118 
COMPARATIVE ZOOLOGY. 
blood is exposed to the influence of the air, and, therefore, 
the more active the respiration and the purer the blood. 
Thb lungs are relatively largest in Reptiles, and smallest in 
Mammals. But in the cold-blooded Amphibians and Rep¬ 
tiles, the air-cells are few and large; in the warm-blooded 
Birds and Mammals, they are exceedingly numerous and 
minute . 66 In Birds and Mammals, the blood in the capil¬ 
laries is exposed to the air on all sides; in the Reptiles, 
on one only. Respiration is most perfect in Birds; they 
require, relatively to their weights, more air than Mam¬ 
mals or Reptiles, and most quickly die for lack of it. In 
Birds, respiration is not confined to the lungs; but, as in 
Insects, extends through a great part of the body. Air- 
sacs connected with the lungs exist in the abdomen and 
under the skin of the neck, wings, and legs. Even the 
bones are hollow for this purpose; so that if the wind- 
Fig. S5.— Lungs of a Frog: a, hyoid 
apparatus; ft, cartilaginous ring at 
root of the lungs; c, pulmonary 
vessels; d, pulmonary sacs, having 
this peculiarity common to all cold¬ 
blooded air-breathers, that the tra¬ 
chea does not divide into bronchial 
branches, but terminates abruptly 
by orifices which open at once into 
the general cavity. A cartilaginous 
net-work divides the space into lit¬ 
tle sacs, on the walls of which the 
capillaries are spread. 
Fig. SG. —Distribution of Air-tubes in Mam¬ 
malian Lungs : a, larynx; b, trachea; c, d, 
left and right bronchial tubes; e, /, g, the 
ramifications. In Man the subdivision con¬ 
tinues until the ultimate tubes are one twen¬ 
ty-fifth of an inch in diameter. Each lobule 
represents in miniature the structure of tbe 
entire lung of a Frog. 
