GASTRIC DIGESTION 159 



the lower constrictors in turn and it enters the esophagus. 

 (Meanwhile the soft palate has prevented it from going upward 

 and the epiglottis from entering the larynx.) 



The muscles of the pharynx and upper esophagus, although striped, 

 are not absolutely under the control of the will; we may or may not 

 choose to swallow, but once begun the act completes itself, being beyond our 

 power to interrupt. 



In the lower portion of the esophagus (about one-third) the 

 muscles are of the unstriped variety (this is the first appearance 

 of unstriped muscle in the alimentary tract, from this part on it 

 has no other kind) . The normal movements of unstriped muscle 

 are exactly suited to the requirements of the digestive process: 

 they are deliberate and slow instead of forcible and sudden, in 

 consequence of which they accomplish not only the passage but 

 the softening of the food by the admixture of mucus and water, 

 thus facilitating the contact of the digestive fluid with the whole 

 mass. Through the cardiac sphincter of the esophagus it enters 

 the stomach. It is first swept to the fundus, which serves as a 

 storehouse while successive portions of the food are acted upon. 

 In the stomach the mechanical process consists in the action of 

 the muscle coats, which move the food about that it may be 

 still more softened and thoroughly mixed with gastric juice. The 

 contractions of the muscles of the stomach go on in a wave-like 

 manner toward the pylorus, alternately constricting and relaxing 

 the walls of the cavity. 



In the stomach the chemical process consists in conversion of 

 proteid foods into peptones and amino-acids. The digestive 

 fluid is gastric juice. The enzymes are pepsin and rennin. These 

 act upon proteins after they have been acidulated, and finally 

 reduce them to peptones. (Some protein food may be absorbed 

 as peptone but the greater part is reduced to still simpler forms, 

 as amino-acids, etc.) 



Pepsin cannot act in alkaline fluids; gastric juice is acid (hydro- 

 chloric acid is essential). 



In the digestion of meats, the acid softens the connective tissue 

 fibers (which are already partially gelatinized by cooking) and 

 thus prepares them for the action of the pepsin. Eggs are digested 

 in the same manner but more easily, having so little connective 

 tissue. 



