278 VETERINARY PHYSIOLOGY 



the rate at which it escapes the changes in the state of the 

 arterioles may be made out. This perfusion method is much 

 used in studying the action of drugs. (Practical Physiology.} 



5th. Since the state of the arterioles influences the arterial 

 pressure, 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. 



Normal State of Arterioles. If an arteriole in some 

 transparent tissue be examined, it will be found to main- 

 tain a fairly uniform size, but to undergo periodic slow 

 changes in calibre. If the ear of a white rabbit be studied, 

 it will be seen to undergo slow changes, at one time appear- 

 ing 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 

 respirations. They appear to be due to the periodic rhythmic 

 contraction which is a characteristic property of visceral 

 muscle fibres. This rhythmic action is better marked in 

 some vessels than in others. 



Vaso-motor Mechanism. If the sciatic nerve of a frog be 

 cut, the arterioles in the foot at once dilate. If the sciatic be 

 stimulated, the arterioles become smaller. The same results 

 follow if the anterior roots of the lower spinal nerves, from 

 which the sciatic takes origin, be cut or stimulated. 



We must, therefore, conclude (1) that the central nervous 

 system exerts a constant tonic influence upon the arterioles, 

 keeping them in a state of semi-contraction ; and (2) that this 

 influence may be increased, and thus a constriction of the 

 arterioles caused, and in this way the flow of blood from 

 arteries to capillaries obstructed and the arterial pressure 

 raised ; and (3) that this influence may be diminished, so 

 that the arterioles dilate and allow an increased flow into the 

 capillaries from the arteries, and thus lower the arterial 

 pressure. 



These mobile arterioles, under the control of the central 

 nervous system, constitute a vaso-motor mechanism, which 



