CLASS SECOND. 



BIRDS. 



All the animals of this class are Oviparous ; that is, they produce their 

 young by eggs. They are distinguished from the other divisiona of the 

 animal kingdom, by being covered with feathers. To aid in their classifica- 

 tion, the feathers have been distinguished by dififerent names, according to 

 the particular part of the animal which they cover. 



Birds have a double respiration and circulation,— these are for adapting 

 the animals for flight. Another admirable contrivance for this purpose is 

 the general diffusion of air through their bodies. Their lungs are ncit di- 

 vided, but adhere to the ribs, and are composed of a membrane penetrated 

 by orifices, which permit the free passage of the air into the cavities of the 

 chest, lower abdomen, the arm-pits, and even to the bones. Birds respire 

 not only by branches of the aorta, but also by means of the pulmonary 

 artery. 



Birds are true bipeds, and can only lift things from the ground by means 

 of their bill. The neck is much elongated, as well as the beak, which is 

 covered with a horny substance. They are always destitute of teeth. The 

 upper mandible is a prolongation of the intermaxillary bones : these are 

 prolonged into two arches ; the internal one of which consists of the palate 

 bones, and the external one of the maxillary and jugal bones ; the mandible 

 being united to the cranium by elastic laminse. The two mandibles move 

 upon each other by means of an intermediate bone placed at their articula- 

 tion. 



The organ of smell is situated at, or near the base of the bill, generally 

 hidden by the feathers. 



The tongue has but little muscular substance, and is supported by a pro- 

 duction of the hyoid bone. Most birds seem not to be possessed of a fine 

 degree of taste. 



The eyes of birds are so constructed, that their sight is very acute, whe. 

 ther near or at a distance. In addition to the ordinary eyelids, there is a 

 membrane which acts like a curtain to cover the eye. 



Birds which fly by day seldom have an external ear ; but nocturnal birds 

 possess that organ, although not nearly so much developed as that of quad, 

 rupeds. It is generally covered by barbed feathers, which are more fringed 

 than those of their body. 



The Brain of birds agrees in character with that of the other vertebrate 

 oviparous animals, but remarkable for its volume, being proportionally 

 greater, and even exceeding, in many cases, that of the mammalia. This in- 

 creased bulk is attributable to tubercles, which are analogous to the cor. 

 pora striata, and not to the hemispheres themselves, which are rather 

 compressed and without circumvolutions. The cerebellum is largely deve. 



