150 



OSTEOLOGY. 



developed, lies close below the infra-orbital groove, and it is not till later that they 

 become separated by the growth of the maxillary sinus, which first makes its 



appearance as a shallow fossa to the medial side of the 

 orbito-nasal element about the fourth month. In the 

 adult bone the course of the infra-orbital canal and 

 foramen indicates the line of fusion of the orbito-nasal 

 and zygomatic elements, whilst the position of the 

 anterior palatine canal serves to determine the line of 

 union of the incisive with the palatine elements. In 

 addition to the foregoing centres, Rambaud and Renault 

 describe another, the infra -vomerine, which, together 

 with its fellow, is wedged in between the incisive and 

 the palatine elements beneath the vomer, thus explaining 

 the Y-shaped arrangement of the foramina of Stensen, 

 which open into the incisive foramen. 



The premaxillae, which in most vertebrates are in- 

 dependent bones lying in front of the maxillse, constitute 

 in man and apes the portions of the maxilla which 

 lie in front of the incisive foramen, and support the 

 superior incisor teeth. Each premaxilla is developed 

 from two centres : a facial, which ultimately contains 

 the incisor and canine teeth, and forms the anterior part 

 of the hard palate, as well as the anterior half of the 

 frontal process of the complete maxilla (Fawcett) ; and 

 a palatine centre (infra vomerine of Rambaud and 

 Renault) which forms the medial wall of the correspond- 

 ing canal of Stensen. The former develops very early, 

 either before or after the maxilla (Mall), and fuses almost 



~ at once with the maxilla along the alveolar margin ; the 

 A, Lateral side ; B, Medial side ; C, , , , & , .,. ' 



Under side, a, Nasal process ; 6, latter appears about the twelfth week, and soon fuses 

 Orbital plate ; c, Anterior nasal spine ; with the facial centre. The line of fusion of the pre- 

 d t Infra-orbital groove ; e, Infra-orbital maxillae with the maxillae proper can be readily seen 

 foramen; /Anterior palatine groove; in young s k u lls, and occasionally also in the adult. It 

 a, Palatine process ; h, Premaxillary J , . , J , , 



suture ; t, Alveolar process. corresponds to a suture which passes on the palate 



obliquely laterally and forwards, from the incisive 



foramen to the interval between the lateral incisor and the canine tooth. In cases 

 of alveolar cleft palate the adjacent bones fail to unite along the line of the suture. 

 In some instances, however, the cleft passes outwards between the central and lateral 

 incisor teeth, and this condition suggests the explanation that the premaxillary element is 

 derived from two centres a lateral and a medial. The researches of Albrecht and 

 Warinski support this view. The latter anatomist further observes that the lateral 

 cleavage may lead to a division of the dental germ of the lateral incisor tooth, and so 

 explain the occurrence of the supernumerary incisor which is occasionally met with. In 

 this way the different varieties of cleft palate are readily explained ; median cleft palate 

 being due to failure of union between the two premaxillary bones. Lateral cleft palate 

 may be of two types : the cleft in one case passing forwards between the central and 

 lateral incisor, and being due to the non-union of the two elements from which the 

 premaxilla is primarily developed ; the other, in which the cleft passes between the 

 lateral incisor and the canine, or between the lateral incisor and a supernumerary in- 

 cisor, owing to the imperfect fusion of the premaxilla laterally with the maxilla. 



FIG. 159. OSSIFICATION OF THE 

 MAXILLA. 



Ossa Palatina. 



The palate bone, of irregular shape, assists in the formation of the lateral 

 wall of the posterior part of the nasal cavity, the posterior portion of the hard 

 palate, the orbit, the pterygo^-palatine, the infra-temporal, and the pterygoid fossae. 

 It consists of horizontal and vertical parts, united to each other like the limbs of 

 the letter L. At their point of union there is an irregular outstanding process, 

 called the pyramidal process, whilst capping the summit of the vertical part and 

 separated by a deep cleft are two irregular pieces of bone, called the sphenoidal and 

 orbital processes. 



