THE MECHANICAL FACTORS OF DIGESTION 



413 



waves do not pass into the duodenum, the muscular continuity between 

 the parts being interrupted by a ring of fibrous tissue. The time of 

 recurrence of the waves varies in different animals, and also with 

 the nature of the food. In cats, there are generally four to six per 

 minute; in dogs, about four; in man, three. Fat diminishes the 

 number per minute, carbohydrate increases them. 





o 



tim 

 o 



C 



o 



A 



\ 



2 



Hours 



FIG. 207. To SHOW RETARDING EFFECTS OF FATS UPON FOOD LEAVING 

 STOMACH. (Cannon.) 



Light line = curve of mashed potato; heavy line = curve of mutton fat; dash line = 

 curve of potato and mutton fat mixed. 



The opening of the pyloric orifice is co-ordinated with the acidity 

 and the consistency and nature (Fig. 204) of the gastric cont3nts. 

 Generally speaking, carbohydrates leave first, proteins next, and fats 

 last (Fig. 205). Carboj^ydrates as a rule leave the stomach quickly, 

 but if fed with alkali their exit is retarded (Fig. 205). Feeding proteins 

 with acid, on the other hand, hastens their normally slow exit 

 (Fig. 206). Fats retard the exit of other foods (Fig. 207). When 

 the acid contents of the stomach reach the duodenum, the pyloric 



FIG. 208. RECORD SHOWING CESSATION OF RHYTHMIC REGURGITATIONS OF FLUID 

 FROM STOMACH INTO (ESOPHAGUS AFTER ACIDIFYING GASTRIC CONTENTS AT A . 

 (Cannon.) 



Upstroke = outflow, small oscillations due to respiration. Time in half-minutes. 



aperture closes. Thus, acid on the pyloric side opens, on the duodenal 

 side shuts the pyloric orifice, the regulation probably depending on 

 a local reflex mechanism. 



The cardiac orifice is normally kept shut. In l^he resting condition 

 of the stomach the sphincter resists a tension of about 25 cm. of H 2 O. 

 We are, therefore, not conscious of the rancid contents of the stomach. 



