130 The Vertebrate Organ Systems 



of the abdominal cavity. At the point where it joins the large in- 

 testine is another valve. The lowest portion of the large intestine acts 

 as a temporary storage region for the undigested remains of food. 

 These remains, known as feces, are expelled through the anus. 



The Mouth. — Among the vertebrates, the mouth is a highly de- 

 veloped structure with many specialized organs within it. The an- 

 terior boundary of the mouth is marked in most vertebrates by the 

 lips, which are formed of soft connective tissue and epidermis. They 

 are well supplied with touch organs, and the lower one is more mobile 

 than is the upper. Lips are not present in all vertebrates, for in some, 

 such as the birds, reptiles, and some mammals, horny beaks have 

 developed. 



As the lips terminate at about the region of the premolars in man, 

 a cheek region is formed. Among rodents and some monkeys these 

 cheeks are utilized for temporary storage as the expanded cheek pouches. 



Within the mouth, the chief organs are the teeth and tongue. 

 The teeth are variously developed in the different groups of vertebrates. 

 Among most of them, the teeth are of the homodont type, that is, they 

 are all alike. It is only among the mammals that the hetcrodont con- 

 dition is found. Here there are four types of teeth : the incisors, ca- 

 nines, premolars, and molars. They are variously developed among 

 the members of the different orders of mammals, correlated with the 

 food habits of the group. 



The tongue, also, is variously developed in different vertebrate 

 groups. Essentially its use is to handle the food within the mouth ; 

 however, it may serve many other purposes. In man, for instance, it 

 is important in speech. Among the lizards, frogs, and toads, it 

 is adapted for capturing prey. The woodpecker has an enormously 

 elongate tongue for probing into holes. The sticky long tongue of 

 the anteater is useful for collecting its favorite item of diet. 



In man, the tongue is supported at the rear by the hyoid apparatus 

 of the visceral skeleton; at the front, it is attached to the floor of 

 the mouth by the frenulum. The fleshy tongue is composed of epi- 

 thelial, muscular, and connective tissues. Over the surface are scattered 

 sensory endings known as taste buds. 



Dorsally the mouth is limited by the bony hard palate, the sur- 

 face of which is covered with transverse ridges of cornified epithelium 

 which aid in manipulating and holding the food. These ridges are 

 best developed among the carnivores and hoofed mammals. Poste- 



