280 ZOOLOGY. 



Birds seldom attain a great size, and their bodies are light 

 in consequence of the penetration of the air into their interior. 

 They do not vary much in their internal structure. 



427. Feathers are analogous to hair, but are more com- 

 plex in structure. A horny tube is first observed, pierced at 

 its extremity, a stalk surmounting this tube. Finally, barbs, 

 growing from the sides of the stalk ; these are fringed with 

 barbules ; and these again are sometimes fringed with others 

 still smaller. 



Fig. 221. Galeated Cassowary. 



The secreting organ of the feather is called the capsule. It 

 would seem that so long as the feather grows, or is being 

 developed, the capsule increases in length, and that in pro- 

 portion an its base elongates, the extremity dies, and dries up 

 so soon as it has formed the corresponding portion of this 

 appendage. Each of these small apparatuses is composed of a 

 cylindrical sheath, covered internally by two tunics, united by 

 oblique septa, and of a central bulb. The substance of the 

 feather is formed on the surface of the bulb, and to form the 

 barbs it is moulded, as it were, into the spaces which the 



