DIGESTION. 57 



middle, and inferior constrictors of the pharynx, which are concerned 

 in deglutition. Lymphoid tissue is very abundant at the upper back 

 part of the pharynx, and a number of lymph-follicles lie between the 

 orifices of the Eustachian tubes, forming the pharyngeal tonsil. 



(ESOPHAGUS. 



This tube extends from the fifth cervical down to the ninth dorsal 

 vertebra. It is about nine inches long and less than an inch in 

 diameter. It is narrowest at its commencement and gradually en- 

 larges. It has three coats: the outside, muscular; a middle coat, 

 fibrous; and an internal, or mucous, coat. The muscular coat has 

 a layer of longitudinal fibers and a layer of circular fibers ; the upper 

 end of the oesophagus has striated fibers, while the lower half has 

 plain, unstriped fibers. The mucous coat is paler than that of the 

 pharynx and mouth. In ordinary circumstances the mucous mem- 

 brane is in longitudinal folds. It contains minute papillae and a 

 squamous epithelium. The nerves of the oesophagus are the vagus 

 and the sympathetic. 



THE MECHANICAL PROCESSES OF DIGESTION OCCURRING 

 IN THE MOUTH, PHARYNX, AND (ESOPHAGUS. 



MASTICATION. 



This is a voluntary act whereby the food is comminuted by the 

 teeth, jaws, and muscles concerned in this act, aided by the tongue, 

 palate, cheeks, and lips. The bulk of the work is accomplished by 

 the biting and grinding movements of the lower teeth against the 

 upper ones. 



From the manner of its articulation with the skull the lower jaw 

 is capable of performing three primary movements, together with 

 combinations of these same, viz. : up and down, side to side, with 

 projection and retraction. The muscles concerned in producing these 

 movements are the masseter, temporal, and internal pterygoids, which 

 raise the lower jaw; the inferior maxillary division of the fifth nerve 

 innervates them. The depression of the jaw is accomplished mainly 

 through the action of the digastric, aided considerably by gravity. 

 The side-to-side, or lateral, movements are due to the separate action 

 of the external pterygoids. Their united contraction gives projection 

 of the lower mandible, to be retracted by a part of the temporal 

 muscle. The innervation of the pterygoids is also by the inferior 

 division of the fifth. 



