CHEMISTRY OF DIGESTION AND NUTRITION. 217 



less, it will be noticed that the proportion of proteid in some of"the~vegetables 

 is not at all insignificant. They are characterized by their excess in carbohy- 

 drates rather than by a deficiency in proteids. The composition of peas and 

 other leguminous foods is remarkable for the large percentage of proteid, 

 which exceeds that found in meats. Analyses such as are given here are 

 indispensable in determining the true nutritive value of foods. Nevertheless, 

 it must be borne in mind that the chemical composition of a food is not alone 

 sufficient to determine its precise value in nutrition. It is obviously true that 

 it is not what we eat, but what we digest and absorb, that is nutritious to the 

 body, so. that, in addition to determining the proportion of food-stuffs in any 

 given food, it is necessary to determine to what extent the several constitu- 

 ents are digested. This factor can be obtained only by actual experi- 

 ments ; a number of results bearing upon this point have been collected which 

 will be spoken of later. It may be said here, however, that in general 

 the proteids of animal foods are more completely digestible than are those 

 of vegetables, and with them, therefore, chemical analysis comes nearer to 

 expressing directly the nutritive value. 



The physiology of digestion consists chiefly in the study of the chemical 

 changes which the food undergoes during its passage through the alimentary 

 canal. It happens that these chemical changes are of a peculiar character. 

 The peculiarity is due to the fact that the changes of digestion are effected 

 through the agency of a group of bodies known as enzymes, or unorganized 

 ferments, whose chemical action is different from that of the ordinary reagents 

 with which we have to deal. It will save useless repetition to give here 

 certain general facts that are known with reference to these bodies, reserving 

 for future treatment the details of the action of the specific enzymes found in 

 the different digestive secretions. 



Enzymes. Enzymes, or unorganized ferments, or unformed ferments, is 

 the name given to a group of bodies produced in animals and plants, but not 

 themselves endowed with the structure of living matter. The term unorganized 

 or unformed ferment was formerly used to emphasize the distinction between 

 these bodies and living ferments, such as the yeast-plant or the bacteria. 

 " Enzyme," however, is a better name, and is coming into general use. 

 Enzymes are to be regarded as dead matter, although produced in living 

 protoplasm. Chemically, they are defined as complex organic compounds con- 

 taining nitrogen. Their exact composition is unknown, as it has not been 

 found possible heretofore to obtain them in pure enough condition for analysis. 

 Although several elementary analyses are recorded, they cannot be considered 

 reliable. It is not known whether or not the enzymes belong to the group of 

 proteids. Solutions of most of the enzymes give some or all of the general 

 reactions for proteids, but there is always an uncertainty as to the purity of 

 the solutions. With reference to the fibrin ferment of blood, one of the 

 enzymes, observations have recently been made which seem to show that it 

 at least belongs to the group of combined proteids, nucleo-albumius (for 

 details see the section on Blood). But even should this be true, we are 



