THE CIRCULATION OF THE BLOOD. 375 



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

 or augment the activity of a special vaso-dilatator center and for this reason 

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



The Influence of Emotional States. The vaso-constrictor centers 

 are capable of being influenced in their activities by emotional states, 

 doubtless as a result of the arrival of nerve impulses from the cortex of the 

 cerebrum. Thus it is well known that fear causes a contraction of the blood- 

 vessels of the head and face and that shame causes a dilatation of the same 

 vessels. With the cessation or the disappearance of the emotional state, 

 the blood-vessels return to their former degree of contraction. The vaso- 

 dilatator centers in the medulla and in the sacral region of the spinal cord are 

 influenced in a similar manner by emotional states. 



The Depressor Nerve. A striking illustration of the depressor or in- 

 hibitor action of afferent nerves upon the vaso-constrictor center is furnished 

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

 nerve." In the rabbit, Fig. 176, 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. 177. FALL OF BLOOD-PRESSURE FROM EXCITATION OF THE DEPRESSOR NERVE. The 

 cylnder 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. 177). The fall in 

 pressure, however, is not due to this cause, for it 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. 



