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 OEsophagus.  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 oesophagus 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.

The oesophagus, or gullet, is a tube about nine inches long, reaching from the throat to the stomach.  It lies behind the windpipe, pierces the diaphragm between the chest and abdomen, and opens into the stomach.  It has in its walls muscular fibers, which, by their worm-like contractions, grasp the successive masses of food swallowed, and pass them along downwards into the stomach.

138.  Deglutition, or Swallowing.  The food, having been well chewed and mixed with saliva, is now ready to be swallowed as a soft, pasty mass.  The tongue gathers it up and forces it backwards between the pillars of the fauces into the pharynx.

If we place the fingers on the “Adam’s apple,” and then pretend to swallow something, we can feel the upper part of the windpipe and the closing of its lid (epiglottis), so as to cover the entrance and prevent the passage of food into the trachea.

There is only one pathway for the food to travel, and that is down the oesophagus.  The slow descent of the food may be seen if a horse or dog be watched while swallowing.  Even liquids do not fall or flow down the food passage.  Hence, acrobats can drink while standing on their heads, or a horse with its mouth below the level of the oesophagus.  The food is under the control of the will until it has entered the pharynx; all the later movements are involuntary.

[Illustration:  Fig. 51.—­A View into the Back Part of the Adult Mouth.  (The head is represented as having been thrown back, and the tongue drawn forward.)