166 THE DIGESTIVE SYSTEM 



is similar to that found in the parotid. (Fig. 103.) There are two 

 types of cells in the alveoli or secreting end-pieces. A serous type 

 of cell similar to that found in the parotid may be found forming 

 many of the alveoli. Another type of cell is present also, the 

 mucous cell, with lighter, clearer cytoplasm, which takes a basic 

 stain and has basally located nuclei. (Fig. 104.) Outside of some 

 of these mucous cells are crescents of serous cells. The secretory 

 ducts are more extensive in the submaxillary than in the parotid 

 gland. In man, serous cells predominate, but in many other 

 animals, such as the woodchuck and the cat, the mucous cells 

 predominate. 



Suhlingval Glands.— The sublingual glands are located at the 

 base of the tongue and, like the parotid and submaxillary, are divided 

 into lobes and lobules. The intercalated ducts are lacking as may 

 also be a large proportion of the secretory ducts. Most of the 

 glandular end-pieces (in the cat) are composed of cells of the mucous 

 type, w^ith more numerous crescents of serous cells than in the 

 submaxillary. 



The parotid, submaxillary, and sublingual glands are uniformly 

 present only in mammals. There are few, if any, glands associated 

 with the mucous membrane of the mouth cavity of fishes, but 

 mucous glands appear in the oral cavity of Amphibia. In reptiles, 

 mucous glands are located in the mouth ca^dty near the edge of 

 the jaws, in the tongue, in the wall of the mouth cavity, and also 

 under the tongue. The poison glands of snakes are specialized 

 labial glands. ]\Iucous glands are well represented in this region 

 in herbixorous mammals that feed largely on dry grass. 



Pharynx.— The pharynx is posterior to the oral cavity. The 

 pathways of food from the oral cavity to the esophagus, and of air 

 from the nasal passages to the larynx, cross in the pharynx so that 

 this chamber is part of both the digestive and respiratory systems. 

 Embryologically, it is associated with gills, with the trachea and 

 lungs, with the thyroid gland, parathyroid glands, and the thymus, 

 although presenting a great deal of variation in different ^■erte- 

 brates. It is somewhat funnel-shaped, exjjanded anteriorly and 

 tapered down posteriorly. 



THE ALIMENTARY CANAL. 



Througiiout the alimentary or digestive canal, but not including 

 the month or anus, there is a similar structural plan for the wall, 

 which is com])osed throughout of four coats of tissues. 



