THE CIRCULATION OF THE BLOOD 163 



blood than usual; at other times the vasoconstrictors are 

 free to exert their tonic stimulation and so regulate the flow 

 of blood to the organs. 



28. The regulation of arterial tone by hormones ; adrenaline. 

 This has already been described on page 65. It will be 

 recalled that the presence of adrenaline in the circulating 

 blood directly excites the arterioles to constrict; that this 

 action on the arterioles is greater in some regions (for ex- 

 ample, the abdominal organs) than in others (for example, 

 the skeletal muscles and skin) ; that the rate and force of 

 the heart beat are influenced; that the adrenal glands are 

 excited to secrete by nervous impulses which are dispatched 

 from the central nervous system during states of emotional 

 excitement (fear and anger) and, we may now add, when- 

 ever the blood is deficient in oxygen. There are also reasons 

 for thinking that the internal secretion of the pituitary body 

 (p. 67) may likewise play some role in regulating arterial 

 tone and possibly in the distribution of the blood among the 

 organs. This is a new field of physiology and the present 

 state of our knowledge justifies only this brief reference 

 to it. Enough is known, however, to show that hormones 

 cooperate with the vasomotor nerves in regulating the flow 

 of blood to the organs. 



29. Importance of the vascular adjustments in daily life. 

 It is not possible within the limits of the present work to 

 enter further into the mode of action of these factors of vas- 

 cular coordination. Our main purpose is to show the student 

 that proper coordination is as important in adapting the work 

 of the heart and blood vessels to the hourly needs of daily 

 life as it is in producing purposeful movements of the skeletal 

 muscles. Every change of occupation and activity, every 

 change of surrounding conditions of temperature, moisture, 

 wind, etc., necessitates some special adjustment of the vas- 

 cular system; and this adjustment is dependent upon the 

 same sort of coordinating action which we have already 



