100 COMPARATIVE ZOOLOGY. 
or angular, in Fishes; oval in Reptiles, Birds, and the 
Camel tribe; and circular in the rest of Mammals. They 
are double-convex when oval, and double-concave when 
circular. . ; 
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 more abundant in venous than ar- 
terial blood, in the sickly and ill-fed than in the healthy 
and vigorous, in the lower Vertebrates than in Birds and 
Mammals. 
There is less blood in cold-blooded than in warm-blood- 
ed animals; and the larger the animal, the greater is the 
Fia. 65.—Capillary Circulation in the Web of a Frog’s Foot, X 100: a,b, small veins; 
d, capillaries in which the oval corpuscles are seen to follow one another in sin- 
gle series; c, pigment-cells in the skin. 
