





166 THE BODY AT WORK 



nitrogenous compounds, they are carried to the liver, formed 

 into urea, and passed to the kidneys for elimination. If car- 

 bonic acid, it is carried to the lungs for exhalation. The water 

 formed by combination of hydrogen and oxygen may escape 

 from the lungs, the kidneys, or the skin. 



Two or three pounds of mixed foods are consumed every day. 

 By the blood they are carried to the tissues, whence an equiva- 

 lent quantity of waste that is to say, oxidized material is 

 removed. About 1J pounds of oxygen is required to burn the 

 day's fuel. 



The problems of respiration are twofold. In the first place 

 we have to consider the physics and chemistry of the combina- 

 tion of haemoglobin with oxygen, and of the elimination of 

 carbonic acid from the blood in the lungs ; secondly we have 

 to explain the transference of oxygen from haemoglobin to the 

 tissues, and the reception in the blood of carbonic acid pro- 

 duced by the tissues. 



The apparatus by which air is brought into relation with 

 the blood consists of lungs and windpipe. At its upper end, 

 where it joins the portion of the alimentary tract common to 

 deglutition and respiration, the special respiratory tube is 

 protected by the larynx. The nasal chambers belong to the 

 respiratory tract ; the gullet, or pharynx, is common to the 

 two functions. 



The mucous membrane which lines the nose and windpipe 

 is kept moist in order that it may catch particles of dust 

 drawn in with the air. At the same time the nasal chambers 

 serve to warm the air, and to add moisture to it if it be too 

 dry ; for the lining epithelium of the lungs would suffer if 

 dry air came in contact with it. The wall-surface of the 

 nasal chambers is increased by the projection of folded and 

 chambered " turbinate bones." The importance of warming 

 the air before it is admitted to the lungs is remarkably illus- 

 trated in the case of certain sea-birds. The nasal chambers of 

 the frigate-bird, and of some other birds which resemble it, are 

 exceptionally complicated. Since the animal is devoid of any 

 sense of smell, and the air which it breathes must be nearly satu- 

 rated with moisture, the only function which can be assigned 

 to these convoluted passages is that of warming inspired air. 



The larynx will be more minutely described when it is con- 



