472 APPLIED BIOLOGY 



which are produced by the muscles of the pharynx and esoph- 

 agus. These movements are in the beginning voluntary 

 (i.e., under control of the will) ; but it is well known that 

 when food has started down the esophagus, the muscular 

 walls of that organ contract (narrow the diameter) involun- 

 tarily, and force the food into the stomach. The fact that 

 food is forced down and does not run because of gravity ex- 

 plains how an acrobat can perform the feat of drinking water 

 while standing on his head. Horses and cows are examples 

 of animals which regularly drink and eat -with the mouth- 

 cavity much lower than the stomach. 



(D) Take a piece of rubber tubing about two feet long, suspend 

 vertically, plug the lower end with a cork, and fill with water. Now 

 begin at the plugged end and grasp the tubing between a thumb and 

 finger and then move your hand upward so as to contract the tube 

 in succession. It is evident why the water appears to run up-hill in 

 the tube. Similarly, the muscles which are in the walls of the 

 esophagus contracting in succession from the pharynx toward the 

 stomach force the food along the esophagus. 



The muscle-fibers in the walls of the stomach are arranged 

 transversely, longitudinally, and obliquely. The part of 

 the stomach next to the esophagus contracts and steadily 

 presses upon the contained food; but the posterior half 

 undergoes a series of wave-like constrictions. The result 

 is that the food is broken up into smaller masses and well 

 mixed with the digestive fluids. These movements have 

 been studied by feeding dogs and cats with bismuth sub- 

 nitrate, a drug which physicians often prescribe to allay 

 gastric irritation. This drug mixes with the food in the 

 stomach and renders the whole mass so opaque to the X-rays 

 that the shape of the stomach is easily seen with the aid of 

 a fluoroscope, and photographs can be made similar to those 

 often taken of the skeleton within the body. 



From time to time the pylorus opens, allowing liquid 

 foods (often called chyme) to pass into the intestine. As 



