DIGESTIVE SYSTEM 



733 



mammals such as the monotremes, sirenians, and cetacea, there is only 

 one dentition, while in some groups there are an indefinite number of 

 successive dentitions. In such animals as Guinea pigs, and in some bats, 

 the milk dentition is lost even before birth. 



Practically all fishes with few exceptions have teeth, and these ex- 

 tend not only to the lining bones of the mouth, but in some even to the 

 hyoid and branchial arches. These latter are known as pharyngeal teeth. 



Fig. 430. 



Biting mechanism of the rattlesnake. la, and Ib, position of the ap 

 paratus when mouth is shut. Ha, and lib, position of the apparatus when 

 mouth is opened widely, showing the spheno-pterygoid muscle (P.e.) con- 

 tracted, the pterygoid (Pt) pulled forwards, the transverse bone or 

 ectopterygoid (Tr) pushing the maxillary (M) rotating it and thereby 

 causing the poison-fang (J) to assume an upright position. Di, Digastric 

 muscle, the contraction of which lowers, or opens the lower jaw ; G, the 

 groove or pit characteristic of the Crotaline snakes ; J, poison fang ; M, 

 maxillary ; P, palatine ; P.e, spheno-pterygoid ; Pm, premaxillary ; Pt, 

 pterygoid ; Q, quadrate ; Sq, squamosal ; T, a, insertion of the an- 

 terior temporal muscle, by contraction of which the mouth is shut ; Tr, 

 transversum or ectopterygoid ; X, origin and insertion of a muscle and a 

 strong ligament, contraction of which draws the maxillary and its tooth 

 back into the position of rest and assists in shutting the mouth. (After 

 Gadow.) 



The teeth may be cone shaped or flat, sometimes they even form large 

 plates as though a number of primitive teeth had grown together. Teeth 

 may be anchylosed to the summit of the jaws, attached to their inner 

 side, or have their roots implanted in grooves or pockets as in the human 

 being. The grooves in the jaw, in which teeth grow, are called alveoli.* 



*Mammals are said to be monophyodont if they develop only one set of teeth, and diphyodont 

 if they develop two. However, even in momophyodont mammals, a second set usually develops, 

 although this set later becomes absorbed or remains in a vestigial condition. 



When all the teeth are uniform they are said to be homodont, while if they vary in shape they 

 are heterodont. 



Teeth are said to be acrodont, if anchylosed to the summit of the jaws, pleurodont, if fastened 

 to the jaw's inner side, and thecodont, if the roots are implanted in alveoli. 



Teeth have also received names according to their function or their peculiar physical appear- 



