long enough to cause the death of the animal 

 unless artificial respiration is maintained, but usually the heart 

 beat soon breaks through the complete inhibition. The "inner 

 stimulus" in such cases increases in strength sufficiently to overcome 

 the opposing inhibitory influence, and this circumstance may be 

 regarded as an argument against those views that trace the origin of 

 the "inner stimulus" to some of the products formed during the ca- 

 tabolism of contraction. Moderate stimulation of the vagus, suffi- 

 cient simply to slow the rate of beat, can be maintained without dimi- 

 nution in effect for very long periods; indeed, as is explained in the 

 next paragraph, the heart beat is kept partially inhibited more or 

 less continuously through life by a constant activity of the 

 vagus. In the cold-blooded animals, especially the terrapin, 

 the heart may be kept completely inhibited for hours by stimu- 

 lation of the vagus. Mills reports that he has kept the heart 

 of the terrapin in this condition for more than four hours, f 

 Most observers state that complete inhibition can be maintained 

 for a longer time when the stimulus is applied alternately to 

 the two vagi, but it is possible that this result is due to the fact 

 that continuous stimulation applied to a nerve usually results 

 in some local loss of irritability. 



Reflex Inhibition of the Heart Beat — Cardio-inhibitory 

 Center. — The inhibitory fibers may he stimulated refiexiy by action 

 upon various sensory nerves or surfaces. One of the first experi- 

 mental proofs of this fact was furnished by Goltz's often-quoted 

 "Klopfversuch."! In this experiment, made upon frogs, the ob- 

 server obtained standstill of the heart b}- light, rapid taps on the 

 abdomen, and the effect upon the heart failed to appear when the 

 vagi were cut. In the mammals every laboratory worker has had 

 numerous opportunities to observe that stimulation of the central 

 stumps of sensory nerves may cause a reflex slowing of the heart 

 beat. The effect is usually very marked when the central stump 

 of one vagus is stimulated, the other vagus being intact. The 

 vagus carries afferent fibers from the thoracic and abdominal 

 viscera, and most observers state that the heart may be reflexly 

 inhibited most readily by simulation of the surfaces of the ab- 



* See Hough, "Journal of Physiology," 18, 161, 1895. 



t "Journal of Physiology," 6, 246. 



JGoltz, "Virchow's Archiv f. pathol. Anatomie, etc.," 26, 11, 1863. 



