486 



Vertehrata. 



young animals it is scarcely indicated ; the enamel (except in teeth growing from 

 persistent pulps) appears to be completely developed only when the tooth comes 

 into use. Before falling out, the milk teeth are usually absorbed to a certain 

 extent in the lower portions by special cells lying in the sun-ounding connective 

 tissue. 



The number of teeth will be indicated here according to the following plan : 

 i I (= 3 incisors above, 2 below on each side), c i (= 1 canine above, below), 

 m f (^ 6 molars above, 5 below), or pm f (=: 3 premolars above, 3 below), 

 m f (^ 3 molars above, 2 below). If the actual teeth present are to be expressed, 

 the following formula is used, the signs above the line indicating the teeth of 

 the upper jaw, those below, those of the lower jaw : 



P" P' 



Some of the Mammalia are altogether edentulous^ but where 

 this is the case with the adult {e.g., Whale-bone Whale) teeth may- 

 have been present in embryonic life ; or even in youth, such are, 

 however, never cut ; they are absorbed. 



The presence of an upper and a lower lip is characteristic 

 of the Mammalia. These are large, muscular folds of skin covering 

 the edges of the jaws and continuous with each other laterally ; 



#''f"^*^-V. 



tn hg 



Fig. 396. Longitudinal section through the head and neck of a Dog, decreased. 

 b hyoid, e opening of eustachian canal into the pharynx, g train (only suggested), h frontal 

 sinus, hg hard palate, Tc epiglottis (lies above the edge of the soft palate), I lyssa {cf. foot 

 note, p. 487)), m turbinals, p anterior, p' posterior edge of the pharynx, r spinal cord, s vocal 

 cords, sp CBSophagiis, t tonsil (see foot note, p. 487), tr trachea, v soft palate, z tongiie, 

 2 axis, 4 fourth cervical vertebra. — Orig. 



