522 THE MECHANICAL PROCESSES OF DIGESTION. [CH. xxxv. 



the hard seeds which constitute their food. But in the stomachs 

 of man and other Mammalia the movements of the muscular coat 

 are too feeble to exercise any such mechanical force on the food ; 

 neither are they needed, for mastication has already done the 

 mechanical work of a gizzard ; and experiments have demonstrated 

 that substances are digested even enclosed in perforated tubes, 

 and consequently protected from mechanical influence. 



The normal actions of the muscular fibres of the human 

 stomach appear to have a three-fold purpose: (i) to adapt the 

 stomach to the quantity of food in it, so that its walls may be in 

 contact with the food on all sides, and, at the same time, may 

 exercise a certain amount of compression upon it; (2) to keep 

 the orifices of the stomach closed until the food is digested ; and 

 (3) to perform certain peristaltic movements, whereby the food, 

 as it becomes chymified, is gradually propelled towards, and 

 ultimately through, the pylorus. In accomplishing this latter 

 end, the movements without doubt materially contribute towards 

 effecting a thorough intermingling of the food and the gastric juice. 



When digestion is not going on, the stomach is uniformly con- 

 tracted, its orifices not more firmly than the rest of its walls ; but, 

 if examined shortly after the introduction of food, it is found 

 closely encircling its contents, and its orifices are firmly closed 

 like sphincters. The cardiac orifice, every time food is swallowed, 

 opens to admit its passage to the stomach, and immediately again 

 closes. The pyloric orifice, during the first part of gastric diges- 

 tion, is usually so completely closed, that even when the stomach 

 is separated from the intestines, none of its contents escape. 

 But towards the termination of the digestive process, the 

 pylorus offers less resistance to the passage of substances from 

 the stomach ; first it yields to allow the successively digested 

 portions to go through it ; and then it allows the transit even of 

 undigested substances. It appears that food, so soon as it enters 

 the stomach, is subjected to a kind of peristaltic action of the 

 muscular coat, whereby the digested portions are gradually moved 

 towards the pylorus. The movements are observed to increase 

 in rapidity as the process of chymification advances, and are 

 continued until it is completed. 



The contraction of the fibres situated towards the pyloric end 

 of the stomach seems to be more energetic and more decidedly 

 /peristaltic than those of the cardiac portion. Thus, it was found 

 in the case of St. Martin, that when the bulb of a thermometer 

 was placed about three inches from the pylorus, through the 

 gastric fistula, it was tightly embraced from time to time, and 

 drawn towards the pyloric orifice for a distance of three or four 



