100 



COMl'ARATIVli ZOOLOGY. 



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

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

 although they are soft and elastic, so that they squeeze 



Fig. 05.— Ci>mpiirative Sixe mid Shape of the red Corpnscles of varions Animals. 



through very nari-ow passages. They are oval, circular, 

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

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

 are double-convex v?hen 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 

 liealthy and vigorous, in the lower Vertebrates than in 

 Birds and Mammals. Tlieir number is subject to great 



