THE CIRCULATION OF THE BLOOD. 333 



maintained by the cooperative antagonism of the vaso-motor centers 

 can be increased or decreased by nerve impulses reflected to the cord 

 and medulla from the periphery or from the brain. The effect may 

 be general, or local and confined to the area from which the im- 

 pulses arise. The following experiments may be cited as illustrations : 



Stimulation of the central end of a divided posterior root' of a 

 spinal nerve gives rise to increased vascular contraction, as shown 

 by the rise of blood-pressure. The same holds true for any sensory 

 nerve. Stimulation of the central end of the divided sciatic will give 

 rise to opposite results, according to the strength of the stimulus, 

 weak stimuli producing dilatation, strong stimuli producing contrac- 

 tion of the vessels. Stimulation of the central end of the divided 

 vagus gives rise to dilatation of the vessels of the lips, cheeks, and nasal 

 and palatal mucous membranes. Stimulation of the auricular nerve 

 in the rabbit will, according to the strength of the currents, give rise 

 to either contraction or dilatation. Stimulation of the tongue is 

 followed by dilatation of the vessels of the submaxillary gland. 



In explanation of these different results it has been assumed that 

 in the afferent nerves there are two classes of fibers, one which in- 

 creases, the other decreases the activity of the vaso-constrictor centers. 

 The first is termed pressor, the second depressor. Inasmuch as the 

 vascular dilatation is often greater than the dilatation which follows 

 division of the vaso-motor fibers themselves, it has been assumed 

 by some that the general vascular tonus, as well as its variations from 

 time to time, is the resultant of the simultaneous activity and varia- 

 tions in activity of both vaso-constrictor and vaso-dilatator centers ; 

 that in the afferent nerves there are fibers which when stimulated 

 augment, for example, the vaso-constrictor center and inhibit the 

 vaso-dilatator centers, or the reverse. The result, either contraction 

 or dilatation, which follows stimulation of their peripheral termina- 

 tions will depend on the character of the physiologic stimulus. 



In a similar manner the vaso-motor centers are influenced by 

 emotional states, fear causing a contraction, shame a dilatation, of 

 the vessels of the face and neck. It is probable that these effects are 

 due to the transmission of nerve impulses from the cortex to the vaso- 

 motor centers in the medulla. 



The Depressor Nerve. In the rabbit there is a small nerve 

 formed by the union of a branch from the trunk of the vagus with a 

 branch from the superior laryngeal. The peripheral distribution of 

 this nerve is over the wall of the ventricle. The same nerve is found 

 in many other animals. In some, 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 either on the heart or the vessels. Stimulation of the 

 peripheral end has neither an accelerating nor an inhibiting action 



