278 DIGESTION 



great majority of the tubes are clothed from base to mouth with cells 

 which have gone through a process of mucous degeneration, and have taken 

 the form of goblet cells i. e., swollen structures with the nuclei in the basal 

 end (Fig. Ill, B). The contents of these cells behave exactly like those of 

 the typical mucous cells (Heidenhain). 



Likewise in the glands of the small intestine are found goblet cells be- 

 tween the true epithelial cells; but they are often wanting. Frequently they 

 occur separately in the region of the upper end of the tube, rarely also at 

 the lower end. 



From these facts it follows that the cells during rest undergo mucous 

 degeneration; in activity the mucus is discharged and the cell itself often 

 perishes in the process. Just to what extent the latter takes place has not 

 been finally determined. At the base of the epithelium are found here and 

 there small round cells which are looked upon as substitutes for the cells 

 which disintegrate. 



THIRD SECTION 



MOVEMENTS OF THE ALIMENTARY CANAL 



1. MASTICATION 



The movements of mastication are for the purpose of dividing the food 

 mechanically and of saturating it with saliva, so that a bolus may be pre- 

 pared suitable for swallowing. Mastication is accomplished by movements 

 of the lower jaw against the upper, the morsel of food being placed by appro- 

 priate movements of the tongue and cheeks, between the two rows of teeth, 

 and being ground up by the latter acting against each other under the force 

 of the powerful jaw muscles. 



The teeth and the mode of chewing are unmistakably adapted to the kind 

 of food eaten, so long as regard is had to the animals whose natural food is 

 exclusively animal or exclusively vegetable. In man the teeth and movements 

 of the jaw present no pronounced characteristics, doubtless because, with the 

 assistance of the art of cooking, he has learned how to subsist upon so many 

 different articles of diet. Raw meat cannot be properly reduced by the teeth of 

 man ; but if properly boiled or roasted so that the connective tissue binding the 

 muscle fibers together is loosened, it is easily masticated. Likewise the seeds 

 of cereals, which are otherwise incapable of being attacked by the human digest- 

 ive apparatus, are rendered fit for food by boiling or by baking. Among all 

 the articles of diet which are generally accessible to the higher animals, strictly 

 speaking only grass and hay are incapable of serving man as food. 



The actual grinding of the food is attended to primarily by the molar 

 teeth, those in the front of the mouth serving only for biting off morsels of 

 suitable size. The lower jaw is drawn upward by the masseter and temporal 

 muscles, forward and upward by the internal pterygoids, forward by the 

 external pterygoids. It is drawn downward by the digastric, the mylohyoid 

 and the geniohyoid, and backward by the posterior belly of the digastric. 



