404 THE ELIMINATION OF WASTE PRODUCTS. 



by a whole series, often long continued, of short spasmodic expirations, the 

 glottis being freely open during the whole time, and the vocal cords being 

 thrown into characteristic vibrations. 



In crying, the respiratory movements are modified in the same way as in 

 laughing ; the rhythm and the accompanying facial expressions are, how- 

 ever, different, though laughing and crying frequently become indistin- 

 guishable. 



CHAPTER III. 



THE ELIMINATION OF WASTE PRODUCTS. 



335. We have traced the food from the alimentary canal into the blood, 

 and, did the state of our knowledge permit, the natural course of our study 

 would be to trace the food from the blood into the tissues, and then to follow 

 the products of the activity of the tissues back into the blood and so out 

 of the body. This, however, we cannot as yet satisfactorily do ; and it will 

 be more convenient to study first the final products of the metabolism of the 

 body, and the manner in which they are eliminated, and afterward to return 

 to the discussion of the intervening steps. 



Our food consists of certain food-stuffs, viz., proteids, fats, and carbo- 

 hydrates, of various salts, and of water. In their passage through the blood 

 and tissues of the body, the proteids, fats, and carbohydrates are converted 

 into urea (or some closely allied body), carbonic acid, and water, the nitrogen 

 of the urea being furnished by the proteids alone. Many of the proteids 

 contain sulphur, and also have phosphorus attached to them in some com- 

 bination or other, and some of the fats taken as food contain phosphorus ; 

 these elements ultimately undergo oxidation into phosphates and sulphates, 

 and leave the body in that form in company with the other salts. 



Broadly speaking, then, the waste products of the animal economy are 

 urea, carbonic acid, salts, and water. These leave the body by one or other 

 of three main channels, the lungs, the skin, and the kidney. Some part, it 

 is true, leaves the body by the bowels, for, as we have seen, the feces contain, 

 besides undigested portions of food, substances which have been secreted into 

 the bowel, and are, therefore, waste products ; but the amount of these is so 

 small that they may be neglected. 



The lungs serve as the channel for the discharge of the greater part of 

 the carbonic acid, and a considerable quantity of water ; this discharge we 

 have just studied. Through the skin there leave the body a comparatively 

 small quantity of salts, a little carbonic acid, and a variable but on the whole 

 large quantity of water. 



The kidneys discharge all or nearly all the urea and allied bodies, the 

 greater portion of the salts, and a large amount of water, with an insignifi- 

 cant quantity of carbonic acid. They are especially important, since by 

 them practically all the nitrogenous waste leaves the body ; and to them we 

 will turn first. 



336. 



ourselves 



THE COMPOSITION AND CHARACTERS OF URINE. 



. These are so fully dwelt upon in special works that we may confine 

 here to salient points. The healthy urine of man is a clear yellowish 



