92 GENERAL STRUCTURE OF BIRDS. 
a manner as to exhibit the chief of them. The cavities of 
the trunk is completely divided, by the muscular partiti 
termed the diaphragm, into two portions—the thorax, con- 
taining the heart and lungs; and the abdomen, containing 
the digestive apparatus. It is chiefly by the alternate con- 
traction and relaxation of this muscle, that the act of 
breathing is performed in Mammals, as will be apc 
hereafter (§ 331). 
78. In Birps there is a much closer conformity to one 
general plan than we find among Mammals. The covering of | 
feathers, by which we ordinarily distinguish the members of 
this class, prevails universally ; and there is no wide depar- 
ture from the typical form. This class belongs to the 
oviparous division of the Vertebrata ; the young being reared 
from eggs. But it is distinguished from Reptiles, which are 
also oviparous and air-breathing, by being warm-blooded ;. 
and by having a very energetic instead of a very slow circu- 
lation. The warmth of the maternal body, moreover, is im- 
parted to the egg in the act of incubation ; and without the 
heat thus communicated (unless it be supplied from some 
other source) the embryo cannot be developed. The covering 
of feathers is given, not only to keep in the heat of the body, 
which is even greater than that of Mammals, but also 
to afford the required surface for the wings, on ‘which the 
Bird is supported and propelled through the air. The 
feathered portion of the wings is stretched out upon the 
bones which answer to those of our arm, and is moved by i 
muscles, The wings are very small, or are entirely absent, i 
the Ostrich and a few other birds, which present the neares 
approach to the Mammalia in their internal structure ; and 
these cannot rise from the ground, but run swiftly along it, 
by means of their powerful legs. In the Penguin, also, th 
wings are small ; and they are used as fins, by the assistance 
of which this bird, which can neither walk nor fly with 
rapidity, can swim very quickly through the water. 
79. Generally speaking, Birds are characterized by th 
extraordinary power of motion which they possess, and b 
the great acuteness of the sense of sight, by which thei 
movements are chiefly directed. They are also remarkable fo 
their instinctive actions, which are chiefly related to their c. 
of their young, for whom they usually construct a protectiv 
