THE MUSEUM 
OF 
NATURAL HISTORY. 
ZOOLOGY, 
CLASS II.— BIRDS. 
Although the numerous and varied tribes of Birds 
undoubtedly come next in order after the Mammalia, 
we cannot point to any member of the latter class, 
which, in its general characters, really makes an ap- 
proach to the birds. Some of the lowest mammals 
certainly present some resemblance to the oviparous 
Vertebrata in a physiological point of view; but the 
peculiarities exhibited by these rather indicate a rela- 
tionship to the class of reptiles, and thus, in our classi- 
fications, the Birds form, as it were, a supplementary 
class, interpolated between the two similarly-quadi'uped 
groups of Mammalia and reptiles. 
This view is remarkably in accordance with geolo- 
gical facts, as far as we can judge from the present state 
of our knowledge; the preponderating reptilian Fauna 
of the secondary period gives place, in the gi-adual 
evolution of organic nature, to the similarly prepon- 
derating mammalian Fauna of the tertiary epoch, 
whilst the traces of birds, such as they are, occur 
simultaneously with these from a very early period of 
time.* 
Birds, lilce mammals, are warm-blooded, air-breathing, 
vertebrate animals, and, like them, possess a heart com- 
posed of four distinct cardties and voluminous minutely 
cellular lungs. The latter organs, however, present 
some differences from the corresponding parts in the 
Mammalia ; they are not lobed, and, instead of being 
freely suspended in the cavity of the chest, they are 
attached to the hmer surface of the dorsal part of this 
cavity. Moreover, the surface of the .lung, instead of 
forming a closed sac, as in the Mammalia, is perforated 
by several large apertures, passing down through its 
substance to the main branches of the air-tubes, and, 
on the other hand, communicating externally with an 
extensive system of air-sacs, which penetrate to nearly 
every part of the body of the bird, and even occupy the 
internal cavities of those hollow bones, which, in the 
Mammalia, and indeed in the young bird, are filled 
with marrow. By this arrangement the air taken into 
* The foot-prints of birds occur in the new red sandstone, 
the earliest of the secondary formations, both in Europe and 
America. They are accompanied by similar traces of gigantic 
Batrachiau reptiles. 
the lungs may, to a considerable extent, penetrate, as 
it were, into the very substance of the bird’s body, 
a circumstance of no small importance in reducing 
its specific gravity, and rendering it capable of being 
readily supported in the air by the action of the wings. 
So ready is the communication between these air-sacs 
and the lungs, that birds have even been known to 
breathe through a fractured ^vmg-bone, when the ordi- 
nary air-passages have been closed by compression.* 
Another anatomical character by which birds are 
distinguished from mammals, consists in the absence 
of the diaphragm or muscular partition, which, in the 
latter, separates the cavity of the chest from that of 
the abdomen, and which, by its movements, assists 
greatly in respiration ; the alternate enlargement and 
diminution of the cavity of the chest by which this 
function is executed in birds, is efl'ected by the alter- 
nate elevation and depression of the broad flat sternum. 
As the power of fliglrt is the principal general charac- 
teristic of the whole class of birds, we naturally expect 
to find the structure of the skeleton specially modified 
for the accomplishment of this object; and so com- 
pletely are these expectations fulfilled, that it is impos- 
sible ever to mistake the skeleton of a bird for that of 
any other form of animal (see Plate 31). Nevertheless 
the parts described as occurring in the skeleton of a 
mammal, may invariably be traced distinctly. The dis- 
tinctions of head, neck, and trunk are always clearly 
visible ; but the tail is very short, and the proportions 
of the different regions of the body are usually quite 
different from those which prevail among the Mam- 
malia. 
The skull is generally of small size, and its cavity is 
much smaller in proportion tiian in the Mammalia. 
This indicates a smaller brain, and less general uitelli- 
* Although it is by no means certain that the air pervading 
the body of the bird by means of these air-sacs, is in any way 
subservient to respiration, it is impossible not to recognize in 
this provision lor the passage of air amongst the tissues of the 
body an analogy with that arrangement of the respiratory 
apparatus in insects, which, in like manner, serves to render 
the bodies of those animals sufficiently light to enable them to 
exercise the power of flight. In this, as in some other respects, 
the analogy between birds and insects is unmistakable. 
