THE DEVELOPMENT OF THE JAW. 181 



A full half of the length of the crowns of the central 

 incisors, about half that of the laterals, and the tips only of 

 the canines are calcified ; the first temporary molars are 

 complete as to their masticating surfaces ; the second tem- 

 porary molars have their cusps more or less irregularly 

 united, in many specimens the four cusps being united into 

 a ring of dentine, the dentine in the central depression of 

 the crown not being yet formed. During the formation of 

 the permanent teeth, very similar relations exist between 

 the amount of calcification in the incisors and canines ; thus 

 when, as sometimes happens, the development of the teeth 

 proceeds very imperfectly up to a certain date, and then 

 changes for the better, it may be that the lower half of the 

 crown of the central incisor, somewhat less of the lateral, 

 and the extreme tip of the canine will be honeycombed, 

 while the remainder of the tooth will be perfect, thus per- 

 petuating an evidence of the stages to which each of these 

 teeth had at that particular period attained. 



Having noted in some detail the characters of the jaws of 

 a nine months foetus, we may pass on to the consideration 

 of those changes which precede the cutting of the deciduous 

 teeth. A general increase in size takes place, new bone 

 being developed at all those points where the maxillse are 

 connected by soft tissue with other bones, as well as from 

 their own periosteum. But the increase in dimensions does 

 not take place in all directions equally, so that material 

 changes of form result. 



In correspondence with the elongation of the tooth sacs, 

 the alveoli become increased in depth, and their edges circle 

 inwards over the tooth sacs; active development of bone 

 takes place in the sutures uniting the two halves of the jaws 

 to one another, which is compensated by the inclination 

 inwards of the alveoli of the central incisors. In the lower 

 jaw the articular process, at first hardly raised above the 

 level of the alveolar border, rises rapidly up, the direction 



