THE CLASS BIEDS. 305 



[There is in the ornithorynchus, on each side a femoral gland and 

 duct leading to the spur on its heel. The secretions of this gland 



Fig. 281a. 



were at one time supposed to be poisonous. The apparatus is of 

 the most singular character, and was first described by me in the 

 Tr. Wern. Soc. R. K.] 



THE CLASS BIRDS. 



426. The class birds is one of the best defined and most 

 distinct, whether viewed with reference to the exterior or 

 interior. Birds are oviparous vertebrate animals, with a 

 double and complete circulation ; to which may be added, that 

 the respiration is aerien and double; which means, that instead 

 of being confined to the lungs, as in mammals, the air pene- 

 trates throughout the body, and even into the interior of the 

 bones ; their blood is hot, as in mammals. Finally, they are 

 covered with feathers, and their pectoral extremities have the 

 form and character of wings. 



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. 



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



