THE VASOMOTOR NERVES. 569 



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

 vessels of a given organ are constricted the flow through 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 

 recently generalized the facts of this kind, and has suggested that 

 in addition to the nervous regulation described in the preceding 

 pages there may be formed chemical substances of a definite char- 

 acter which exert a similar useful regulating action. As examples 

 of this influence, we have the lactic acid produced in muscles during 

 activity and probably also the carbon dioxid produced in this as 

 in other tissues. These substances may act to produce a local 

 dilatation during functional activity and thus provide the organ 

 with more blood at the time that it is needed. On the other hand, 

 the internal secretion of the adrenal glands (adrenalin) and possi- 

 bly also of the infundibular portion of the pituitary gland have 

 the reverse effect, causing a vasoconstriction and thus tending to 

 maintain normal vascular tone. In a similar way it is probable that 

 the distension of the arteries by internal pressure acts as a mechani- 

 cal stimulus which leads to increased tone and thus aids in main- 

 taining a normal arterial tension. 



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



