DIGESTION 179 



splitting into simpler substances. These are found in practice to contain 

 less latent energy than the molecule from which they originated. The 

 hydrolytic process, then, is one of katabolism, the resulting chemical 

 materials being more stable than their mother-substance. That water 

 is required in the reaction is shown by the fact that no known enzyme 

 acts save in its presence. 



Various conditions determine the rate of digestive zymolysis, as has 

 in part already been seen above. Among these are temperature (the 

 normal body-temperature is the optimum), chemical reaction, fluidity or 

 solidity of the substance acted on, the size of the particles of the latter, 

 the concentration of the products of the zymolysis, the concentration 

 of the enzyme, and whether or not the food-substance has been cooked 

 (heated) or not. The necessity of promptly removing the products of 

 the hydrolysis in order that the action of the enzyme may continue 

 Briiche showed in 1862; it is a fact often demonstrated since. The im- 

 possibility of thus removing the products of the action when carried on 

 artificially outside a living body is one of the chief difficulties in experi- 

 ments on digestion under these circumstances. In the organism these 

 products are promptly removed by absorption into the circulation. No 

 explanation of this hindrance is at present available, but it is probably 

 chemical and dependent on some sort of inhibiting reaction involving 

 the enzyme. The slowing influence exerted by the excessive heating of 

 the food-substance to be acted upon is probably a matter of general solu- 

 bility; the heating of starch is clearly a step toward organic solution, but 

 the cooking of proteids renders them in general somewhat less soluble. 

 For the mode of action of ptyalin, see below (page 188), where hydrolysis 

 in the stomach is briefly discussed. 



Deglutition. The next mechanism and set of movements which we 

 have to consider in an orderly study of digestion are those of the process 

 of swallowing, technically called deglutition. These have long been the 

 subject of research on the part of anatomists and physiologists, for 

 both the mechanism and its action are very complex. 



The pharynx is a muscular, membranous, funnel-shaped tube con- 

 necting the mouth and nasal fossae with the esophagus below it. The 

 superior constrictor of the pharynx is composed of cross-striated fibers, 

 the middle constrictor of both smooth and cross-striated fibers, and the 

 inferior wholly of smooth or un-striated muscle. The esophagus, a 

 muscular tube about 23 cm. long and 2 cm. in diameter, connects the 

 pharynx above with the stomach below. The point at which it passes 

 through the diaphragm is its narrowest part. Like the pharynx the 

 esophagus has three coats, the outermost being muscular and con- 

 sisting of two layers of fibers, of which the external are longitudinal and 

 the internal circular. As in the pharynx also, the upper part of the esoph- 

 agus is of cross-striated fibers chiefly and the lower part of smooth 

 fibers. The muscles of the upper part are supplied by the recurrent 

 laryngeal nerve, while the vagus supplies the muscles of the lower part 

 of the tube. The pharyngeal muscles are supplied by branches from the 



