Chap. 11 FOODS AND NUTRITION 183 



eat and watch and run habit. But no matter how we try to hurry it, the human 

 esophagus never speeds up like that of a dog; on the contrary like an elevator, 

 it takes its own time. At man^' a modern table the primitive habit of eat and 

 watch and run continues. 



The human esophagus is strictly a passageway. Usually we swallow down- 

 ward, but it is quite possible to swallow upward while standing on one's head. 

 Any acrobat can demonstrate this and every day horses and cows drink up- 

 ward at a sharp angle. Even if its esophagus rises perpendicularly to the milk, 

 this does not hinder a drinking weasel (Fig. 11.13). 



Fig. 11.13. Weasel drinking milk with its esophagus at right angles to the milk. 

 (Courtesy, American Museum of Natural History, New York.) 



In other animals the esophagus may be distended into a sac which holds the 

 extra food and acts as a waiting-room for gastric digestion. Cattle and other 

 ruminants have such temporary storage sacs: the largest one is the rumen 

 which in an average-sized steer has a capacity of about 30 gallons; the others 

 are the reticulum and omasum. In the market the lining of the reticulum is 

 known as honey-comb tripe (Fig. 11.14). After a period of eating and 

 swallowing into the rumen, cattle, sheep, deer, and other ruminants lie down 

 to chew their cuds. At that time contractions of the esophagus go into reverse 

 and bring one bolus after another of the slightly fermented grass to the mouth 

 where it is chewed and again swallowed, this time permanently. 



The crops of birds, especially of domestic fowl, are lateral enlargements of 

 the esophagus. The chicken that goes to roost with a full crop sleeps on while 



