DIGESTION AND RESPIRATION 



517 



lubricate the food, and this is still the major function of our saliva. 

 The saHva of most mammals and of a few other tetrapods contains 

 digestive enzymes and the chemical breakdown of food begins in the 

 mouth. Ptyalin, which must be activated by chloride ions present in 

 the saliva, is an amylase that hydrolyzes starch to the double sugar mal- 

 tose. 1 he small amount of maltose present splits some of the maltose, 

 yielding the single sugar glucose. The poison glands of reptiles and 

 the glands of vampire bats that secrete an anticoagulant are other spe- 

 cialized oral glands. 



The tongue of frogs and anteaters is specialized as a food gathering 

 device, and that of snakes is part of the olfactory mechanism (p. 46), 

 but its chief function in most vertebrates is to manipulate food in the 

 mouth and to aid in swallowing. The tongue pushes the food between 

 the teeth, so that the food is thoroughly masticated and mixed with 

 saliva. Then the food is shaped into a ball, a bolus, and moved by the 

 tongue into the pharynx. 1 he tongue bears numerous microscopic taste 

 buds, and the human tongue is of great importance in speech. 



Nasal cavity 



E^ct. naris 



Se-c. palate 

 Ton. 6 lie ~ 

 Su.blingix.al gland 

 SiLbma>cillary gland 

 Vocal cord 



Trachea' 



Gall bladd&ir 



Liv^r (lif tcdzxp) 

 Common bile duct 



Duodenixm. 



Transverse colon 



Ascendind Colon 



Ca.e.cum 

 Appendioc 



Parotid gland 



Soft palate 

 Pharynx 



Epiglottis 

 Esophad U-S 



Storaach 



Spleen 



Pancreas 



■Jejunum. 



Descending colon 

 Ileum. 



Rectum. 



Figure 26.2 The digestive system of man. 



