THE CIRCULATION 261 



such as the web of the frog's foot, or the wing of the bat, or 

 the mesentery, it is possible to measure the diameter of the 

 arterioles by means of an eye-piece micrometer, and to study 

 their dilatation and contraction. 



2nd. The engorgement of the capillaries brought about by 

 dilatation of the arterioles manifests itself also in an increased 

 size of the part. Every one knows how on a hot day, when 

 the arterioles of the skin are dilated, it is difficult to pull on 

 a glove, which, on a cold day, when the cutaneous arterioles 

 are contracted, feels loose. By enclosing a part of the body 

 in a case with rigid walls filled with fluid or with air which is 

 connected with some form of recording tambour, an increase 

 or decrease in the size of the part due to the state of its 

 vessels may be registered. Such an instrument is called a 

 plethysmograph. 



3rd. When the arterioles to a part are dilated and the 

 blood is flowing freely into the capillaries, the part becomes 

 warmer, and by fixing a thermometer to the surface conclu- 

 sions as to the condition of the arterioles may be drawn. 



4tth. By streaming blood through the vessels and observing 

 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, 

 Chap. X.) 



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 

 respiration. They appear to be due to the periodic rhythmic 

 contraction which is a characteristic property of non-striped 



