528 THE MECHANICAL PROCESSES OF DIGESTION [OIL XXXV. 



In swallowing liquids the two mylo-hyoid muscles form a 

 diaphragm below the anterior part of the mouth. The stylo-glossi 

 draw the tongue backwards and elevate its base ; the two hyo-glossi 

 act with these, pulling the tongue backwards and downwards. The 

 action of these muscles resembles that of a force-pump projecting the 

 mass of fluid down into the oesophagus ; it reaches the cardiac orifice 

 with great speed, and the pharyngeal and oesophageal muscles do not 

 contract on it at all, but are inhibited during the passage of the fluid 

 through them (Kronecker). 



This is proved in a striking way in cases of poisoning by corrosive 

 substances like oil of vitriol ; the mouth and tongue are scarred and 

 burnt, but the pharynx and oesophagus escape serious injury, so 

 rapidly does the fluid pass along them; the cardiac orifice of the 

 stomach is the next place to show the effects of the corrosive. 



There is, however, no hard-and-fast line between the swallowing 

 of solids and fluids : the more liquid the food is, the more does the 

 force-pump action just described manifest itself. 



MOVEMENTS OF THE STOMACH. 



The gastric fluid is assisted in accomplishing its share in digestion 

 by the movements of the stomach. In graminivorous birds, for 

 example, the contraction of the strong muscular gizzard affords a 

 necessary aid to digestion, by grinding and triturating 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 it has been demonstrated that substances are 

 digested even when enclosed in perforated tubes, and consequently 

 protected from mechanical influence. 



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 into the stomach, and immediately again closes. The pyloric 

 orifice, during the first part of gastric digestion, is usually so com- 

 pletely 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. The peristaltic action 

 of the muscular coat, whereby the digested portions are gradually 



