VENOUS AND ARTERIAL BLOOD. 203 



is evident that this change of character has been produced 

 during the passage of the blood through the tissues ; and so 

 important is the alteration, that the blood which has been 

 subjected to it is not fit to pass again into the arteries of the 

 body, until it has been renewed by exposure to air in the 

 Lungs. In their vessels, the contrary change of which the 

 nature will be presently explained ( 253) is effected, the 

 dark hue of venous blood giving place to the bright red of the 

 arterial fluid ; this is again changed during the passage of the 

 blood through the body, to be again restored in the lungs. 

 The same is the case in regard to Fishes, whose gills perform 

 the same function as the lungs of air-breathing Vertebrata. 

 And among the Invertebrated classes, although the deteriora- 

 tion of the blood in its passage through the body is not made 

 manifest by any change of colour, yet its renewal by exposure 

 to air in the respiratory organs is not less requisite. 



228. Hence the continual movement of the blood is neces- 

 sary for two purposes in particular; -first, to convey the 

 nutritive materials from the place where they are received and 

 prepared, to that in which they are appropriated, and thus 

 to afford to every organ a constant supply of the materials 

 which it requires ; and, second, to carry this fluid, at regular 

 intervals, to certain organs by whose instrumentality it may 

 be exposed to the influence of the air, so as to regain the 

 qualities it has lost, and part with what it has taken-up to its 

 prejudice. But there are many other objects fulfilled by it, 

 which will unfold themselves as we proceed. 



Properties of the Blood. 



229. When the circulating blood of a red-blooded animal 

 is examined with a microscope, it is seen to consist of two 

 distinct parts ; a clear and nearly colourless fluid, to which 

 the name of liquor sanguinis (or liquor of the blood) is given ; 

 and of an immense number of rounded particles floating in 

 this fluid, which are often termed the globules of the blood. The 

 shape and size of these particles are, for the most part, very 

 uniform in animals of the same species ; but in no instance 

 are they globular ; and it is better, therefore, to term them 

 corpuscles. In Man and most other MAMMALS, they are 

 nearly flat discs, resembling pieces of money, but usually 

 exhibiting a slight depression towards the centre (fig. 115). 



