40 EVENINGS AT THE MICROSCOPE 



With tliis fine needle I make a minute prick through, 

 the skin of my hand. A drop of blood oozes out, with 

 which I smear this slip of glass. The slip is now on the 

 stage of the instrument, with a power of 600 diameters. 

 You see an infinite number of small roundish bodies, of a 

 clear yellowish color, floating in a colorless 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, blood- disks; since their form 

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

 The slightness of their color is dependent on their extreme 

 tenuity: when a large number lie over each other the ag- 

 gregated color is very manifest, as it then becomes either 

 a full dark red, or bright rich scarlet; 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 recognized 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 Eeptiles their 

 form is elliptical, and the surfaces are flat, or slightly con- 

 vex. This distinction, then, will at once enable us to de- 

 termine Mammalian blood. 1 But to determine the various 

 tribes of this great class among themselves, we must have 

 recourse to another criterion that of dimensions. 



1 The Camels among Mammalia, and the Lampreys among Fishes, are excep- 

 tions to the above rule; the former having elliptical and convex blood-disks, 

 and the latter circular, and slightly concave. 



