THE CARDIAC NERVES. 517 



Escape from Inhibition. Strong stimulation of the vagus 

 may stop the entire heart, but the length of time during which 

 the heart may be maintained in this condition varies in different 

 species and indeed to some extent in different individuals.* In 

 some animals cats, for example the strongest stimulation of the 

 nerve serves 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 section, 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 stimulation of the vagus. 

 Mills reports that he has kept the heart of the terrapin in this 

 condition for more than four hours.| 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 be stimulated reflexly 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."t In this experiment, made upon frogs, the ob- 

 server obtained standstill of the heart by 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 sensory fibers from the thoracic and abdominal 



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



t "Journal of Physiology," 6, 246. 



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



