152 



ANIMAL BIOLOGY 



Furthermore, saliva contributes to the digestion of starches by 

 an enzyme termed ptyalin, which in the alkaline medium con- 

 verts starch into a sugar, maltose. But only a small proportion 

 of the starch is digested by ptyalin during the rapid passage of 



Salivary glands 

 Pharynx 



Thyroid gland 

 Thymus gland 



Lungs 



Diaphragm""" 

 Liver 



Gall bladder 



Mouth 



Trachea 



Large intestine - 



Vermiform appendix 



Esophagus 



Stomach 

 -Pancreas 



Small intestine 



Rectum 



Fig. 110. — Diagram of the human alimentary canal and its derivatives. 

 The pharynx shows the embryonic position of five pairs of gill pouches, the 

 second pair probably giving rise to the tonsils, and the third and fourth to 

 the thymus glands. 



food through the mouth region, and the activity of the enzyme 

 ceases soon after it reaches the stomach where the alkaline reac- 

 tion of the mouth gives place to an acid reaction. 



So the mouthful of food, masticated, moistened, and with some 

 of its starch digested, is rolled by the tongue and passed along the 

 pharynx into the esophagus. Henceforth it is beyond voluntary 

 recovery, because it is pushed along by involuntary rhythmical 



