CHANGES OF FOOD IN THE ALIMENTARY CANAL. 



143 



4. Of the Changes which the Food undergoes during its passage along 



the Alimentary Canal. 



101. The object of the Digestive process, as already pointed out, is to 

 reduce the Alimentary matters to a condition in which they can be intro- 

 duced by Absorption into the Circulating system. This reduction is partly 

 effected, as we have seen, by Mechanical means ; but it is chiefly due to the 

 Chemical agencies which are brought to bear upon the ingested substances, 

 during their transit through the mouth, the stomach, and the upper portion 

 of the intestinal tube. The first of these is exerted by the Salivary fluid, 

 which is incorporated with the food in the act of mastication, and of which 

 a large quantity descends with it to the stomach. For the secretion of this 

 fluid, it will be remembered that three pairs of glands of considerable size 

 are provided, namely, the parotid, the sublingual, and the submaxillary. 

 But in addition to these there is a small pair situated just at the tip of the 

 tongue, whilst a very important part of the fluid is furnished by the lingual 

 and palatine glands, and by the numerous follicular glands lodged in and 

 beneath the buccal mucous membrane. The larger Salivary glands are all 

 constructed upon the type of tubular glands, the principal features of which 

 are shown in Figs. 60 and 61. The excretory ducts (d, Fig. 61), which are 



FIG. GO. 



FIG. 61. 



FIG. GO. Lobule of Parotid Gland of newborn infant, injected with mercury. Magnified oOdiameters. 

 FIG. 61. Submaxillary Gland of the Dog. a, mucous cells ; b, protoplasm cells ; c, demilune of Gian- 

 nuzzi ; d, transverse section of an excretory duct, with its peculiar columnar epithelial cells. 



lined by columnar epithelium, divide and subdivide till they terminate in a 

 series of follicles, which contain (to a greater extent in the case of the sub- 

 maxillary than of the others) large transparent non-granular cells, with au 

 eccentric nucleus (a a) ; and in addition a mass of what appears at first sight 

 to be granular protoplasm, with interspersed nuclei, the "demilune" of Gi- 

 anuuzzi 1 (c), but later investigations show that this is composed of many 

 closely-compressed cells. Other vesicles contain these protoplasm cells alone. 2 

 Ewald and others have observed a fine plexus surrounding the individual 



1 Bericht d. Sachs Gesellsch. d. Wiss., 1866, p. G8. 



2 Pfliiger, Schultze's Archiv, Bd. v, p. 203. 



