1000 DIGESTION 



Thus, it has been ascertained that the mucosa of the dog, if spread out 

 in a single layer, covers more than one-half of the body-surface. 



In man, the muscular stratum of the alimentary canal is made up of 

 smooth muscle tissue which is arranged in two layers, an outer longi- 

 tudinal and an inner circular. This arrangement is departed from in 

 the mouth, pharynx and stomach, where oblique fibers are added; 

 moreover, the mouth, pharynx, upper part of the esophagus, and end 

 of the rectum, contain numerous strands of striated muscle. Inter- 

 nally, the circular layer of smooth muscle tissue lies in relation with 

 areolar tissue, containing blood-vessels, lymphatics and nerves. It 

 forms the submucous coat. This in turn is clad with epithelium, 

 constituting the continuous mucous lining of the entire digestive tract. 

 Externally, the longitudinal layer of smooth muscle tissue is enveloped 

 by a thin serous layer, the peritoneum. The mechanical processes 

 associated with digestion, are mastication, deglutition, and the churn- 

 ing movements of the stomach, small intestine and large intestine. 



Mastication. — The articulation between the mandible and max- 

 illary bone is classified as a double condyloid joint. Owing to the 

 looseness and strength of its capsular ligament, the articular surfaces 

 of these bones may be moved freely upon one another, allowing the 

 mandible to execute three types of movements which may be classi- 

 fied as (a) depression and elevation, (h) projection and retraction, and 

 (c) deviation from side to side. Its raising is effected by the combined 

 contraction of the temporal, masseter, and internal pterygoid muscles, 

 and its depression by gravity and the action of the digastric muscle 

 in conjunction with the mylohyoid and geniohyoid. At this time, 

 the hyoid bone is fixed by the contraction of the'omohyoid and sterno- 

 hyoid muscles. When both external pterygoids contract simulta- 

 neously, the jaw is protruded. The opposite movement is effected by 

 the internal pterygoids. The contraction of only one set of these 

 antagonistic muscles gives rise to a deviation of the jaw toward one 

 side or the other. 



The grinding motions of mastication consist chiefly in a lowering 

 and raising and a lateral deviation of the jaw, the food being kept 

 between the molar teeth by the action of the tongue, the orbicularis 

 oris and the buccinators. The action of these parts is controlled by a 

 reflex center which is situated in the medulla oblongata and includes 

 the nuclei of the motor nerves innervating the aforesaid muscles, 

 namely, those of the trigeminal, facial and hypoglossal nerves. On 

 the afferent side, this center is connected with different receptors, and 

 particularly with the spindles of the muscles concerned in this act. 

 By this means, the force and character of the movements of the jaw are 

 reflexly regulated. The closure of the lips and depression of the tongue 

 and jaw during inspiration may give rise to a negative pressure in the 

 oral cavity, approximating 25 to 50 cm. H2O. 



The importance of mastication differs in different animals. In 

 the carnivora, the food is rapidly projected through the mouth and is 



