THE CIRCULATION OF THE BLOOD 



389 



the vaso-constrictor center will still be excited; if they act preponder- 

 antly on the depressor fibers the center will be inhibited, though in either 

 case not to the same degree as in the first instance. 



3. The peripheral stimuli may act on the terminals of the pressor fibers alone. 

 The nerve impulses thus developed are transmitted to both centers but 

 their effect will be to excite the activity of the vaso-constrictor centers 

 and to inhibit the activity of the vaso-inhibitor centers; or the peripheral 

 stimuli may act on the terminals of the depressor fibers alone. The 

 nerve impulses thus developed are likewise transmitted to both centers 

 but their effect will be to inhibit the activity of the vaso-constrictor centers 

 and to excite the activity of the vaso-inhibitor centers. In this view it is 

 assumed that both centers are 

 continuously active and in a 

 state of tonus, and that for 

 the necessary vascular reac- 

 tion one center must be 

 stimulated, and the other be 

 inhibited. As to which cen- 

 ter will be stimulated or in- 

 hibited, will probably depend 

 on the character of the stim- 

 ulus. Hence, the general 

 vascular tonus as well as its 

 variations from time to time, 

 in whole or in part, is the 

 resultant of the simultaneous 

 activity and variations in 

 activity of these two mutually 

 antagonistic but cooperative 

 centers. 

 Local Special Vaso-dilata- 



tor Centers. The vaso-dilatator 



centers in the medulla oblongata 



and in the sacral region of the 



spinal cord, which give origin to 



nerve-fibers that regulate the 



blood-supply to the salivary 



glands on the one hand and to 



the generative organs on the 



other hand as previously stated 



(page 383) are ordinarily in a 



condition of relative inactivity 



and hence the blood-vessels as- 



car.a. 

 inf.c.y. 



FIG. 178. DIAGRAM SHOWING THE ORIGINAND 

 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., caro tid artery; dig. m., digastric mus- 

 cle; hyp. n., hypoglossal nerve; sup. c. g., superior 

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



sume a caliber just sufficient to 

 supply the materials necessary 

 to maintain the nutritive activi- 

 ties of the organs concerned. If, however, these centers are aroused [to 

 activity, either by nerve impulses descending from the cerebrum in conse- 

 quence of psychic states of an affective or emotional character, or by nerve 

 impulses transmitted to them through afferent nerves, the blood-vessels of 



