CHAP AR oh 
Tue INTERNAL SKELETON. 
HAVING now completed our brief survey of the skin and 
its appendages—the external or eao-skeleton of Birds— 
we must now turn to that which is ordinarily alone called the 
skeleton—namely, the internal or endo-skeleton. 
This mainly consists of bone, though, in part also, of cartilage 
and membrane. The bony substance, or osseous tissue*, of birds 
is very dense and strong and yet very light. Their bones are 
whiter than those of other animals, and are more or less 
generally permeated with air. In the Penguins, however, 
none of the bones contain air, while in the Ostrich many 
of the bones which in other birds contain air are filled with 
marrow. With one or two other exceptions, the bones of 
the upper arm and of the thigh always contain air. All the 
bones, except those of the toes, may contain air—or be pneu- 
matic (as it is called)—as in the Pelican and Gannet; and even 
the bones of the toes are pneumatic in the Hornbill, and all 
its bones are so save the jugal 7. 
There are several reasons why it is desirable that the student 
should pay very particular attention to the study of the internal 
skeleton. 
In the first place, a knowledge of that system of parts which 
supports and sustains all the others—the bony framework or 
skeleton—is most useful, because many of its characters are 
made use of in Ornithological classification and the defining 
of groups. It is, besides, the part least perishable, and our 
only guide to the nature and affinities of birds which can now 
* Every different kind of substance of which the living body is composed 
(e.g. bone, gristle, muscle, fat, &e.) is called a “tissue.” 
+ For the “ jugal” see below, p. 180. 
