142 The Vertebrate Organ Systems 



various local movements confined to special regions, but these are super- 

 imposed on the generalized wave of contraction which sweeps over the 

 tract. These waves, beginning in the upper portion of the esophagus 

 and continuing through the rectum, are known as peristalsis. This 

 movement is the most important in keeping the food going along the tract. 



Fig. 43. — Diagram of the interrelationships of the hormones involved in gas- 

 trointestinal coordination. (From Prosser: Comparative Animal Physiology, W. B. 

 Saunders Company.) 



In the mouth and pharynx the mechanical movements are voluntary 

 in nature. In the mouth these movements consist of the chewing mo- 

 tions which by means of the teeth and tongue break the food into small 

 particles and mix it with the saliva. In addition, the food is formed 

 into small balls, known as boluses, due to the gluey substance, mucin, 

 of the saliva. By movements of the tongue, the boluses are gradually 

 moved into the pharynx where they are swallowed. Each swallowing 

 movement is followed by a peristaltic wave in the esophagus. 



This peristaltic wave is the only movement observable in the 

 esophagus. As each wave reaches the stomach, the cardiac sphincter re- 

 laxes and permits the food or liquid to enter. 



The peristaltic waves of the stomach may be started by the swal- 

 lowing of food; however, they are not just a continuation of those of 

 the esophagus. They arise at a point midway between the cardiac and 

 pyloric valves, and gradually start originating nearer the cardiac end. 



