TRANSPORT OF MATERIALS 147 



that the arterioles of the ear itself are dilated, while those of the 

 rest of the body are constricted, so that a rise of arterial pressure 

 results. It will be seen that this is the most effective means 

 possible of obtaining a more copious blood supply to any organ. 



The arterioles can also be made to dilate or constrict by the 

 action of various drugs or chemical substances. The constricting 

 effect of adrenaline has already been referred to. Some of the 

 chemical products of active cells have a dilating effect, the chief of 

 these being the hydrogen-ions from carbon dioxide. Hence an 

 increase of activity automatically brings about a better supply of 

 blood. 



77/i? Capillaries. Although the walls of the fine network of 

 blood vessels that connects the small arterioles with the small 

 venules consist of simple protoplasmic cells and possess no muscular 

 coat, there is evidence that these vessels can be made narrower or 

 wider. This must occur owing to the constituent cells becoming 

 thicker and less flattened when contraction takes place, somewhat 

 as an amoeba or leucocyte does when stimulated. Whether they are 

 under the control of nerves is not yet decided, but certain chemical 

 agents have been shown to relax them. A substance called " hista- 

 mine," which is derived from a complex amino-acid constituent of 

 protoplasm by removal of carbon dioxide, has a dilating effect on 

 the capillaries, but a constrictor one on the arterioles. Under its 

 action, therefore, an accumulation of blood in a nearly stagnant 

 state is liable to take place in the capillaries, and such a condition, 

 by which blood is removed from active circulation, is of importance 

 in the condition of "shock" after wounds or surgical operation. 

 Substances with an action similar to that of histamine would be 

 produced in the destructive changes going on in damaged cells. 



A condition in which the capillaries are dilated, but with very 

 little blood current through them, owing to constriction of arterioles, 

 is seen in the blue skin sometimes produced by cold. The fact that 

 the skin is dark in colour, not white, shows that the capillaries 

 in it are full of blood, but this blood is almost stationary. Its colour 

 shows that oxygen has been removed, and the coldness of the skin 

 shows that the circulation through it has nearly ceased. The skin 

 may also be full of blood, but red and hot, as when exposed to 

 warmth or heated by friction. In this case, the arterioles are dilated, 

 and the capillaries are passively distended by the raised pressure 

 thus produced inside them. Thus a full current of blood passes 

 through them. 



There is evidence that when organs are not in activity the whole 

 of their capillary vessels are not filled with blood. Some are empty 

 and more or less invisible. These may be filled with red blood when 

 the arterioles dilate, or with venous blood when they themselves 



