474 RESPIRATION. 



receptacles in the thoracic and abdominal cavities, which, in 

 addition to this use, prove highly favourable to flight and 

 swimming. The chief receptacles are one inter-clavicular 

 placed in front of the lungs and between the furculum, one 

 median and two lateral thoracic occupying the anterior and 

 lateral aspects of the chest; two hepatic, extending from the 

 thorax into the pelvis; two abdominal, placed beneath the 

 latter. These communicate through small orifices with air 

 cavities inside the bones, and especially the dorsal vertebrae, 

 the vertebral ribs, the sternum, the scapula, coracoid and 

 furculum, the humerus, the pelvic bones, and the femur. 

 So freely does the interior of the bones communicate with 

 the bronchia, that in some birds, if the femur or humerus 

 is fractured, it is impossible for the bird to rise in flight. In 

 such a case, too, breathing can be imperfectly carried on 

 through the cavity in the bone. Some birds of prey can, by 

 expelling the air suddenly from these internal cavities, in- 

 crease their specific gravity, and dart with great velocity on 

 their prey. 



CHEMICAL CHANGES IN EESPIEATION. 



THE BLOOD. 



The food which an animal assimilates is transformed into 

 blood, and from this material is derived for the nourishment 

 of the tissues. In the process of nutrition, blood, however, 

 becomes charged with the products of decomposition, which 

 have to be excreted by various organs, and in part by the 

 lungs. The exchange of impurities for the vivifying elements 

 of pure air constitutes in essence the function of respiration. 

 Such a function cannot, therefore, be understood if we do not 

 describe the blood. This liquid, as we have already seen, 

 flows through the body in arteries, capillaries, and veins, 



