CARDIAC INHIBITION AND ACCELERATION 329 



inhibition. Very decided changes in the frequency of the heart may 

 also be produced with the aid of the cutaneous end-organs, their 

 activation being effected either by cold or warm water, mechanical 

 impacts, massage, effervescent water, and other stimuli. The fact that 

 some of these afferent impulses most easily elicit inhibitor and others 

 accelerator phenomena, has been explained by assuniing that they may 

 be more intimately connected either with the cardio-inhibitor or 

 with the cardio-accelerator mechanism. In the case of the carbonated 

 water bath, however, the mechanical stimulus, consisting in the bump- 

 ing of the globules of the gas against the integument, may be aug- 

 mented by a direct effect of the carbon dipxid upon the cardiac center. 

 It seems entirely probable that some of it may be absorbed and then 

 act as a stimulant not only to the respiratory but also to the cardio- 

 vascular system. 



As has been emphasized above, the cardiac center is also the re- 

 cipient of sensory impulses which arise either in the membranous 



Fig. 173, — Record of the Caeotid Blood-pressure in Rabbit During Stimulation 

 OF THE Depressor Nerve. 



structures enveloping the heart, or in this organ itself. The fibers 

 conducting the impulses from the heart are attributes of the vagus 

 system, and have been designated by Ludwig and Cyon,^ their dis- 

 coverers, as the depressor nerve. These fibers become clearly recogniz- 

 able upon the arch of the aorta, whence they reach the vagus center 

 either by pursuing an independent course along the carotid artery 

 (rabbit), or by intermingling with the vagosympathetic fibers (dog). 

 In the rabbit, this nerve is isolated most easily in the neck, where it 

 forms an anatomical entity next to the inner border of the cervical 

 sympathetic and the trunk of the vagus. Centrally to this point 

 it divides into two slender bundles, one of which enters the cervical 

 portion of the va^us directly, and the other, the superior laryngeal 

 branch of this nerve. The fibers of both branches then intermingle 

 with the other vagal fibers. 



1 Berichte der sachs. Akad. der Wissensch., 1866. 



