456 



ORGAN SYSTEMS OF MAN 



esopKogus 



Fig. 17-5. In fish (upper left) the food and water portal of entry is the mouth. In land-dwellers the air and food 

 passageways cross as shown here in the amphibian (upper right) and man. The left lower figure shows the 

 condition of the larynx during breathing and the lower right figure shows it during the act of swallowing food. 



routed back along the efferent nerves to the 

 sahvary glands (Fig. 17-4) which are stim- 

 ulated to pour saliva into the mouth 

 through ducts. This is not the only way 

 salivation (secreting saliva) can be in- 

 duced, as any hungry person knows who 

 smells, sees, or even thinks about food. 

 Such responses are called conditioned re- 

 flexes and are acquired by learning. From 

 past experience one knows the appearance, 

 odor, and taste of food and so one responds 

 reflexively by increasing the salivary output 

 when any or all of these sense organs are 

 stimulated. Salivation is an important re- 

 sponse because it prepares the mouth cavity 

 for food before it is introduced. 



Swallowing 



The problem of getting food from the 

 mouth cavity to the stomach in vertebrates 

 has an interesting evolutionary history. The 

 difficulty arose when animals migrated onto 

 land and the breathing and food paths were 

 forced to coincide. Water, the oxygen-con- 

 taining medium, passes into the mouth and 

 out the gill clefts in fish, while the food en- 

 ters the mouth also and merely continues 

 straight ahead into the esophagus (Fig. 

 17-5). No difficulty was experienced with 

 such a mechanism. However, when verte- 

 brates moved out on the land the oxygen- 

 containing medium, air, took a new path- 



