1566 HUMAN ANATOMY. 



Girls. 



Ages. 9 10 II 12 13 14 15 16 Total. 



Lateral incisors 24 8 4 . . .... . . 36 



First bicuspids 56 13 2 i i . . . . 73 



Second bicuspids 51 16 2 2 71 



Canines . . 30 34 12 5 . . i . . 82 



Second molars 5 44 80 288 249 66 14 746 



{ It seems possible from the method employed that, especially in the case of the second molars, 

 the tables may err on the side of overstating the age. ) Livy's researches show that in the 

 first dentition the first molars, incisors, and canines come through first in the lower jaw. In 

 most cases the bicuspids come first in the upper. The second molars come first in the lower jaw, 

 unless their appearance is delayed, in which case the order is uncertain. The date of the appear- 

 ance of the second molar can be only an appro.ximate guide to the age. When it is present the 

 child is unlikely to be under twelve. The change in the shape of the jaw — namely, the lengthening 

 necessary for a longer row of larger teeth, as well as the widening required to make room for the 

 canines — begins in the course of the second dentition and continues after its close, as the second 

 molar does not at once assume its permanent position in regular line with the rest. It was 

 pointed out in the section on the growth of the face that the greatest activity of growth takes 

 place at the pauses of dentition. The roots of the permanent teeth are by no means fully 

 developed at their eruption. With their perfection the sockets are formed around them by the 

 harmonious moulding of the parts involved. 



Homologies. — There are two chief evolutionary theories of the origin of the mammalian 

 teeth : one, the concrescence theory, is that they are formed by the growing together of originally 

 separate cones, the primitive reptilian teeth. This view is supported by Rose^ and Kiikenthal,^ 

 at least for the bicuspids and molars. Cope,* whom Osborn* has followed, advanced the 

 differentiation theory, according to which the many cusps of the molars have arisen as outgrowths 

 from a primitive cone. This is based on comparative anatomy and paleontology. According to 

 this, there was first the cone, in the upper jaw called tha proiocotie and in the lower \he prolo- 

 conid. Two secondary cusps ne.xt appeared respectively before and behind it : the paracone 

 and metacone of the upper teeth and the paraconid and nietaconid of the lower. The next 

 change is for these to move to the labial side in the upper jaw and to the lingual in the lower. 

 Thus the primitive cone and these two secondary ones form the points of a triangle with the base 

 outward in the upper jaw and inward in the lower. A prolongation, the talo>i or heel, is next 

 developed on the posterior end of the tooth, and rises into a single cusp, the hypocone in the 

 upper jaw and the hypoconid in the lower. The last, however, has two secondarv cusps spring 

 from it, the entoconid and the hypoconid. According to this theory, the paraconid of the lower 

 teeth has disappeared in the human molars owing to want of room consequent on the develop- 

 ment of tlie talon of the upper teeth. The following table shows the homologies of the cusps 

 of the human molars according to Osborn. 



Upper Molars. 



Anterior lingual Protocone. "j 



Anterior buccal Paracone. \ Forming the triangle. 



Posterior buccal Metacone. ) 



Posterior lingual i Hypocone. The talon. 



Remnant of triangle. 



Lower Molars. 



Anterior buccal Protoconid. 1 



Anterior lingual Metaconid. I 



Posterior buccal Hypoconid. ] 



Posterior lingual Entoconid. [■ The talon. 



Posterior Hypoconulid. J 



Rose has advanced, in support of his theory of concrescence, that calcification begins sepa- 

 rately for each cusp. Osborn points out that Rose has shown that they ossify ver\"nearly in 

 the order of their alleged evolution. Schwalbe ^ professes himself unable to' decide on the 

 relative merits of the two theories. 



Variations. —Variations of the cusps and of the fangs have been described with the teeth. 

 Those of number affect chiefly the incisors and molars. An additional incisor may occur on 

 one or both sides in either dentition, not very rarely in the upper jaw, but extremely so in the 

 lower, the condition in the latter being more stable. Extra upper incisors are often more or less 

 displaced to the rear and implanted obliquely. They are particularly common in cases of cleft 

 palate ; not impossibly the presence of additional teeth predisposes to the non-union of the 



' Anatom. Anzeiger, Bd. vii., 1892. 



^ Jenaische Zeitschrift, Bd. xxviii.. 1893. 



* Journal of Morphology, 1888. 1889. 



* American Naturalist, 1888, and International Dental Journal, 1895. 



* Anatom. Anzeiger, Bd. ix., 1894. 



