ALIMENTATION 107 



b. Amylase, secreted by the pancreas, converts starch into 

 sugar (maltose). Ptyalin and amylase have a similar action on 

 starch. 



c. Glycogenase, produced in the liver, converts glycogen to 

 dextrose. It is also found in muscle. 



3. Inverting enzymes, secreted by the small intestine princi- 

 pally, act in an alkaline or neutral medium ; they convert double 

 sugars (disaccharides) into simple sugars (monosaccharides). 



a. Sucrase secreted by the small intestine; converts cane 

 sugar to dextrose and levulose. 



b. Maltase, secreted by the small intestine, salivary glands, 

 and pancreas, converts maltose to dextrose. 



c. Lactase, secreted by the small intestine, converts lactose 

 (milk sugar) into dextrose and galactose. 



4. Lipolytic enzyme, acting on fat in an alkaline or neutral 

 medium. 



a. Lipase, secreted by the pancreas, acts in conjunction 

 with bile to split neutral fats into fatty acids and glycerin, in the 

 small intestine. 



5. Coagulating enzyme. 



a. Rennin, secreted by the stomach in an inactive form, 

 prorennin, which is activated by the acid of the stomach, converts 

 the casein of milk into an insoluble protein, the curd, which is then 

 acted upon by pepsin. 



Foodstuffs. — The useful constituents of food consist of the 

 following foodstuffs: water, inorganic salts, proteins, fats, and 

 carbohydrates. These substances are foods in the sense that 

 they yield energy or serve as the raw material for the replace- 

 ment of the losses involved in metabolism. The living state is 

 accompanied by continuous physical and chemical changes in the 

 cells of the body, as a result of which, energy, largely in the form 

 of heat, is evolved along with carbon dioxide, water, inorganic 

 salts, and other excretory products. 



Calorimetry. — By means of a respiration calorimeter it is 

 possible to demonstrate that the energy of heat and work is 

 obtained from the potential energy of the food. The potential 

 energy of foodstuffs can be determined by burning or oxidizing 

 them in a calorimeter, which is an apparatus by means of which 

 the heat liberated during oxidation may be accurately measured. 

 Naturally only organic foodstuffs are involved in this considera- 



