360 FOOD AND DIGESTION 



first act is inhibited, and the contraction wave reaches the stomach six 

 seconds after the commencement of the second act. 



During the act of deglutition the posterior nares are closed through the 

 action of the levator palati and tensor palati muscles, which raise the velum; 

 the palato-pharyngei, drawing the posterior pillars of the fauces together; 

 and the azygos uvulae, which raises the uvula thus forming a complete cur- 

 tain. Otherwise the food would pass into the nose, as happens in the case 

 of cleft palate. At the same time the larynx is closed by the adductor 

 muscles of the vocal cords and the descent of the epiglottis, the larynx being 

 drawn upward as a whole through the action of the mylo-hyoid, genio-hyoid, 

 thyro-hyoid, and digastric muscles. The presence of the epiglottis is not 

 necessary for the completion of the act of deglutition. 



Nervous Mechanism of Deglutition. The sensory nerves engaged 

 in the reflex act of deglutition are branches of the fifth cerebral, supplying the 

 soft palate; the glosso-pharyngeal, supplying the tongue and pharynx; the 

 superior laryngeal branch of the vagus, supplying the epiglottis and the glottis. 

 The motor fibers concerned are branches of the fifth, supplying part of the 

 digastric and mylo-hyoid muscles and the muscles of mastication; the facial, 

 supplying the levator palati; the glosso-pharyngeal, supplying the muscles 

 of the pharynx; the vagus, supplying the muscles of the larynx through the 

 inferior laryngeal branch; and the hypoglossal, the muscles of the tongue. 

 The nerve center by which the muscles are harmonized in their action is 

 situated in the medulla oblongata. It cannot be definitely circumscribed, 

 but is in the general level of the vagus origin. The movements of the esoph- 

 agus are co-ordinated by the complex of sensory and motor fibers of the 

 fifth and the ninth to twelfth cranial nerves, which all take some part in this 

 complicated reflex. 



Cannon has demonstrated that the smooth muscle of the lower end of 

 the esophagus and around the cardiac orifice is maintained in contraction 

 by a local reflex mechanism. This prevents regurgitation of the foods. 

 The local apparatus is brought into action by the stimulation of sensory cells 

 in the mucous membrane of this region of the stomach by the acid of the 

 gastric secretion. The reflex is assumed to be a local one taking place through 

 the intrinsic nervous mechanism. This acid closure of the cardiac sphincter 

 is to be compared with the similar mechanism for the pylorus, see page 356. 



DIGESTION IN THE STOMACH. 



The stomach of man and the carnivora is the dilated portion of the ali- 

 mentary canal following the esophagus. The esophagus enters the stomach 

 at the cardiac end and the pyloric end of the stomach is continuous with the 

 duodenal part of the intestine. It varies in shape and size according to its 

 state of distention. It is supplied with nerves from the vagus and from the 

 sympathetic and receives a special artery, the gastric artery. 



