DIGESTION 141 



The Specific Action of Enzymes. The number of enzymes in the 

 digestive fluids and in the various tissues of the body has given rise to the 

 idea, which has been confirmed by experiment, that an enzyme exerts its 

 action on but one substrate, in other words, that its action is specific. Thus 

 an enzyme that would transform starch into sugar would not be capable of 

 causing a cleavage of fat into a fat acid and glycerin; an enzyme that would 

 cause a cleavage of saccharose would not be capable of causing a cleavage of 

 lactose. So with all other enzymes. Each seems to be specially adapted to 

 catalyze but one substrate under given conditions. Various other features 

 of enzymes, their mode of action, their origin from preexisting substances, 

 the methods by which they are made active, the conditions under which 

 they are most active, etc., will be mentioned in connection with a considera- 

 tion of the fluids and tissues in which they are present. 



MOUTH DIGESTION 



The digestion of the food as it takes place in the mouth comprises 

 a series of physical and chemic changes, the result of the action of the 

 teeth, the tongue, and the saliva. The mechanic division of the food 

 and the incorporation of the saliva with it are termed respectively mastication 

 and insalivation. 



MASTICATION 



Mastication is the mechanic division of the food, and is accomplished 

 by the teeth under the movements of the lower jaw, brought about by the 

 alternate contractions of different groups of muscles. Complete mechanic 

 disintegration of the food is important for its subsequent solution and 

 chemic transformation ; for when finely divided it presents a larger surface 

 to the action of the digestive fluids and thus enables them to exert their 

 respective actions more effectively and in a shorter period of time. 



The Teeth. In man passing from childhood to adult life two sets 

 of teeth make their appearance. The first set constitutes the temporary, 

 deciduous, or milk teeth; the second set constitutes the permanent teeth, 

 which should last with proper care through life or to an advanced age. 



The temporary teeth, twenty in number, ten in each jaw, though smaller 

 than the permanent teeth, have the same general conformation. They 

 are divided into four incisors, two cuspids or canines, and four molars for 

 each jaw. 



The permanent teeth, thirty-two in number, sixteen in each jaw, are 

 divided into four incisors, two cuspids or canines, four bicuspids or pre- 

 molars, and six molars for each jaw. 



Each .tooth may be said to consist of three portions: (i) the crown, 



water. In this instance after a certain percentage of these substances has thus united the prod- 

 ucts of the reaction begin to recombine with the formation of the original compounds until the 

 opposing tendencies are in equilibrium, a state in which they remain so long as the conditions 

 remain unchanged. Again, if maltose and water be mixed, a reaction occurs which eventuates in 

 the formation of dextrose. In time the dextrose molecules combine to form maltose and water 

 until a condition of equilibrium is established. If the yeast enzyme be added the reactions both 

 analytic and synthetic are increased in velocity. The enzyme, however, does not initiate, but 

 merely hastens a reaction already taking place. 



