Human physiology / by William Furneaux.

  • William Samuel Furneaux
Date:
1909
Catalogue details

Licence: In copyright

Credit: Human physiology / by William Furneaux. Source: Wellcome Collection.

    108/270 page 96
    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 Solids-j Ptyalin .... 14 I Other matters ... 22 10,000 Ptyalin (Gr. pt-uo^ 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 saliva 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 ’ 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.
    digitised image 1
    digitised image 2
    digitised image 3