DIGESTION. 29 



muscles ; the bolus is grasped by the superior constrictor muscle of the 

 pharynx and rapidly forced into the oesophagus. 



The food is prevented from entering the posterior nares by the uvula 

 and the closure of the posterior half-arches (the palato pharyngei muscles) ; 

 from entering the larynx by its ascent under the base of the tongue and 

 the action of the epiglottis. 



In the $d stage, the longitudinal and circular muscular fibres, contracting 

 from above downward, strip the bolus into the stomach. [For nervous 

 mechanism of Deglutition, see Medulla Oblongata.] 



Gastric Digestion. The stomach is a dilation of the alimentary canal, 

 13 inches long, 5 inches deep, having a capacity of about 5 pints; there 

 can be distinguished a cardiac and pyloric orifice, a greater and lesser 

 curvature, a greater and lesser pouch. 



It possesses three coats : 



1 . Serous, a reflection of the peritoneum. 



2. Muscular, the fibres of which are arranged longitudinally, transversely 

 and obliquely. 



3. Mucous, thrown into folds, forming the rugae. 



Imbedded in the mucous coat are immense numbers of mucous and 

 true gastric glands. In the pyloric end of the stomach are found the 

 mucous glands, which are lined with columnar epithelium throughout their 

 extent. In the cardiac end are found the true peptic glands (Fig. 3), the 

 ducts of which are also lined with columnar cells, while the secretory parts 

 are lined with two distinct varieties of cells. One variety consists of small 

 spheroidal, granular cells, which border the lumen of the gland, and are 

 known as the chief cells ; the other variety consists of large, oval, well- 

 defined granular cells, much less abundant, and are situated between the 

 basement membrane of the gland and the 'chief cells. From their position 

 they have been termed parietal cells. During the intervals of digestion the 

 chief cells are pale, and the hyaline substance of which they are composed 

 is finely granular. During the stage of active secretion the cells become 

 swollen and turbid, and are then said to be rich in pepsin. Toward the 

 end of digestion the granules disappear, the cells become pale and return to 

 their former size. 



During the intervals of digestion, the mucous membrane of the stomach 

 is pale and covered with a layer of mucus. Upon the introduction of food, 

 the blood vessels dilate and become filled with blood, and the mucous 

 membrane becomes red. At the same time small drops of a fluid, the 

 gastric juice, begin to exude upon its surface, which gradually run together 

 and trickle down the sides of the stomach. 



