THE ALIMENTARY CANAL 



This is a long and complicated tube extending from the mouth to the anus. 

 Excepting the two ends, each of which is at first a pouch from the ectoblast, it is 

 developed from the entoblast with a mesoblastic envelope. It consists of the 7no7ith, 

 pharynx, and cesophagus above the diaphragm, and of the stomach and small and 

 large intesthies below it. There are many accessory organs connected with it whose 

 primary function is to assist in the process of nutrition. The chief ones above the 

 diaphragm are the teeth, the tongue, and ihe. salivary glands; those below it are 

 glands of various kinds, mostly so small as to be contained in the mucous membrane. 

 But two distinct organs, the liver d^rxd \h& payicreas , belong to this class, both being 

 originally outgrowths from the gut. The trachea and lungs have a similar origin, 

 but their physiological function is so different that they are treated of under a separate 

 heading. 



The general structural plan of the digestive tube, presenting in places great mod- 

 ifications, is : (i) a lining of mucous membrane ; (2) a submucous layer of areolar 

 tissue, into which glands may penetrate from the former ; (3) a double layer of non- 

 striped muscular fibres, of which, as a rule, the inner is circular and the outer longi- 

 tudinal ; (4) below the diaphragm, a serous covering from the peritoneum, which, 

 although originally complete, is in the adult wanting in certain parts. 



The length of the alimentary canal is, on the average, not far from 9 m. (ap- 

 proximately 30 ft.), of which not more than 45 cm. (about 18 in.) is above the 

 diaphragm. A preliminary sketch of the divisions above the diaphragm may be con- 

 venient. The vestibule of the mouth is the space between the lips and cheeks exter- 

 nally and the jaws and teeth internally. The (potential) cavity of the month is within 

 the arches of the gums and teeth. It is bounded above by the hard palate and its 

 backward continuation the soft palate. The greater part of the floor is occupied by 

 the tongue. There is a free horseshoe-shaped space beneath the tongue within the 

 lower jaw, called the alveolar-lingual groove or, better, the sublingual space. The 

 pharynx joins the mouth at the anterior pillar of the fauces, a fold passing outward 

 and downward from the soft palate to the tongue. The pharynx extends from the 

 base of the skull to the lower border of the larynx. The upper part, the naso- 

 pharynx, is behind the nasal chambers which open into it, the oro-pharynx is behind 

 the mouth, and the laryngo- phatynx behind the larynx. At the lower border of the 

 larynx it is followed by the cesophag2is, a long tube which, piercing the diaphragm, 

 opens into the stomach. 



THE MOUTH. 



The framework of the mouth is made by the hard palate and the alveolar 

 processes of the upper jaw, by the greater part of the body (including the alveolar 

 processes) of the lower jaw and part of the ramus, and by the hyoid bone, to which 

 may be added the mylo-hyoid muscle forming the floor. 



When the lips are opened and the lower jaw dropped, the mouth is a true cavity 

 extending to the pharynx ; when these parts are closed, the tongue fills practically the 

 whole space. It is convenient, however, to speak of the cavity of the mouth. This 

 space is subdivided into the vestibule or preoral cavity and that of the oral cavity or 

 mouth proper. The former is the region between the closed lips and cheeks in front 

 and the closed jaws and teeth behind. When the lips are closed, it communicates with 

 the mouth proper only by a small passage behind the wisdom-teeth, in front of the 

 ramus of the jaw. 



THE LIPS, CHEEKS, AND VESTIBULE. 



The orifice of the mouth (rima oris) is a transverse sHt of variable length, 

 bounded by projecting folds, — the lips. These, like the cheeks, with which they are 

 continuous, are composed of complicated layers of muscle, covered externally by 

 skin and internally by mucous membrane. 



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