CHAPTEE III. 



The Inteenal Skeleton. 



TTAVlNGr now completed our brief survey of the skin and 

 -*--*- its appendages — the external or exo-skeleion of Birds — 

 we must now turn to that which is ordinarily alone called the 

 skeleton — namely, the internal or endo-sktleton. 



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 Horubill, and all 

 its bones are so save the jugal t. 



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 

 inade use of in Omithologieal 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 lincl of sub8tance of which the liring body is composed 

 (e. g. bone, gristle, muscle, fat, &c.) is called a " tissue." 

 t For the "jugal" see below, p. 180. 



