CARBOHYDRATES. 59 



of great importance; and a detailed discussion is justifiable, furthermore, 

 because it is very probable that also in the animal organism chemical 

 changes take place by means of which a substance belonging to one class 

 is changed into a compound of another, as, for example, a fat into a car- 

 bohydrate or conversely. 



The animal organism is, in its entire existence, dependent upon this 

 carbonic acid assimilation by the plants, for in this way it obtains all the 

 organic compounds of complicated structure. Energy originating in the 

 sun is thus obtained by the animal in the form of potential energy, from 

 which the animal derives its kinetic energy and ability to perform work. 

 The assimilation by the plant not only serves to furnish organic material 

 for the animal organism, but to a certain extent it furnishes the oxygen 

 which it requires for obtaining the kinetic energy again by combustion; 

 the assimilation process being one of reduction in which oxygen is con- 

 stantly being evolved. The oxygen in this way returns to the general 

 cycle of the elements. This oxygen, after taking part in the metabolism 

 of the animal, escapes chiefly as carbon dioxide and water, both of which are 

 again utilized by the plant for the formation of other organic substances. 



Let us turn now to those carbohydrates which are most important as 

 food for the animal organism, namely, grape-sugar, cane-sugar, and starch. 

 From the last two compounds the animal obtains all the carbohydrates 

 that it needs. Let us follow these sugars on their way through the ali- 

 mentary canal to their absorption and final assimilation. As an example, 

 we shall choose starch, because it is here that the relations are the most 

 complicated, and we shall be able to treat of the behavior of the 

 simpler sugars in connection with the separate phases in the breaking 

 down of the starch. 



First of all starch or, strictly speaking, the food containing it 

 is ground up with the saliva by the act of chewing. The saliva con- 

 tains a diastatic ferment, ptyalin, 1 which converts the starch into dextrins 

 and finally largely into maltose. 2 The latter is inverted by means of a 



1 Ptyalin is not found in the saliva of all animals, e.g., that of the carnivora. It 

 would be well to drop the name ptyalin as it tends to give one the impression that 

 there is only one ferment found in the saliva. At present we know only of its 

 action, which coincides with that of various other ferments found in the animal and 

 vegetable kingdoms. It is better, therefore, to speak of a diastatic or amylolytic fer- 

 ment. We would be justified in using a special name for the ferment of unknown com- 

 position only when it has an unusual action; thus, for example, if the diastase in saliva 

 led to different cleavage-products of starch than do the ferments of other origin. 



2 The earlier assumption, that glucose is formed directly, has been shown to be false. 

 Cf. J. Seegen: Zent. med. Wissensch. 14, 849 (1876), and Pfliiger's Archiv. 19, 106 

 (1879). Otto Nasse: ibid. 14, 473 (1877). Musculus and v. Mering : Z. physiol. 

 Chem. 1, 395 (1877-78). Ibid. 2, 403 (1878-79); ibid. 4, 93 (1880). von Mering: ibid. 

 5, 185 (1881). Brown and Heron: Ann. 199, 165 (1879); 204, 228 (1880). Kiilz and 

 Vogel: Z. Biol. 31, 108 (1895). 



