

THE BLOOD 107 



little or no heat production is going on. This is the case 

 with the connective tissues and with the skin. 



At the surface the blood loses heat, at least under all 

 conditions which can be called normal. The cooler blood 

 then returns to the large vessels, merges with the heated 

 blood from the muscles and glands, and brings the 

 temperature of the mixture to an average which seldom 

 varies materially. The means by which this standard tem- 

 perature is maintained will be discussed at length in a 

 later chapter. It will be well to point out even at this 

 time that the skin is subject to considerable changes of 

 temperature, and that our sense of being warm or cold 

 depends entirely on the condition prevailing at the sur- 

 face. If we turn again to our illustration of a house heated 

 by a "circulation" of hot air or water, we shall recognize that 

 here, as in the living body, there is a constant escape of heat 

 to the environment. The temperature of a pane of glass 

 in a window of the house will be influenced both by the 

 internal and the external state of affairs. The glass may be 

 warmed as hot air is wafted against it from within or cooled 

 by a gust from without. So the skin may be warmed by a 

 waxing of the blood-current close beneath it or chilled by a 

 passing draft. 



Blood may be described as a red fluid, but inspection 

 with the microscope shows at a glance that its redness is 

 Hot due to a coloring-matter in solution and uniformly dis- 

 tributed, but to the presence of minute solid bodies in sus- 

 pension. These are the red corpuscles. The liquid in 

 which they are swept about is the plasma. The corpuscles 

 make up something less than one-half the total volume. 

 In view of this, it is surprising that the blood can have such 

 a free-flowing character and find its way through the cap- 

 illaries so readily. One would anticipate that such a 

 mingling of solid particles with fluid would result in the 

 formation of a highly viscid mass. That the actual condi- 

 tion is so different must be due to the absolute smoothness 

 and the great pliability of the corpuscles. 



The Red Corpuscles. The individual corpuscle is usu- 



