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 
