CH. XXXVII.] MOVEMENTS OF THE STOMACH 553 



branch; and the hypo-glossal, the muscles of the tongue. The 

 nerre-centres by which the muscles are harmonised in their action, 

 are situated in the medulla oblongata. 



Stimulation of the vagi gives rise to peristalsis of the oesophagus. 

 The cell-stations of these fibres are in the ganglion trunci vagi. 

 Division of both pneumogastric nerves produces paralysis of the 

 oesophagus and stomach, and firm contraction of the cardiac orifice. 

 These nerves therefore normally supply the oesophagus with motor, 

 and the cardiac sphincter with inhibitory fibres. If food is swallowed 

 after these nerves are divided, it accumulates in the gullet and never 

 reaches the stomach. 



In discussing peristalsis on a previous occasion (p. 141), we 

 arrived at the conclusion that it is an inherent property of muscle 

 rather than of nerve; though normally it is controlled and influenced 

 by nervous agency. This nervous control is especially marked in the 

 oesophagus; for if that tube is divided across, leaving the nerve 

 branches intact, a wave of contraction will travel from one end to the 

 other across the cut. 



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 stomach 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 by 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 later 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 



