260 



ELEMENTARY ANATOMY. 



[LESS. 



such notches are more marked, but these attain their maxi- 

 mum of development in the lower jaw of the Flying Lemur 

 (Galeopithecus], where each incisor has the appearance of a 

 comb, being notched by parallel notches down to the very 

 base of the crown. (Fig. 228). 



Rarely, as in certain Insectivorous Beasts (e.g. Hemi- 

 centetes), the upper incisor may bear more than one cusp. 



FIG. 229. SIDE \ r iE\v OF SKULL OF HEMICENTETES. 



An excavated incisor of a different kind is familiar to us 

 in the Horse, where each incisor has a deep median depres- 

 sion, the " mark," which has a form such as would be pro- 

 duced by the sudden inflection of the surface of the tooth so 

 as to form a deep pit. This " mark " 

 continues to be visible till the tooth 

 has worn down by use beyond the 

 point to which the inflection extends. 

 The dark colour of the " mark " is 

 due merely to the accumulation ot 

 fragments of food and foreign bodies 

 within the pit. 



When the canine teeth are large 

 the incisors are generally small, as 

 we see in the Dog and Cat, where 

 there are three above and three 

 below on each side of each jaw. 

 This is the typical number of Mam- 

 malian incisors, but they may be as 

 many as f on each side, as in the 

 American Opossums. 



Incisors may, on the contrary, be 



altogether wanting though other teeth are present, as in 

 the Armadillos, except one kind. They may be quite want- 

 ing in the upper jaw but present in the lower, even in man's 

 own order, e.g. Lepilemur. 



FIG. 230. VERTICAL SEC- 

 TION OF A HORSE'S IN- 

 CISOR, showing the depth 

 of the vertical fold forming 

 the " mark" m. 



p, the pulp cavity. 

 (After Rousseau.} 



