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). 
