Thus, while the food is being chewed, another process is going on by which starch is changed into sugar. The saliva also moistens the food into a mass for swallowing, and aids in speech by keeping the mouth moist. The activity of the salivary glands is largely regulated by their abundant supply of nerves. Thus, the saliva flows into the mouth, even at the sight, smell, or thought of food. This is popularly known as “making the mouth water.” The flow of saliva may be checked by nervous influences, as sudden terror and undue anxiety. Experiment 56. _To show the action of saliva on starch_. Saliva for experiment may be obtained by chewing a piece of India rubber and collecting the saliva in a test tube. Observe that it is colorless and either transparent or translucent, and when poured from one vessel to another is glairy and more or less adhesive. Its reaction is alkaline to litmus paper. Experiment 57.Make a thin paste from pure starch or arrowroot. Dilute a little of the saliva with five volumes of water, and filter it. This is best done through a filter perforated at its apex by a pin-hole. In this way all air-bubbles are avoided. Label three test tubes _A, B_, and _C_. In _A_, place starch paste; in _B_, saliva; and in _C_ one volume of saliva and three volumes of starch paste. Place them for ten minutes in a water bath at about 104° Fahrenheit. Test portions of all three for a reducing sugar, by means of Fehling’s solution or tablets.[21] _A_ and _B_ give no evidence of sugar, while _C_ reduces the Fehling, giving a yellow or red deposit of cuprous oxide. Therefore, starch is converted into a reducing sugar by the saliva. This is done by the ferment ptyalin contained in saliva. 137. The Pharynx and Œsophagus. The dilated upper part of the alimentary canal is called the pharynx. It forms a blind sac above the level of the mouth. The mouth opens directly into the pharynx, and just above it are two openings leading into the posterior passages of the nose. There are also little openings, one on each side, from which begin the Eustachian tubes, which lead upward to the ear cavities. The windpipe opens downward from the pharynx, but this communication can be shut off by a little plate or lid of cartilage, the epiglottis. During the act of swallowing, this closes down over the entrance to the windpipe, like a lid, and prevents the food from passing into the air-passages. This tiny trap-door can be seen, by the aid of a mirror, if we open the mouth wide and press down the back of the tongue with the handle of a spoon (Figs. 46, 84, and 85). Thus, there are six openings from the pharynx; the œsophagus being the direct continuation from it to the stomach. If we open the mouth before a mirror we see through the fauces the rear wall of the pharynx. In its lining membrane is a large number of glands, the secretion from which during a severe cold may be quite troublesome.