THE CIRCULATION OF THE BLOOD 



385 



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. Thevaso-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 379) are ordinarily in a 

 condition of relative inactivity 

 and hence the blood-vessels as- 

 sume a caliber just sufficient to 

 supply the materials ^ necessary 

 to maintain the nutritive activi- 

 ties of the organs concerned. ^,, , 



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 

 the P salivary glands and of the organs of generation, at once actively dilate 

 and the amount of blood they transmit so great that on emerging from the 

 capillaries it still retains its arterial hue. The same phenomena arise , 

 the efferent pre-ganglionic fibers, e.g., the chorda tympam and the nerve c 

 Jacobson, and the nervus erigens or pelvic nerve are stimulated 

 25 



FIG 180. DIAGRAM SHOWING THE ORIGIN 

 AND RELATION OF THE DEPRESSOR NERVE IN THE 

 R \BBIT. 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 mus< 

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

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



