192 HUMAN PHYSIOLOGY 



changing color but if it is boiled with the products of 

 salivary digestion it is bleached and deposits a red or 

 yellow sediment of the oxids of copper. 



When a cow is chewing her cud we may suppose that 

 there is salivary digestion in the mouth of the ruminant. 

 The average human being is not apt 'to hold food long 

 enough in the mouth to allow much transformation; 

 the question is, rather, how long saliva can continue to 

 act in the stomach. We can discuss this to better 

 advantage when we have dealt with the motor phenomena 

 exhibited by that organ. 



Swallowing. Food is transferred from the mouth to 

 the stomach by a coordinated series of movements which 

 give an excellent example of the reflex principle in an 

 elaborate form. First, the material is thrust back 

 into the throat by the practical obliteration of the 

 mouth cavity. Then the muscular bands in the wall of 

 the pharynx contract in order from above downward, 

 squeezing the food into the esophagus. At this moment 

 breathing has to be suspended and the passages closed 

 against the possible entrance of food. This is ac- 

 complished for the nasal connection by the drawing 

 back of the soft palate, a mobile partition between the 

 mouth and the upper part of the pharynx. The larynx 

 is shielded by being pulled forward under the root of 

 the tongue and, at the same time, has an additional 

 safeguard through the folding down upon it of a leaf- 

 like lid, the epiglottis. 



The first stages in swallowing are swiftly executed. 

 When the food is once within the esophagus breathing 

 can be resumed and the bolus advances more slowly. 

 It is thrust down the esophagus by a travelling contrac- 

 tion, successive regions of the tube closing in behind the 

 moving mass as one could manipulate a rubber tube 

 with the thumb and finger to send a glass bead through 

 it. A propulsive movement of this sort is an example of 

 peristalsis. Careful analysis has shown that the mov- 

 ing ring of contraction is preceded by a zone of ex- 



