100 



COMPARATIVE ZOOLOGY. 



ular, or sac-like, in all animals; but they are constantly 

 changing. The form of the red disks is more permanent, 

 although they are soft and elastic, so that they squeeze 



FIG. 65. Comparative Size and Shape of the red Corpuscles of various Animals. 



through very narrow passages. They are oval, circular, 

 or angular, in Fishes; oval in Reptiles, Birds, and the 

 Camel tribe ; and circular in the rest of Mammals. They 

 are double-convex when nucleated, and double-concave 

 when circular and not nucleated. 



Blood is always heavier than water; but is thinner in 

 cold-blooded than in warm-blooded animals, in herbivores 

 than in carnivores. The blood of Birds, which is the hot- 

 test known, being 10 higher than Man's, is richest in red 

 corpuscles. In Man, they constitute about one half the 

 mass of blood. The white globules are far less numerous 

 than the red; they are relatively more abundant in venous 

 than arterial blood, in the sickly and ill-fed than in the 

 healthy and vigorous, in the lower Vertebrates than in 

 Birds and Mammals. Their number is subject to great 



