Chap. XV] DIGESTIVE PROCESSES 291 



Within the body the change takes place at body temperature, 

 and is due to the presence of organic ferments or enzymes present 

 in all of the digestive juices. 



Enzymes. — An enzyme is an organic ferment produced by living 

 cells, and is capable of effecting chemical change without itself 

 undergoing alterations in the process. Each enzyme has a definite 

 action at a suitable temperature, and will only work in a medium 

 of definite reaction, either acid or alkaline. Further, the products 

 of the action must be removed, and in the body this is accomplished 

 by absorption. 



CHANGES THE FOOD UNDERGOES IN THE MOUTH 



Mastication. — \\Tien solid food is taken into the mouth it is 

 cut and ground by the teeth, being pushed between them again 

 and again by the muscular contractions of the cheeks and the 

 movements of the tongue until the whole is thoroughly crushed 

 and ground down. 



Insalivation. — During the process of mastication saliva is 

 poured in large quantities into the mouth, and mixing with the 

 food moistens it and reduces it to a soft, pulpy condition, A cer- 

 tain amount of air caught in the bubbles of the saliva also becomes 

 entangled in the food, and this facilitates the penetration of the 

 gastric juice. 



Secretion of saliva. — The secretion of saliva is the result of 

 reflex stimulation of the nerves connected w^ith the salivary glands. 

 The movements of mastication, the taste and odor of food, act as 

 stimulants to the sensory or afferent nerves which carry these im- 

 pulses to a nerve centre in the brain (probably in the medulla 

 oblongata), and from thence motor impulses are transmitted 

 through efferent nerves to the gland. Psychical acts may also 

 influence the secretion of saliva, as for example the thought or 

 smell of food, or a feeling of nausea may stimulate the secretion, 

 and anger, fear, or worry may inhibit it. 



Saliva. — Saliva is a mixture of the secretions of all three pairs 

 of salivary glands, as well as of the small glands of the mucous 

 membrane of the mouth. It consists of water, some mucus, 

 and an enzyme called ptyalin. It has a specific gravity of 1.004 

 to 1.008 and an alkaline reaction. The amount secreted in twenty- 

 four hours is estimated to be from one to two quarts. 



