CHEMICAL CHANGES IN PLANTS AND ANIMALS. 647 



immediately broken down without actually being incorporated by the 

 tissues ("circulating protein"). This is evidenced by the marked 

 increase in the nitrogenous excretion which appears shortly after 

 protein ingestion. It is important to notice that carbohydrate and 

 fat can replace one another in the diet, but that neither can replace 

 protein to more than a slight extent. Indeed, it has very recently 

 been shown that not all proteins have the same nutritive value (Os- 

 borne and Mendel). Thus, an animal can thrive upon a single pro- 

 tein, such as casein, egg-albumen, or glutenin (wheat). It will live, 

 but will not grow to maturity upon gliadin (wheat) or hordein (bar- 

 ley) alone. It will not even live upon zein (maize). This difference 

 in typical proteins is undoubtedly due to differences in the amino- 

 body content of the protein. Zein contains no lysine or tryptophane. 

 It has long been known that the albuminoid substance, gelatin, con- 

 taining no tryptophaue and no tyrosine, cannot replace protein. 



It is believed that these deficiencies in amino-acids make it im- 

 possible for the organism to build up its native protein from such 

 material. 



The specific waste products of protein (meat) metabolism, the 

 nitrogenous excreta, appear mainly in the urine, and will for the 

 most part be considered in that connection, while the formation of 

 urea and certain other substances will be discussed with the liver. 



Respiration. The most important changes in respired air are the 

 changes in the quantities of oxygen and carbon dioxide. Pure air, 

 after being dried, contains, by volume, of oxygen 20.8 per cent., of 

 nitrogen 79.2 per cent., and a quantity of carbon dioxide (0.04 per 

 cent.) so small that it need not be considered. When 100 volumes of 

 air have been breathed once, it gains a little more than four parts of 

 carbon dioxide and loses a little more than five parts of oxygen ; so 

 that the composition of 100 volumes of inspired air, when expired, is, 

 after being dried, oxygen 15.4 parts, nitrogen 79.2 parts, and carbon 

 dioxide 4.3 parts by volume. 



Much the greater portion of the oxygen lost from respired air 

 enters into combination with the haemoglobin ; a small portion is 

 absorbed by the blood-serum. The immediate source of the carbon 

 dioxide is the blood, in which it exists partly in simple solution and 

 partly in a loose combination with hemoglobin. 



The blood is the common carrier of the body : from the alimentary 

 canal it receives ultimately all the food material ; from the lungs it 

 receives oxygen ; these it carries to the tissues for their sustenance ; 



