166 THE BLOOD AND ITS CIRCULATION. 



gastric is a paralyzing nerve of the heart (Weber and Budge). 

 We shall observe similar facts in the innervation of the 

 vessels. 



Moderating JVerves of the Heart. Budge, Weber, and 

 Cl. Bernard (1848) discovered, almost at the same time, 

 that excitation of the entire pneumo-gastric nerve, or of its 

 peripheral extremity only, has the effect of retarding the 

 motion of the heart ; thus in the dog, an animal whose heart 

 beats irregularly and very rapidly, such excitation serves to 

 regulate the cardiac pulsation. Different explanations have 

 been given of this phenomenon ; some have considered that 

 retardation of the motion of the heart is caused by exhaustion 

 succeeding too violent excitation of the pneumo-gastric nerve : 

 a nerve leading to a muscle could be looked upon only as an 

 exciting agent of this muscle, and the exhaustion of the nerve 

 seemed to explain the retardation which follows excitation. 

 This explanation, however, does not apply to the retardation 

 which follows excitation of the peripheral extremity of a 

 nerve which has been previously cut ; and it fails especially 

 when we consider that, by simply cutting the pneumo-gastric 

 nerve, the rapidity of the pulsations of the heart is greatly 

 increased. Since observation of similar phenomena in other 

 parts of the nervous system, has lately made the idea familiar 

 to us of nerves possessing paralyzing properties, it is gener- 

 ally admitted that the pneumo-gastric nerve is a moderating 

 nerve of the heart : section of this nerve suppresses the mod- 

 erating influence, and, consequently renders the pulsation 

 more rapid ; excitation increases the moderating influence, 

 and thus retards the pulsation. The theories by which it 

 has been sought to explain the foregoing experiment, while 

 denying the moderating, paralyzing function of the pneumo- 

 gastric nerve, are in many cases extremely complicated, and 

 we will only mention here the facts most recently furnished 

 by experiment. Legros and Onimus, who examined the 

 effects produced by excitation of the pneumo-gastric nerve 

 by intermittent currents of electricity, have shown that, 

 under these conditions, the pulsations become fuller and less 

 frequent in exact proportion to the number of intermissions; 

 the number of intermissions required to produce stoppage of 

 the heart is smaller when the animal is weakened or chilled, 

 or in a state of hibernation. Arloing and Tripier have re- 

 marked that excitation of 'the right pneumo-gastric nerve 

 has more effect on the action of the heart than that of the 

 left. (It should be added that study of the comparative 



