266 



PHYSIOLOGY. 



excites the centers of the vagus and slows the heart. Increase of intra- 

 cranial pressure lessens 1 the rate of the heart. 



Death from plunging into cold water is a case of reflex inhibition 

 from the nerves of the skin. Irritation of the gastric mucous mem- 

 brane is also an example of a sensory irritation depressing the cardio- 

 inhibitory power of the vagus, the heart beating faster. 



Afferent Nerve of the Heart. ( Depressor Nerve of Ludwig and von 

 Cyon.) This nerve in the rabbit usually arises from two branches, one 

 from the trunk of the vagus and the other from the superior laryngeal. 



Sup-, lar. a 



Depressor 



S.C.G. 



Vagus- 



^ lar. n.- 



..Vagus 



SupT.Cerv.Gang 

 -Depressor 

 --Cerv. symp.n. 



Vago.symp 



RABBIT 



DOC 



Fig. 83. Diagram of the connections of the Depressor Nerve in the 

 Rabbit and Dog, according to Cyon. It will be noticed that in the lat- 

 ter animal the depressor nerve runs in the vagus trunk for the greater 

 part of the course. (STARLING.) 



It ends in the heart, and, according to some, in the aorta's origin. It 

 is found in man and other animals. When its central end is stimu- 

 lated, there is a fall of arterial tension to about half its former level. 

 After the stimulation is arrested, the tension returns to normal. With 

 this fall of arterial tension the beats of the heart are slowed; but if 

 the vagi are divided, there is no change in the frequency of the heart, 

 which shows that the lessening of the number of heart-beats is due to 

 stimulation of the cardio-inhibitory center. Even after curarization, 

 irritation of the central end of the depressor lowers the arterial tension. 

 If the splanchnics are previously divided, stimulation of the depressor 

 still has some effect, 



