:6o 



ELEMENTS OF BIOLOGY 



107). In addition to these, the wall of the stomach has a third, 

 oblique layer. These muscles are derived from the inner sheet of 

 mesoderm and are composed of plain or smooth muscle cells. They 

 have the property of constant rhythmic contraction and are not un- 

 der direct control of the will. Once food has been masticated in the 

 oral cavity it is swallowed by a complicated automatic process, and 



SUBMUCOSA ^ 



CIRCULAR 

 MUSCLE 



SEROSA 



^ILLI 



'M'^m^ 



^^W^S^ 



X^^ BLOOD VESSEL 



LONGITUDINAL 

 MUSCLE 



Fig. 107. — A diagrammatic section through the human intestine to show the rela- 

 tions of the muscle and other layers of the wall. 



after entering the oesophagus it is moved along by the contraction of 

 the smooth muscles in the wall of the canal, having passed out of 

 control of deliberate efforts of the will. In the stomach the constant 

 rhythmic contractions of the muscular walls impart a mixing action 

 to the food. These movements of the walls of the digestive canal are 

 termed peristalsis. Thus by peristaltic action the materials in the 

 canal are moved along. Most cathartic agents hasten the movement 

 of the contents of the digestive canal by stimulating the smooth 



