DIGESTION. 195 



CHAPTER VII. 



DIGESTION, MASTICATION, INSALIVATION, AND DEGLUTITION. 



General arrangement of the digestive apparatus Prehension of solids and liquids Mastication Physiological anat- 

 omy of the teeth Anatomy of the maxillary bones Temporo-maxillary articulation Muscles of mastication- 

 Muscles which depress the lower jaw Action of the muscles which elevate the lower jaw and move it laterally 

 and antero-posteriorly Action of the tongue, lips, and cheeks in mastication Summary of the process of masti- 

 cation Parotid saliva Submaxillary saliva Sublingual saliva Fluids from the smaller glands of the mouth, 

 tongue, and fauces Mixed saliva Quantity of saliva General properties and composition of the saliva Action 

 of the saliva on starch Mechanical functions of the saliva Deglutition Physiological anatomy of the parts con- 

 cerned in deglutition Muscles of the pharynx Muscles of the soft palate Mucous membrane of the pharynx 

 (Esophagus Mechanism of deglutition First period of deglutition Second period of deglutition Protection of 

 the posterior nares during the second period of deglutition Protection of the opening of the larynx Function 

 of the epiglottis Study of deglutition by autolaryngoscopy Third period of deglutition Intermittent contrac- 

 tion of the lower third of the oesophagus Nature of the movements of deglutition Deglutition of air. 



THE inorganic alimentary principles are, with few exceptions, introduced in the form 

 in which they exist in the blood and require no preparation or change before they are 

 absorbed ; but the organic nitrogenized principles are always united with more or less 

 matter possessing no nutritive properties, from which they must be separated, and, even 

 when pure, they always undergo certain changes before they become part of the great 

 nutritive fluid. The non-nitrogenized principles also undergo changes hi constitution or 

 in form preparatory to absorption. With the varied forms in which food is taken by 

 different animals, we find great differences in the arrangement of the digestive apparatus, 

 from the simple pouch with a single orifice, which constitutes the entire digestive system 

 of many of the infusorial animalcules, to the immense length of intestine, with its numer- 

 ous glandular appendages, found in the mammalia. In the higher classes of animals, 

 great differences exist in the anatomy of the digestive organs, particularly as regards the 

 length and capacity of the alimentary canal. In the carnivora, in which the food con- 

 tains comparatively little indigestible residue, the intestine is but three or four times the 

 length of the body (i. e. from the mouth to the anus), and the colon, which receives the 

 residue of digestion, is of small capacity ; while in the herbivora, in which the bulk of 

 food, compared with its nutritious principles, is enormous, there are frequently four dis- 

 tinct cavities to the stomach, and the intestine is ten, twelve, and in some (the sheep) 

 twenty-eight times the length of. the body, with a colon of very large size. The food of 

 man is derived from both the animal and the vegetable kingdom, and, in relative length 

 and capacity, the alimentary canal is between that of the carnivora and the herbivora, 

 being from six to seven times the length of the body. 



A full meal probably occupies from two to four hours in its digestion, this depending, 

 of course, upon the kind of food, the fineness of its comminution by mastication, etc. The 

 matters taken into the stomach consist generally of all varieties of alimentary principles, 

 and they are exposed to certain mechanical processes in the mouth and alimentary canal 

 and to the action of various secreted fluids. 



In the mouth, the food is divided, as occasion demands, by the incisor teeth, and 

 is then passed, by the action of the cheeks and tongue, between the molars, where it is 

 subjected to mastication. During this process, it is mixed with the various fluids which 

 compose the saliva and becomes more or less coated with the tenacious secretions of the 

 mucous follicles of the buccal cavity. It is, or should be, reduced in the mouth to a pul- 

 taceous mass, with which the saliva, particularly that from the parotid gland, is thoroughly 

 incorporated. The secretion of the submaxillary and the sublingual gland, being more 

 viscid, has a tendency to coat the exterior of the alimentary bolus. 



By the action of the tongue, the alimentary bolus, after mastication, is passed back to 



