DIGESTION. 43 



What muscles are involved in the act of eating ? 



Biting movements are produced by action of the temporal, mas- 

 seter, and internal pterygoid muscles, opposing the depressors of 

 the jaw ; grinding movements are produced by the alternate action 

 of the external pterygoids. The action is to some extent reflex, 

 though largely voluntary, and is controlled in the medulla through 

 branches of the cranial nerves, motor impulses coming through 

 branches of the trigeminus and (to the tongue) hypoglossal nerves. 



What do we mean by the term " insalivation " ? 



The mixing and thorough moistening of the food-mass with the 

 saliva and mucus in the mouth during the act of chewing is called 

 insalivation. 



What is the saliva? 



The saliva is derived almost entirely from the parotid, submax- 

 illary, and sublingual glands, and is secreted most abundantly 

 during mastication of food. It is a transparent watery fluid, and 

 is somewhat viscid from the mixture of mucus. This viscidity 

 allows it to retain air-bubbles when churned by the action of the 

 tongue and cheeks. It has a sp. gr. of about 1006 and an alkaline 

 reaction. Chemically, it is mostly (99 1 per cent.) water, holding 

 in solution very small amounts of salts and proteids, with the addi- 

 tion of a ferment called ptyalin. The amount secreted in twenty- 

 four hours is estimated to be 1 to 2 quarts, most abundant secretion 

 occurring during mastication. 



What is the use of saliva ? 



It keeps the mouth in a moist condition, and so lubricates the 

 tongue in speaking and in chewing ; dissolves the soluble portions 

 of the food, and in this way brings them in contact with the organs 

 of taste ; mixes with the food, and forms a soft and slippery bolus 

 suitable for swallowing. Saliva from the front of the mouth, con- 

 taining more water, softens the bolus, while the tonsillar and 

 pharyngeal secretions, being mucous, coat it with a slippery sur- 

 face ; and the saliva has a special digestive function by the action 

 of its ferment, ptyalin. 



What is the action of ptyalin ? 



Ptyalin is a ferment (a non-nitrogenous body having an uncer- 

 tain chemical composition), with the special property of converting 

 cooked starch into grape-sugar. While the ferment itself cannot 



