3 86 TEXT-BOOK OF PHYSIOLOGY 



duced electric currents. The organs of generation in consequence of the 

 increased blood flow exhibit changes in volume and rigidity similar to those 

 exhibited previous to the act of coition. 



In the particular instances in which stimulation of the peripheral termina- 

 tions of afferent nerves, associated with the nervus erigens and chorda 

 tympani, is followed by active dilatation of the blood-vessels, it has been 

 assumed that there are afferent nerve-fibers which directly stimulate or 

 augment the activity of the special vaso-dilatator centers and for this reason 

 should be termed "reflex vaso-dilatator nerves" (Hunt). 



The Depressor Nerve. A striking illustration of the depressor or inhibitor 

 action of afferent nerves upon the vaso-constrictor (reflex) center is furnished 

 by the result of stimulation of a branch of the vagus, the so-called "depressor 

 nerve." In the rabbit, Fig. 180, there is a small nerve formed by the union 

 of a branch from the trunk of the vagus with a branch from the superior 



FIG. 181. FALL OF BLOOD-PRESSURE FROM EXCITATION OF THE DEPRESSOR NERVE. The 

 cylinder was stopped in the middle of the curve and the excitation maintained for seventeen min- 

 utes. The line of zero pressure (0,0) should be 30 mm. lower than here shown. (Bayliss.) 



laryngeal. The peripheral distribution of this nerve is over the wall of the 

 ventricle and perhaps to some extent to the structures of the aorta near its 

 origin. A similar anatomic arrangement is met with in the horse, pig, and 

 hedge-hog. In some other animals, as the dog, it is bound up in the vago- 

 sympathetic. In man it is also present, though shortly after its origin it 

 enters the trunk of the vagus. Division of this nerve is without effect 

 on either the heart or the vessels. Stimulation of the peripheral end has 

 neither an accelerator nor an inhibitor action on the heart. Stimulation of 

 the central end is followed by a fall in blood-pressure, frequently to a level 

 below one-half the normal value; at the same time there is a diminution, 

 brought about reflexly, in the rate of the heart-beat (Fig. 181). The fall in 

 pressure, however, is due mainly to the dilatation of the arterioles, the result 

 of an inhibition of the general vaso-reflex center, and partly to the diminu- 

 tion in the rate and force of the heart, the result of a simultaneous stimula- 

 tion of the inhibitor center. This latter factor is of less importance than the 

 former for the fall of pressure occurs equally well after division of all the 

 cardiac nerves. For this reason the nerve was termed the depressor nerve 

 of the vaso-motor center. 



On exposure of the abdominal cavity, it is observed during stimulation of 

 the depressor that there is a notable dilatation of the intestinal vessels. 



