DIGESTION. 



15 



, (i) its inappreciable quantity ; 



(2) the dependence of its activity upon temperature, it 



being most active at about 35° C, and entirely 

 destroyed by boiling ; 



(3) the fact that it apparently undergoes no chapge itself 



while active, since a very small quantity will trans- 

 form an unlimited amount of starch if the sugar be 

 removed as it is formed. 

 It therefore belongs to the class of bodies known to chemists 

 as ferments, and as there are many such active within the 

 body, we have dwelt at some length upon the general 

 properties of this, the first one we meet with. 



The food cut into pieces by the teeth, moistened, and in 

 part changed by the saliva, is swallowed, and passes into the 

 , stomach, where it is mixed with a fluid, the gastric juice, 

 which is manufactured in glands situated in the walls of the 

 stomach. The walls are muscular, and their contractions 

 churn and mix the food with the juice. This juice has no 

 effect upon the starch, and little upon the fat of the food ; 

 its important action is upon the more complex proteids, 

 which, being changed and dissolved, are rendered capable 

 of being absorbed into the blood. There is in gastric juice 

 a quantity of free hydrochloric acid, and a ferment called 

 pepsin, analogous in its conduct and dependence on tem- 

 perature to the ptyalin of the saliva. The acid and the 

 ferment act together in the process of digestion, neither of 

 them by itself has any effect, but the acid stops the action 

 of the ptyalin upon starch. 



[The action of the gastric juice upon milk is peculiajjjtjs 

 fi4«t-i .iMi11 r(l j - f m d tTie P par t is dig ested. Milk contains fat, 

 sugar,^ ^lts, and proteids, especially one called casein ; the 

 c u r dling euusiblb ili the pi'eci pilaLiun Of ih'iB c as«n, and is • 

 not due to the acidity of the juice, for it takes place when 

 the acid is neutralised, but is caused by a ferment called 

 rennet.] 



There is now in the stomach a mixture of solid and dis- 

 solved foods ; those in solution are the proteids digested by 

 the gastric juice, and that portion of sugar transformed by 

 the saliva, besides, of course, any food-stuffs that may have 

 been taken as liquids. The solids are the fats and the rest 

 of the starch, though the fats are partially melted by the 



