OSSIFICATION OF THE UPPER JAW. 71 



tion of the body either results from the union of these, or is an independent 

 growth. The presphenoid is united to the body of the postsphenoid from the 

 seventh to the eighth month. The line of union is indicated for some time bj a 

 hole filled with cartdage, round above, and opening inferiorly into a wide notch, 

 which is recognisable for several years after birth. The body of the presphenoid 

 is for a year or two broad and rounded inferiorly. but becomes gradually narrower 

 and more i^rominent : it is separated at first by a layer of fibro-cartilage from the 

 splienoidal spongy bones. 



The .'<phcnoidal ><poii(iii hnnrs seldom appear till after birth, and are united to 

 the body at the age of puberty. Each is in early life a hoUow pjTamid fonned 

 by the union of three separate laminae, viz.. an inferior, an external, and a 

 superior : the inferior lamina f oi-ms the greater part of Avhat can be distinguished 

 in the adult ; the external is that to which the orbital portion belongs ; while 

 the superior lamina, forming the inner wall and roof of the original sphenoidal 

 sinus, becomes, as the sinus expands, partly absorbed and partly united to the 

 attenuated body of the pre-sphenoid, which is ultimately reduced to the thin 

 septum sphenoidale and the rostrum. 



In tlie ethmoid bone ossification commences in the fourth or fifth month, by 

 the appearance of a nucleus in the orbital plates of the lateral masses gi-adually 

 extending into the turbinated bones. Dming the fii-st year the vertical and 

 cribriform plates are ossified from a single nucleus, which, spreading outwards, 

 unites with the lateral masses about the beginning of the second year. The 

 ethmoidal cells are not fonned tUl the fourth or fifth year. 



The superior maxillary commences to ossify immediately after the lower 

 maxilla and the clavicle from several nuclei, the earliest of which appears in the 



Fig. 61. 



Fig. 61. — Different views of the Superior Maxillary Bone of a Fcetus of four 

 OR FIVE MONTHS (R. Quain). 



A, external surface ; a fis.sure, 1, is seen extending through the orbit into the infra- 

 orbital foramen. 



]j, the internal surface ; the incisor fissure, 2, extends from the foramen upwards 

 through the horizontal plate and some way into the nasal process. 



C, the bone from Ijelow, showing the imperfect alveoli and the incisor fissure, 2', 1, 

 wliich crosses the palatine plate, between the second and third alveolus, and i^asses through 

 the outer jjart of the boae. 



alveolar arch about the sixth or seventh week. Its early growiih has not yet been 

 sufficiently studied. Beclard (MeckePs Ai-chiv, vi. p. 432) states that it consists 

 at first of five pieces, viz. : 1, an alveolar arch ; 2, a palatal jaart ; 3. an orbital and 

 malar ; -1, a nasal and facial ; 5, an incisor part. This, however, does not appear 

 to be a constant arrangement. By the end of the third month these pieces have 

 united together : at the same time two ridges are seen prolonged downwards from 

 the lower surface of the alveolar arch, forming the dcntnl r/roorc. Before buth 

 this groove is divided into alveoli by partitions which arise from its floor. The 

 antiaim begins as a shallow depression, fonned before birth on the inner surface 

 of the bone. This deepens and extends outwards, gradually separating the orbital 

 and palate portions of the bone, which at Ijirth are very close together. 



In all young subjects, and sometimes even in the adult, there is a fissure — 

 the incifio/- Jixsura — passing outwards from the incisor foramen to the alveolar 



