THE MUSCULAR AND NERVOUS MECHANISMS OF 

 THE DIGESTIVE TRACT. 



BY E. H. STARLING. 



CONTENTS : Mastication, p. 313 Sucking, p. 314 Deglutition, p. 314 Move- 

 ments of the Stomach, p. 321 Vomiting, p. 324 Movements of the Intestines, 

 p. 326 Defecation, p. 335. 



MASTICATION. 



IN order to prepare the solid food-stuffs for the action of the digestive 

 juices in the alimentary canal, they are reduced to a fine state of sub- 

 division in the mouth by the process of mastication. In this act the 

 incisors divide the lump of food into smaller portions, which are then 

 ground up between the molar teeth. The necessary movements for these 

 processes are carried out by the muscles acting on the lower jaw, the 

 temporo-maxillary articulation allowing of elevation and depression of 

 the lower jaw, as well as forward, backward, and side-to-side movements. 

 This dental triturating apparatus would be of no avail without some 

 means of keeping it constantly supplied with fresh particles of food. 

 The renewal of the food between the teeth is carried out by movements 

 of the tongue and cheeks. Bilateral paralysis, sensory or motor, of the 

 tongue, renders mastication almost impossible. Paralysis of the cheek 

 (buccinator) on one side causes the food to collect and form a pouch 

 outside the molar teeth. In such cases, the patient assists mastication 

 by emptying the pouch with his finger. 



With regard to the muscles involved in the various movements by 

 which mastication is performed, the lower jaw is raised by the action of 

 the masseter, temporal, and internal pterygoid muscles. Depression 

 of the lower jaw is largely passive, but is also aided by the digastric and, 

 to a slighter extent, by the mylohyoid and geniohyoid muscles. The 

 grinding movements of the jaw sidewards, forwards, and backwards 

 are carried out by contractions of the external pterygoid muscles of the 

 two sides in varying degree. 



Of the muscles affecting the tongue, the genioglossus and transverse 

 fibres move this organ forwards, while the hyoglossi, palato- and stylo- 

 glossi cause retraction. Movement from one side to the other is effected 

 by contraction of the longitudinal muscular fibres. Contraction of the 

 hyoglossi alone will make the upper surface of the tongue convex from 

 side to side ; contraction of the genioglossi will make it concave. 



The act of mastication is a voluntary one, but, like most other co- 

 ordinated muscular actions, is dependent for its guidance on sense 

 impressions. The sensory nerve involved is the fifth. The efferent 



