BLOOD VESSELS 453 



conclusions as to the condition of the arterioles may be 

 drawn. The temperature of the surface of the body is also 

 modified by the activity of the heart ; if the heart begins 

 to fail the temperature tends to fall. 



Uh. By streaming blood through the vessels, gener- 

 ally of a frog, and observing the rate at which it escapes, 

 the changes in the state of the small vessels may be made 

 out. This perfusion method is much used in studying the 

 action of drugs (Practical Physiology). 



oth. Since the state of the arterioles influences the arterial 

 pressure (p. 450), if the heart's action is kept uniform, 

 changes in the arterial blood pressure indicate changes in the 

 arterioles — a fall of pressure indicating dilatation, a rise of 

 pressure, constriction. 



(2) Normal Condition of the Arterioles. — Normally the 

 arterioles are in a state of semi-contraction ; but if the 

 arterioles in some transparent tissue be examined, they will 

 be found to undergo periodic slow changes in calibre. The 

 ear of a white rabbit shows such slow changes ; it seems at one 

 time pale and bloodless, at another time red and engorged. 

 During this latter phase numerous vessels appear which in 

 the former condition were invisible. These slow changes are 

 independent of the heart's action and of the rate of respira- 

 tion. They appear to be due to the periodic rhythmic 

 contraction of the walls of the vessels. 



During the functional activity of a part, a free supply of 

 blood to its capillaries is required. This is brought about 

 by a relaxation of the muscular coats of the arterioles leading 

 to the part, and probably by an active as well as a passive 

 dilatation of the capillaries. When the part returns to rest, 

 the free flow of blood is checked by the contraction of the 

 muscular walls of the arterioles, and probably of the walls 

 of the capillaries. 



The action of the arterioles is well seen under the 

 influence of certain drugs (vaso- dilators and vaso- 

 constrictors). If nitrite of amyl is inhaled by the animal, 

 it will be seen that the skin and mucous membranes 

 become red and engorged with blood, while at the same 

 time the arterial pressure falls. Nitrites cause the muscular 



