THE ALIMENTARY CANAL. 



THIS long and complicated tube, extending from the mouth to the anus, 

 is developed from the entoderm with a mesodermic envelope, except at the 

 two ends, each of which is at first a pouch lined by ectoderm. It consists 

 of the mouth, pharynx and oesophagus above the diaphragm, and of the 

 stomach and small and large intestine below the diaphragm. There are 

 many accessory organs connected with the alimentary canal whose primary 

 function is to assist in the processes of digestion. The most important of 

 these above the diaphragm are the teeth, the tongue and the salivary glands; 

 those below the diaphragm are glands of various kinds, mostly so small as 

 to be contained within the mucous membrane. Two large organs, however, 

 the liver and the pancreas, belong to this class, both being primarily out- 

 growths from the early gut-tube. The general structural plan of the alimen- 

 tary canal, presenting in places, however, great modifications, includes: (i) 

 alining of mucous membrane ; (2) a submucoiis layer of connective tissue 

 into which glands may penetrate from the mucosa; (3) a double layer of un- 

 striped muscle, arranged, for the most part, as an inner circular and an 

 outer longitudinal stratum; and, below the diaphragm, (4) a. serous covering 

 from the peritoneum, which, although originally complete, is in the adult 

 wanting in certain parts. 



THE ORAL CAVITY. 



The Mucous Membrane. The histological transition from the skin 

 covering the exterior of the lips to the 'oral mucous membrane takes place 

 gradually, the two being connected by a broad intermediate zone which 

 approximately corresponds to the red area of the lips. The oral mucous 

 membrane is everywhere covered with stratified squamous epithelium, from 

 .2-. 4 mm. in thickness, which presents the details of the varying strata of 

 cells typical of such structures (page 17). When for any reason the large 

 flat surface cells are not removed, as ordinarily they continually are by abra- 

 sion, they form a whitish semiopaque film that masks the rosy tint of the 

 oral mucosa. The tunica propria consists of closely felted bundles of fibrous 

 tissue and elastic fibres, and passes into the submucous stratum without 

 sharp demarcation. Towards the surface supporting the epithelium, the 

 bundles become more delicate and closer, so that the stroma acquires a less 

 fibrous and more homogeneous appearance. The subepithelial border of the 

 tunica propria is beset with innumerable minute elevations, the papilla, 

 which are especially well developed on the lips, the anterior part of the hard 

 palate and the gums. The papillae of the tongue are special structures and 

 are, therefore, here not considered. Within the elevations, which contain 

 the vascular loops and nerves, the stroma is relatively, compact and homo- 

 geneous. In addition to the ordinary connective tissue cells, leucocytes and 

 mast-cells are frequently encountered within the stroma; the mast-cells, dis- 

 tinguished by their coarse basophilic granules, are particularly abundant in 

 the gum, close to the neck of the tooth. The oral mucous membrane is 

 attached to the surrounding bones and muscles by the submucous layer, a 

 stratum of generally loose fibro-elastic tissue containing the larger blood- 

 vessels, lymphatics and nerve-trunks and the small oral glands. According 

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