INTRODUCTORY 17 



girdle and hind-limb. The shoulder-girdles are unquestionably 

 reptilian, but this is not so patent as are the other characters. 



While in some birds, as in the Hornbills for instance, every 

 bone in the skeleton contains spacious air-chambers, in others, 

 the long bones at any rate — the bones of the wings and legs — 

 are filled with marrow, and this is true even of such skilled 

 performers on the wing as the Swallows. The peculiar char- 

 acters of the shoulder-girdle, wing, and hind-limb will be dis- 

 cussed later in these pages. 



The Respiratory System 



The respiratory system of birds presents some points which 

 demand a brief notice. 



The Lungs 



In the first place the lungs are not suspended freely within 

 the body cavity, but are closely affixed to the dorsal wall of 

 the anterior portion of the thorax. In the second place they 

 differ from the lungs of other vertebrates in that the respira- 

 tory air is not simply drawn into the lungs and expelled again, 

 but is rather drawn through them and passed into a series of 

 large thin- walled chambers lying along each side of the body 

 cavity, and known as the air-sacs. The air stored in these 

 reservoirs serves not only for respiratory purposes, but also 

 as regulators of the temperature, thereby compensating for the 

 lack of sweat glands. 



The Pulmonary System of Air-sacs 



There are five pairs of these air-sacs. The first, or cervical 

 pair, lie one on either side of the base of the neck and may give 

 rise to a number of smaller cells which run up the neck, and 

 even into the head ; while small side branches may penetrate 

 between the muscles of the neck, the vertebrae, and the various 

 cranial cavities : or they may form large inflatable sacs in the 

 region of the throat, as in the Prairie-fowl and Frigate-bird. 



The second pair are known as the interclavicular sacs. In 

 the Ducks they communicate one with another, while in the 

 Storks they combine to form a single chamber. Lateral ex- 

 tensions of these sacs form large axillary chambers' ultimately 

 penetrating the humerus and other of the wing bones, the 



