40 ZOOLOGY. 



The cavities of the chest and abdomen are not se- 

 parated by a diaphragm, this partition being only 

 rudimentary. 



The bronchial tubes communicate by a series of open- 

 ings on the surface of the lungs with a number of air 

 sacs situated partly in the chest, and partly in the 

 abdomen. These sacs are connected with the interior of the 

 bones, which also contain air cells. As has been already 

 stated, the blood is aerated in these air-receptacles as well 

 as in the lungs. By filling these sacs with air, the animal 

 is enabled to reduce its specific gravity at will. 



There are no air-cells in the bones of the penguin and 

 some other aquatic fowls; and only a few of the bones in 

 the ostrich tribe are " pneumatic." 



38. Nervous System. The brain is relatively smaller 

 than in mammals, and is without convolutions. The two 

 hemispheres of the cerebrum are not united by a corpus 

 cattosum. 



39. Senses. The eyes of birds are in front, of a conical 

 shape, and possess a third eyelid. The sense of sight is 

 always very acute. There is no external ear. The 

 nostrils are placed on the sides of the upper mandible. 

 They are sometimes surrounded by bristles, and are 

 sometimes covered by a cartilaginous scale. 



40. Covering. Feathers form the peculiar covering of 

 birds. Being non-conductors of heat, they assist in keep- 

 ing up the high temperature of the body. A feather 

 consists of several distinct parts. First, there is a horny 

 tube, called the quill ; then the shaft, which has a horny 

 surface, with a groove on the under side; the interior 

 being filled with a pith-like substance. The webs are 

 situated at the sides of the shaft. The divisions of the 

 web are called barbs, each of which is covered with a 

 number of barbules. Generally, the barbs are kept 

 together by the barbules, which are provided with hooks 

 for this purpose. In the ostrich tribe, however, the 

 barbs are disconnected. 



There is an oil gland at the base of the tail, with the 



