AVES. 285 
intestine is furnished in most birds with two blind tubes or 
“caeca” (#). These vary considerably in length in different 
birds, and are sometimes wanting ; whilst their exact function 
is still questionable. The large intestine is seldom more than 
a tenth part of the length of the body, and is generally con- 
ducted straight from the ceca to the cloaca. The cloaca 
is a common cavity which in birds, as in Reptiles, receives the 
termination of the intestine and the ducts of the generative 
and urinary organs (c/). 
Respiration is effected in birds more completely, extensively, 
and actively than in any other class of the Vertebrata, and, as 
the result of this, their average temperature is higher than 
in any other Vertebrates. This extensive development of the 
respiratory process is due to the fact that air is admitted in 
Birds not only to the lungs, but also to the interior of a greater 
or less number of the bones, and to a series of air-receptacles 
which are scattered through various parts of the body. The 
lungs are two in number, of a bright-red colour, and spongy 
texture, and they are confined to the back part of the chest. 
They differ from the lungs of Mammals in not being freely 
suspended in a membranous bag (Pleura), but in being fixed 
to the back wall of the chest. The thoracic and abdominal 
cavities are not separated from one another by a complete par- 
tition (midriff or diaphragm) as the Mammals, but the com- 
mon thoracico-abdominal cavity is subdivided by means of 
membranous partitions into a series of cavities or sacs, which 
are termed the “air-receptacles.” These air-sacs are filled 
with air from the lungs, and vary considerably in number and 
size in different birds. They serve not only greatly to reduce 
the specific gravity of the body, but also assist largely in the 
aeration of the blood. Connected with the air-receptacles, 
and supplementing their action in both of these respects, is a 
series of cavities occupying the interior of a greater or less 
number of the bones, and also containing air. In young birds 
these air-cavities in the bones do not exist, and the bones are 
simply filled with marrow, as in the Mammals. In the Pen- 
guin, which does not fly, none of the bones contain air-Cavities 
or are “pneumatic ;” and in the Ostrich only a few of the bones 
contain air. Inthe Pelican and Gannet all the bones of the 
skeleton, except the phalanges of the toes, are permeated by 
air; and in the Hornbill even these are pneumatic. 
