DIGESTION AND RESORPTION 103 



mostly substances of comparatively low molecular weight, 

 readily soluble in water and having a relatively high osmotic 

 pressure and therefore readily diffusible. This difference is 

 most marked in some of the more simple cleavage products of 

 the proteins and least so in the case of the digestive products 

 of the fats. 



146. Uniformity in nutritive material. The feed consumed, 

 especially by herbivora, is of a very heterogeneous character. 

 The proteins and carbohydrates in particular are present in 

 great variety, so that the nature and proportions of the sub- 

 stances out of which the body must draw the material for the 

 construction and maintenance of its tissues may vary greatly 

 at different times. Under the action of the digestive enzyms, 

 however, these diverse substances all yield substantially the 

 same products so that the nutritive solution supplied to the body 

 proper is qualitatively of a very uniform composition, contain- 

 ing chiefly monosaccharids, various acids of the aliphatic series, 

 amino acids derived from the proteins, and the soluble ash in- 

 gredients. By this preliminary action upon the feed in the 

 digestive canal, i.e., practically outside the body, the 

 organism is rendered independent of the particular kinds of feed 

 available, its cells being constantly supplied with uniform 

 nutritive material. 



147. Molecular simplification. It has just been pointed 

 out (145) that digestion from the chemical standpoint consists 

 substantially of a series of hydrolytic cleavages of the nutrients, 

 yielding compounds of lower molecular weight and greater solu- 

 bility and diffusibility. This molecular simplification has, 

 however, a more important aspect which is most strikingly 

 illustrated in the case of the proteins. It was shown in Chap- 

 ter I that the proteins, although very similar in many physical 

 properties, may differ widely from each other as regards molecu- 

 lar structure. This is shown in the first place by the wide 

 variations in the proportions of the constituent amino acids 

 which they yield on hydrolysis (50). Moreover, even were 

 these cleavage products present in the same proportions, the 

 existence of optical isomers and the possible variations in the 

 order and manner of linkage of the amino acids afford almost 

 endless possibilities of isomerism. Studies in immunity have 

 in fact revealed striking specific differences between proteins 



