1048 PHYSIOLOGY 



shown to cause first a diminution of the volume of the hind limb, followed 

 by a marked increase. During the period of obstruction the circulation of 

 the hind limb was interrupted, and there was thus accumulation of carbon 

 dioxide in the tissues and around the blood vessels. This caused a relaxa- 

 tion of the blood-vessel walls and a corresponding increased volume of the 

 limb when the blood was allowed once more to flow by release of the aortic 

 obstruction. 



Hind limb 



B.P. 



Signal 

 Time 10 sees. 



FIG. 481. Effect of temporary compression of the abdominal aorta on the volume of 

 the denervated hind limb. Two compressions, the second not marked by the 

 signal. Blood pressure taken in the femoral artery of one hind limb, the other 

 hind limb being in the plethysmograph. (BAYLiss.) 



THE REGULATION OF THE BLOOD FLOW THROUGH THE 

 CAPILLARIES 



Up to the present we have emphasised only two factors as regulating 

 the blood flow through the peripheral parts of the body, viz. the general 

 blood pressure, and the state of contraction or tone of the arterioles supplying 

 those parts. We have regarded the capillaries as a close meshwork of canals, 

 the calibre of which depends entirely on the extent to which they were 

 distended by the pressure of blood within them. There is no doubt how- 

 ever that both the calibre of and the resistance to the flow of blood 

 through the capillaries are intimately dependent on the nutritive condition 

 of the cells composing their walls. Certain observers have described spon- 

 taneous changes taking place in the 'diameter of the capillaries, and 



