ECONOMY OF THE BODY 123 



and the formation of waste-products. The energy used 

 and the energy lost must be made good again, and that is 

 effected by taking in food. But this food must be prepared 

 within the body and altered both physically and chemically, 

 so that it may be used to the best advantage. This has 

 been already discussed. Many different chemical processes 

 go on in the body, but for the production of energy and the 

 doing of continued work oxidations are essential. Thus the 

 function of respiration implies the capture of oxygen and 

 the elimination of the poisonous carbon dioxide. The 

 filtering out of other waste-products which would clog and 

 poison the system is the function of excretion. Then there 

 is the regulative function in so far as that is effected by the 

 chemical messengers of the ductless glands. Let us turn 

 to these functions of respiration, excretion, and regulation. 



§ 5. Respiration 



The breathing of birds is in several respects peculiar ; 

 it is speciaHsed in relation to flight, to song, and to intense 

 activity in general. Some of the important peculiarities 

 are the following : the lungs though relatively small and 

 hardly distensible, have a large internal surface for gaseous 

 interchange ; the driving out of the air is assisted by the 

 strokes of the wings ; expiration is the active part of 

 the respiratory process, not inspiration as in mammals ; the 

 lungs open into a system of air-sacs which economise the 

 work of breathing and have other uses, though they are 

 quite unimportant in making the bird more buoyant. 



At the back of the tongue on the ventral surface is the 

 glottis, the opening of the windpipe or trachea. It does not 

 show more than a slight hint of the epiglottis, the movable 

 lid characteristic of mammals, familiarly useful in keeping 

 food from going down the wrong way. 



The glottis leads into the larynx, which is supported 

 by the usual (3) cartilages, but is remarkable in having no 

 vocal cords, for in birds these are in the syrinx at the foot 



