Efferent vasomotor 



THE VASOMOTOR NERVES. 611 



General Schema. — The main facts regarding the vasomotor 

 apparatus, so far as the arteries are concerned, may be summarized 

 briefly in tabular form as follows : 



I. Vasoconstrictor fibers — distributed mainly to 

 the skin and the abdominal viscera (splanch- 

 nic area) , all connected with a general center 

 in the medulla oblongata, and in constant 

 f.. . tonic activity. 



nerve fibers. j ^ Vasodilator fibers— distributed especially^ to 



the erectile tissue, glands, bucco-facial region, 

 and muscles; not connected with a general 

 center and not in tonic activity. 



I. Pressor fibers. Cause vascular constriction and 

 rise of arterial pressure from reflex stimula- 

 tion of the vasoconstrictor center — e. g., 

 sensory nerves of skin. 

 II. Depressor fibers. Cause vascular dilatation and 

 ^ „, . . , fall of arterial pressure from reflex inhibition 



Afferent fibers givmg / ^^ ^.j^^ ^^^^j^ activity of the vasoconstrictor 



vasomotor reflexes. ^ center,— e. g., depressor nerve of heart. 



III. Depressor (or reflex vasodilator) fibers. Cause 

 vascular dilatation and fall of arterial pres- 

 sure from stimulation of the vasodilator 

 center, — e.g., erectile tissue, congestion of 

 glands in functional activity. 



It may be supposed that under normal conditions the activity 

 of this mechanism is adjusted so as to control the blood-flow through 

 the different organs in proportion to their needs. When the blood- 

 vessels of a given organ are constricted the flow tlirough that organ 

 is diminished, while that through the rest of the body is increased 

 to a greater or less extent corresponding to the size of the area in- 

 volved in the constriction. When the blood-vessels of a given 

 organ are dilated the blood-flow through that organ is increased and 

 that through the rest of the body diminished more or less. The 

 adaptability of the vascular system is wonderfully complete, and 

 is worked out mainly through the reflex activity of the nervous 

 system exerted partly through the vasomotor fibers and partly 

 through the regulatory nerves of the heart. 



Regulation of the Blood-supply by Chemical and Mechan- 

 ical Stimuli. — From time to time attention has been called to 

 the fact that the calibre of the blood-vessels may be influenced 

 otherwise than through the agency of vasoconstrictor and vaso- 

 dilator nerve fibers. Gaskell, for example, has shown that acids 

 in slight concentration cause a vascular dilatation. Bayliss * has 

 generalized the facts of this kind, and has suggested that in addi- 

 tion to the nervous regulation described in the preceding pages 

 there may be formed chemical substances of a definite character 

 which exert a similar useful regulating action. As examples of 



* Bayliss in "Ergebnisse der Physiologic," 1906, v., 319. 



