MOVEMENTS OF THE ALIMENTARY CANAL 425 



of the teeth and so gain foothold within the tooth substance itself, 

 and we have under way the too-familiar process of tooth decay. 

 Good teeth are so important for efficient mastication, as well as 

 for the appearance of the face, that no pains should be spared to 

 preserve them. Evidently the way to do this is to prevent the 

 accumulation of bacteria in the spaces between them. Thorough 

 cleaning, desirably after each meal, with the occasional use of an 

 antiseptic mouth-wash is fairly but not completely satisfactory. 

 Half yearly inspection and cleaning by a dentist are usually nec- 

 essary to supplement one's own efforts, because of the practical 

 impossibility of keeping every one of the small mouth spaces clear. 

 Such inspection also insures the discovery of decay while the cavi- 

 ties are still small, and makes possible the preservation of the 

 teeth in approximately normal condition for many years. 



Deglutition. A mouthful of solid food is broken up by the 

 teeth, and rolled about the mouth by the tongue, until it is thor- 

 oughly mixed with saliva and made into a soft pasty mass. This 

 mass is sent on from the mouth to the stomach by the process of 

 deglutition, which is described as occurring in three stages. The 

 first stage includes the passage from the mouth into the pharynx. 

 The food being collected into a heap on the tongue, the tip of that 

 organ is placed against the front of the hard palate, and then the 

 rest of the tongue is raised from before back, so as to press the 

 food mass between it and the palate, and drive it back through 

 the fauces. This portion of the act of swallowing is voluntary, or 

 at least is under the control of the will, although it commonly 

 takes place unconsciously. \JThe second stage of deglutition is that 

 in which the food passes through the pharynx; it is the most rapid 

 part of its progress, since the pharynx has to be emptied quickly 

 so as to clear the opening of the air-passages for breathing pur- 

 poses. The food mass, passing back over the root of the tongue, 

 pushes down the epiglottis ; at the same time the larynx (or voice- 

 box at the top of the windpipe) is raised, so as to meet it, and thus 

 the passage to the lungs is closed; muscles around the aperture 

 probably also contract and narrow the opening. The raising of 

 the larynx can be readily felt by placing the finger on the large 

 cartilage forming " Adam's apple " in the neck, and then swallow- 

 ing something. The soft palate is at the same time raised and 

 stretched horizontally across the pharynx, thus cutting off com- 



