374 



TEXT-BOOK OF PHYSIOLOGY. 



It is possible, therefore, that under physiologic conditions, physiologic 

 stimuli act on the peripheral terminations of either the one or the other; 

 according as they do will the center be augmented or inhibited in its activity, 

 and followed by either an increase or a decrease in the degree of the previous 

 vascular contraction. 



Again it may be assumed, from the results of experimentation on afferent 

 nerves, that the physiologic stimuli may act simultaneously on the periph- 

 eral terminations of both classes 

 of fibers and that the vaso-con- 

 stricter center is acted on by the 

 two antagonistic influences. In 

 this assumption the resultant 

 effect on the blood-vessels, viz., 

 increased or decreased contrac- 

 tion, will be the resultant of their 

 action on .the vaso-constrictor 

 center. If the stimuli act pre- 

 ponderantly on the depressor 

 fibers the center will be depres- 

 sed and the vessels will dilate; 

 if they act preponderantly on 

 the pressor fibers the center will 

 be stimulated and the vessels 

 will contract. 



Inasmuch as the vascular 

 dilatation is often greater than 

 the dilatation which follows divi- 

 sion of the vaso-constrictor fibers 

 themselves, it has been assumed 

 by some that the general vascular 

 tonus, as well as its variations 

 from time to time, is the result- 

 ant of the simultaneous activity 

 and variations in activity of both 

 vaso-constrictor and vaso-dilata- 

 tor centers; that in the afferent 

 nerves there are two sets of fibers, 

 one of which when stimulated 

 augments the activity of the vaso- 

 constrictor center and inhibits 



FIG. 176. DIAGRAM SHOWING THE ORIGIN 

 AND RELATION OF THE DEPRESSOR NERVE IN THE 

 RABBIT. Depr. n., depressor nerve; vag. n., vagus 

 nerve; sup. 1. n., superior laryngeal nerve ; inf. c. g., 

 inferior cervical ganglion; sym. n., sympathetic nerve; 

 car. a., carotid artery; dig. m., digastric muscle; 

 hyp. n., hypoglossal nerve; sup. c. g., superior cer- 

 vical ganglion; inf. 1. n., inferior laryngeal nerve. 



the activity of the vaso-dilatator 

 center; the other of which aug- 

 ments the activity of the vaso-dilatator center and inhibits the activity of 

 the vaso-constrictor center. The result, either contraction or dilatation, 

 which follows stimulation of their peripheral terminations will depend on 

 the character of the physiologic stimulus. 



In those particular instances in which stimulation of the peripheral 

 terminations of afferent nerves, associated with the nervi erigentes and 

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



