220 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



action of the enzymes ; it is well illustrated by the action of invertin on cane- 

 sugar, as follows : 



C 12 H 22 O n +H 2 = C 6 H 12 6 + C 6 H 12 6 



Cane-sugar. Dextrose. Levulose. 



In what way enzymes cause the substances they act upon to take up water is 

 unknown. The fact that they are not themselves used up in the reaction pro- 

 portionally to the change they cause formerly influenced physiologists and chem- 

 ists to explain their effect as due to catalysis, or contact action. In its original 

 sense this designation meant that the molecules of enzyme act by their mere 

 presence or contiguity, but it need scarcely be said that a statement of this 

 kind does not amount to an explanation of their manner of action ; to say they 

 " act by catalysis " means nothing in itself. Efforts to explain their action 

 have resulted in a number of hypotheses, a detailed account of which would 

 hardly be appropriate here. It may be mentioned that two ideas have found 

 most general acceptance : one, that the vibrations of the molecules of enzyme 

 set into more rapid vibration the molecules of the substance acted upon, thus 

 leading to the taking up of water and to the subsequent splitting ; the other idea 

 is that the enzyme enters into a definite chemical reaction, in which, however, 

 it plays the part of a carrier or go-between, so that, although the enzyme is 

 directly concerned in producing a chemical change, the final outcome is that it 

 remains in its original condition. A number of chemical reactions of this 

 general character are known, in which some one substance passes through a 

 cycle of changes, aiding in the production of new compounds, but itself 

 returning always to its first condition ; for example, the part taken by H 2 SO 4 

 in the manufacture of ether from alcohol, or the successive changes of haemo- 

 globin to oxyhsemoglobin and back again to hemoglobin after giving up its 

 oxygen to the tissues. Perhaps the most suggestive reaction of this character 

 is the one quoted by Chittenden l to illustrate this very hypothesis as to the 

 manner of action of enzymes, as follows : Oxygen and carbon monoxide gas, 

 if perfectly dry, will not react upon the passage of an electric spark. If, 

 however, a little aqueous vapor is present, they may be made to unite readily, 

 with the formation of CO 2 . The water in this case, without doubt, enters 

 into the reaction, but in the end it is re-formed, and the final result is as 

 though the water had not directly participated in the process. The reactions 

 supposed to take place are explained by the following equations : 



CO + 2H 2 + 2 = CO (OH) 2 + H 2 2 . 

 H 2 O 2 + CO = CO(OH) 2 . 

 2CO(OH) 2 = 2CO 2 + 2H 2 O. 



B. SALIVARY DIGESTION. 



The first of the digestive secretions with which the food comes into contact 

 is saliva. This liquid is a mixed secretion from the six large salivary glands 

 (parotids, submaxillaries, and sublinguals) and the smaller mucous and serous 

 1 Cartwright Lectures, Medical Record, New York, April 7, 1894. 



