THE ALIMENTARY CANAL. 



287 



are changed constitute the milk dentition. The anterior grinders, 

 which with the incisors and canines are replaced, are known 

 as the prcemolars, as opposed to the posterior, true molars, 

 which belong to the permanent dentition, and are not replaced. The 

 true molars only appear after the milk teeth have been replaced, and 

 are distinguished by the size and number of their roots, as well as by 

 the extent of their crowns. Formulae, in which the numbers of 

 incisors, canines, praemolars, and molars in the upper and lower jaws 

 are given, are used to indicate in a simple manner the nature of the 

 dentition, e.g., the dental formula of man is 



212 

 2 1 2- 



2-2 1-1 

 2^2 C TI 



2-2 



S'SH 

 ' 3~3 j 



Alimentary canal. In addition to the hard structures at the 

 entrance to the digestive cavity, 

 soft, movable lips which bound the 

 mouth opening, and a fleshy tongue 

 which is of very various form and 

 lies on the floor of the buccal cavity, 

 are of special importance for the 

 introduction and preparation of the 

 food (fig. 675). In the Monotremata 

 the lips are replaced by the edges of 

 the beak. The tongue, however, is 

 never absent, v but it may be im- 

 movable, and completely fused with 

 the floor of the mouth, as in the 

 Whales. Its front part is mainly 

 tactile in function, but in some cases 

 it is used to seize (Giraffe) or capture food (Ant-eaters). Variously 

 shaped papillae, which are often cornified and bear recurved hooks, 

 project from its upper surface. The papillae, circumvattatce alone have 

 a relation to the sense of taste. The tongue is supported by the hyoid 

 bone and by a cartilaginous rod, which represents the os interglossum 

 (Lytta). The anterior cornua of the hyoid are attached to the 

 styloid processes of the temporal bone, the posterior bear the larynx. 

 Beneath the tongue there is sometimes (most developed in the 

 Insectivora) a single or double projection, which is termed the lower 

 tongue. The sides of the buccal cavity are soft and fleshy, and are 

 not unfrequently in the Rodents, Apes, etc., dilated into wide sacs, 

 the so-called cheek-pouches. The soft palate (palatum molle) must be 

 mentioned as a structure peculiar to the Mammalia ; it constitutes 



FIG. 674. Dentition of Cebm (while 

 changing the teeth) after Owen, t, 

 Incisors ; c, Canines ; pi p2 p3, Prse- 

 molars of the milk dentition ; 7l J2 

 Incisors ; C, Canine ; PI P2 P3, Prae- 

 molars of the permanent dentition;. 

 M 1 M2 M3, Molars. 



