ARTICLES 273 



If the reader will examine, under a low power of the micro- 

 scope, the circulation in the web of the frog's foot — and there 

 is probably no more interesting thing in the whole of physi- 

 ology — he will notice how great is the volume of the capillaries 

 compared with that of the veins and arteries. He will see 

 that the rate of the flow of blood in the arterioles is, accord- 

 ingly, very much greater than that in the capillaries ; and, 

 remembering that the friction is proportional to the velocity, 

 he will realise the great effect that changes in diameter of the 

 arterioles have on the pressure on the arterial side of the system. 

 But he will also appreciate how large a volume of blood can 

 be contained in the capillary region, and that a comparatively 

 small increase in the diameter of the capillaries, if present in a 

 large part of the body, will suck up, as it were, a great propor- 

 tion of the blood present in the circulation. The importance of 

 this fact is that the amount flowing into the heart from the great 

 veins will be less, and therefore also that driven out into the 

 arteries on contraction of the ventricles. Hence there will be a 

 fall in blood-pressure and deficient supply to the tissues gen- 

 erally. Thus, although the capillaries may be wider, the current 

 through them will be diminished by the decrease in driving 

 pressure, in addition to the lower rate of flow involved in their 

 increased width. Under ordinary circumstances, as described 

 by Lister and by Langley, some of the capillaries are more or 

 less empty of blood, and become filled only when dilatation 

 occurs. This is an additional way in which blood is accumu- 

 lated in capillary networks. This fact has recently been 

 further investigated by Krogh, who shows that, in resting 

 muscle, only a small number of the capillaries are filled with 

 blood. In activity, a greater or less proportion of the re- 

 mainder becomes dilated and conveys a current of blood. 

 According to his experiments, it requires a fairly high pressure 

 to open up the collapsed capillaries ; so that, if an increased 

 supply of blood is needed by an organ, it is necessary that an 

 active dilatation of the capillaries should occur, as well as one 

 of the arterioles. It appears that, if the latter alone were to 

 take place, a comparatively small increase in blood-supply 

 would be provided. If the capillaries alone dilate, there is no 

 increase in total supply, or very little, and the blood which 

 continues to flow will be greatly reduced in rate with a ten- 

 dency to stagnation, with rapid loss of its oxygen. This con- 



