286 MOVEMENTS OF THE SMALL INTESTINE. [BOOK ir. 



ments, since the stomach is fullest at the beginning when the 

 movements are slight, and becomes emptier as they grow more 

 forcible. The one thing which does increase pari passu with the 

 movements is the acidity, which is at a minimum when the 

 (generally alkaline) food has been swallowed, and increases steadily 

 onwards. It has not however been definitely shewn that the 

 increasing acidity is the efficient stimulus, giving rise to the 

 movements. 



The nervous mechanism of the gastric movements is at present 

 very obscure. The stomach receives its nervous supply from the 

 vagi and also from the solar plexus, with which the splanchnics are 

 connected. When the vagi are divided, a spasmodic constriction of 

 the cardiac orifice takes place; in other words the tonic action of the 

 sphincter is increased, and food is thus prevented, for a time at least, 

 from leaving the oesophagus. In addition, the natural movements 

 of the stomach itself cease, or become uncertain and irregular, even 

 if food be present. Incomplete movements may be induced by 

 stimulation of the peripheral stumps of the vagi, when the stomach 

 is full, but not so readily if it be empty. The effects of section or 

 stimulation of the splanchnics or of the branches from the solar 

 plexus are uncertain. Nor do we know the exact mechanism by 

 which the pyloric sphincter is used to strain off gradually the 

 more digested portions of the food. The movements of even 

 a full stomach are said to cease during sleep. 



Movements of the small Intestine. Though peristaltic move- 

 ments occur along the whole length of the alimentary canal, from 

 the oesophagus to the rectum, they are more pronounced in the 

 small intestine than elsewhere. When the intestines are watched, 

 after opening the abdomen, circular contractions, that is con- 

 tractions of the circular coat, may be seen travelling lengthways 

 along the intestine and often upwards as well as downwards. 

 Similarly longitudinal contractions, that is contractions of the longi- 

 tudinal coat, may also be seen to travel lengthways. The circular 

 coat being much thicker and stouter than the longitudinal coat, is 

 the more important of the two, and it is by the contractions 

 of the circular coat that in the normal state of things the 

 contents of the intestine are driven along toward the ileo-csecal 

 valve. The contractions of the longitudinal coat appear to be 

 chiefly of use in producing peculiar oscillating movements of the 

 pendent loops in which the intestine is arranged. The rhythmic 

 occurrence of these circular and to-and-fro movements, together 

 with the passive movements caused by the entrance of the fluid 

 contents into or their exit from the various loops, brings about the 

 peculiar writhing of the intestines which has given rise to the 

 phrase peristaltic action. 



The movements, as we have said, take place from above down- 

 wards, and a wave beginning at the pylorus may be traced a long 



