96 ELEMENTARY PHYSIOLOGY 



these glands has several ducts, some of which open into the duct 

 of the corresponding submaxillary gland. 



The fluid secreted by the salivary glands the saliva is a 

 transparent, watery fluid, slightly alkaline * in its action. In the 

 mouth it is always mixed with the secretion of the mucous glands, 

 but if obtained direct from the duct of a salivary gland its compo- 

 sition is as follows : 



Water ..... 9,941 



(Mineral salts ... 23 



Ptyalin .... 14 



Other matters ... 22 



10,000 



Ptyalin (Gr. ptuo, I spit) is a nitrogenous substance, and may 

 be described as the active principle of the saliva, for the value 

 of the saliva as a digestive fluid depends on its presence. 



Many of the food-substances we eat contain a large proportion 

 of starch, which is an insoluble substance. No food-substance can 

 be of any use in the body until it has been brought to the liquid 

 state. Starch, therefore, is useless as a food until it has been dis- 

 solved. The ptyalin has the power of doing this. It converts 

 the insoluble starch into a soluble variety of sugar called grape- 

 sugar. 



In order to illustrate this action, put a little of the solid ptyalin 

 or some saliva, or even a portion of a dried salivary gland, into a 

 vessel with some starch, adding also some water unless the liquid 

 saliva is used. Keep the mixture at a temperature of about 100 

 F., and the conversion of starch into the soluble sugar commences 

 at once. 



In the mouth also this change commences as soon as the 

 saliva mixes with starchy foods, and some of the grape-sugar 

 formed is absorbed at once by the mucous membrane of the 

 mouth, by which means it enters the blood system, and its circu- 

 lation thus commences. The unchanged starchy substances are 

 swallowed, together with some saliva, and the conversion of starch 

 into sugar is continued in other parts ot the digestive tube. The 

 saiiva acts only on starchy foods. 



In addition to the action of the saliva just mentioned (which 

 may be termed a chemical action) it is useful also as a mechanical 

 agent and a general solvent. With the mucus it moistens the 

 mouth, thus facilitating speaking and assisting mastication. It 

 dissolves savoury substances, thus enabling us to taste them. It 



1 An alkali is a substance which has a soapy taste and feel, and has the 

 power of neutralising acids. Acids are distinguished by their sharp, sour 

 taste. Acids turn vegetable blue colours red ; alkalies turn vegetable reds 

 blue. 



