MECHANICAL PHENOMENA. 9 



through wx. The antrum is closed by the pyloric sphincter 

 at p. 



The wall of the stomach consists of three layers of smooth 

 muscle fibres,, an outer longitudinal, a middle circular, and 

 an inner oblique coat. Smooth muscle is characterized 

 physiologically by its power of slow rhythmic contraction 

 and relaxation and its power of prolonged contraction (tonic- 

 ity). The role that these characteristics play in determining 

 the normal movements of the stomach can be best appre- 



(Copied from CANNON: American Journal of Physiology, 1898, 1, p. 360.) 



ciated by a study of the accompanying figures, which indi- 

 cate the changes that occur in the shape of the stomach 

 after an ordinary meal (Fig. 2). 



^- Within five minutes after a meal of bread a slight annular 

 constriction appears near the duodenal end of the antrum 

 and moves peristaltically towards the pylorus. This is 

 followed--ky t several other waves of similar character. Two 

 or three minut^^ter the first movement is seen, very slight 

 constrictions appear near the middle of the stomach (the 

 preantral part) and becoming deeper move slowly toward 

 the pylorus. As digestion goes on the antrum becomes 

 somewhat elongated and the constrictions somewhat deeper, 

 but never until the stomach is nearly empty do they divide 

 the cavity entirely. The waves recur at intervals of almost 

 exactly ten seconds and take about thirty-six seconds to 

 pass from the middle of the stomach to the pylorus. When 

 one wave is just beginning several others are thejefore already 



