BLOOD. 27 



These bodies are what we frequently call the blood- 

 globules, or, more correctly, blood-disks ; since their form 

 is not globular, but thin and flat, like that of a piece of 

 money. The slightness of their colour is dependent on 

 their extreme thinness : when a larger number lie over 

 each other the aggregated colour is very manifest, as it 

 then becomes either full dark red, or bright rich scarlet ; 

 for to these disks blood is entirely indebted for its well- 

 known hue. The blood of all vertebrate animals is com- 

 posed principally of these bodies, which when once seen 

 are easily recognised again : the microscope then readily 

 determines whether any given red fluid or dried stain is 

 composed of blood. 



The disks in the blood of Mammalia, or animals which 

 suckle their young, are circular or nearly so, and slightly 

 concave on both of the surfaces. On the other hand, in 

 Birds, Fishes, and Reptiles, their form is elliptical, and 

 the surfaces are flat, or slightly convex. This distinction, 

 then, will at once enable us to determine Mammalian 

 blood.* But to determine the various tribes of this great 

 class among themselves, we must have recourse to another 

 •criterion, — that of dimensions. 



The blood-disks of Man nearly agree in size with those 

 of the Monkey tribe, of the Seals and Whales, of the 

 Elephant, and of the Kangaroo. Most other quadrupeds 

 have them smaller than in Man ; the smallest of all being 

 found in those animals which chew the cud. The little 

 Musk-deer of Java has disks not more than one-fourth as 

 large as the human, but these are remarkably minute ; 

 no other known animal approaches it in this respect : 

 those of the Ox are about three-fourths, and those of the 

 Sheep little more than half the human average. 



Tables have been made out, showing the comparative 



* The Camels among Mammalia, and the Lampreys among Fishes, 

 are exceptions to the above rule ; the former having ellijotical and 

 convex blood-disks, the latter circular and slightly concave. 



