156 THE HUMAN BODY 



the food can only be squeezed into the oesophagus. The 

 muscular movements concerned in this part of deglutition 

 are all excited without the intervention of the will; food 

 touching the mucous membrane of the pharynx produces 

 quite involuntarily the proper action of the swallowing 

 muscles.* Indeed, many persons after having got the 

 mouth completely empty cannot perform the movements 

 of the second stage of deglutition at all. On account of 

 the involuntary nature of the contractions of the pharynx 

 any food which has once entered it must be swallowed; the 

 isthmus of the fauces forms a sort of Rubicon; food that 

 has entered the pharynx must be swallowed, even although 

 the swallower learned immediately that he was taking poi- 

 son. The third stage of deglutition is that in which the 

 food is passing along the gullet, and is comparatively slow. 

 Even liquid substances do not fall or flow down this tube, 

 but have their passage controlled by its muscular coats, 

 which grip the successive portions swallowed and pass 

 them on. Hence the possibility of performing the appar- 

 ently wonderful feat of drinking a glass of water while 

 standing upon the head, olten exhibited by jugglers; peo- 

 ple forgetting that one sees the same thing done every 

 day by horses and other animals which drink with the 

 pharyngeal end of the gullet lower than the stomach. 



The Gastric Juice. The food having entered the stom- 

 ach is exposed to the action of the gastric juice, which is a 

 thin colorless or pale yellow liquid of a strongly acid re- 

 action. It contains, beside water and some salts and mu- 



How are the movements of the second stage of deglutition excited? 

 What is the third sta^e of deglutition? Is it fast or slow? How is 

 it that jugglers can drink while standing on the head? 



Describe the gastric juice. 



* The process Is what is known as a reflex action. See Chap. XX. 



