BLOOD. 29 



the stage of the instrument, with a power of 600 diam- 

 eters. You see an infinite number of small roundish 

 bodies, of a clear yellowish colour, floating in a colour- 

 less fluid, but so numerous, that it is only here and 

 there, as near the edges of the smear, that you can 

 detect any interval in their continuity. 



These bodies are what we frequently call the blood- 

 globules, or, more correctly, hlood-disJcs ,' since their 

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

 money. The slightness of their colour is dependent on 

 their extreme tenuity : when a larger number lie over 

 each other the aggregated colour is very manifest, as it 

 then becomes either a full dark red, or bright rich scar- 

 let ; for to these disks blood is entirely indebted for its 

 well-known hue. All vertebrate blood is composed 

 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 are circular, or 

 nearly so, and slightly concave on both of the surfaces. 

 On the other hand, in Birds, Fishes, and Keptiles 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 Camels among Mammalia, and the Lampreys among Fishes, are 

 exceptions to the abore rule ; the former having elliptical and convex 

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



