448 MAMMALIAN ANATOMY 



of the superior triangle has been pushed continually forward and been 

 reduced to a very small size. Why should this occur ? Why should 

 not the corresponding tubercles of the inner side of the lower crown 

 have been pushed backward, since action and reaction are equal ? The 

 reason is clear : The superior tubercle is on the internal apex of the 

 trigon, and is supported by but one root, while the resistant portion of 

 the inferior crown is the base of the trigonid, and is supported by two, 

 thus offering twice the resistance to the pressure that the superior does. 

 But why should the anterior part of the inferior tooth move forward, 

 even if it be in the direction of least resistance? This is due to the 

 regular increase in size of the teeth themselves, an increase which can 

 be traced from the beginning to the end of the series. And this 

 increase is the usual result of use." 



"The excess of the forward pressure of the inferior teeth against the 

 superior over any backward pressure has left the posterior internal 

 cusp of the triangle of the inferior molar (metacouid) without contact 

 or consequent functional use. It has, consequently, gradually disap- 

 peared, having become small in the highest Canidae, and wanting in 

 some Mustelidse and all Felidse. The heel of the same tooth has had 

 a similar history. With the diminution in size of the first superior 

 tubercular, with which it comes in opposition in mastication, its func- 

 tional stimulus also diminished, and it disappeared sometimes a little 

 sooner (Felicia?) and sometimes a little later (Hyrenidse) than that tooth. 



" The specialization of one tooth to the exclusion of others as a 

 sectorial appears to be due to the following causes. It is to be ob- 

 served in the first place that when a carnivore devours a carcass it cuts 

 off masses with its sectorials, using them as shears. In so doing it 

 brings the part to be divided to the angle or canthus of the soft walls 

 of the mouth, which is at the front of the masseter muscle. At this 

 point the greatest amount of force is gained, since the weight is thus 

 brought immediately to the power, which would not be the case were 

 the sectorial situated much in front of the masseter. On the other 

 hand, the sectorial could not be situated farther back, since it would 

 then be inaccessible to a carcass or mass too large to be taken into the 



mouth. 



* 



" The position of the sectorial tooth being thus shown to be de- 

 pendent on that of the masseter muscle, it remains to ascertain a 

 probable cause for the relation of the latter to the dental series in 



