SALIVARY AND GASTRIC DIGESTION 



191 



ceptional relaxation. In about 5 seconds from the 

 time the food leaves the mouth it reaches the cardiac 

 opening into the stomach. 



When liquid is swallowed the original impulse given by 

 the reduction of the mouth cavity may send it all the 

 way to the stomach through the passive esophagus. 

 It then arrives almost instantly but a peristaltic wave 

 may be expected to follow at the usual slow rate. In 

 many cases the water finds the cardia in a contracted 

 state and is arrested above it until the wave catches up. 



FIG. 41. An exaggerated representation of peristalsis. I and II 

 are successive views of the same portion of the alimentary tube: 

 P is the zone of contraction shifting downward and always preceded 

 by the zone of unusual relaxation (N). Ill is an imaginary section 

 through II, showing the food bolus (6) slipping along in advance of the 

 contracting region, its advance being facilitated by the relaxation below. 



When the negative, or relaxation, phase of the peristalsis 

 comes along it seems to involve the tissues around the 

 cardia and during the consequent slackening the fluid 

 enters the stomach. 



The Movements of the Stomach. A bird has a thin- 

 walled crop in which food is stored to be delivered slowly 

 to the gizzard. This is a highly muscular organ which 

 contracts rhythmically upon its contents and has its 

 effect reinforced by the gravelstones inside. Man does 

 not have a crop and a gizzard but his stomach is so 



