THE DEVELOPMENT OF THE HUMAN BKULL. 159 



Of the facial bones, the premaxilla is developed within that 

 part of the naso-frontal process which forms the anterior 

 boundary of the mouth. The maxilla, the palatine, and ptery- 

 goid bones are produced within the maxillary process — the 

 former from its external, the latter from its internal part. The 

 internal pterygoid is, even before birth, united with the external 

 pterygoid, the latter being simply an outgrowth downwards of the 

 alisphenoid. None of these bones are at present known to be 

 developed from cartilage, and the lachrymal and jugal are, 

 similarly, membrane bones: 



The cartilaginous rods within the first and second visceral 

 arches undergo very remarkable changes. That of the first 

 arch becomes modified into an upper portion, the future incus, 

 and a lower portion, articulated with this, the future malleus, 

 from which the rest of the cartilage is continued, as " Meckel's 

 cartilage," along the inner side of the visceral arch (Fig. 64). 



The ineudal and malleal portions of the cartilage are, at 

 first, proportionally very large, but their growth soon becomes 

 arrested, and, a centre of ossification appearing in each, they 

 become the incus and malleus. As the root of the first visceral 

 arch is close to the outer and front part of the periotic capsule, 

 so the incus and malleus have a corresponding position, and 

 the tympanic bone, which is developed around the circumfer- 

 ence of the modified first visceral cleft (which becomes con- 

 verted into the auditory meatus, the tympanum, and the 

 Eustachian tube), necessarily lies outside them, so that Meckel's 

 cartilage passes between the tympanic bone and the periotic 

 capsule, in its course from the malleus forwards and down- 

 wards. In front, the tympanic circlet marks the limit of its 

 ossification. So far, it constitutes the p'ocessus gracilis (Pg., 

 Fig. 64), while, beyond this point, it eventually becomes ob- 

 literated. Very early, however, ossification takes place in the 

 membrane of the first visceral arch, adjacent to the middle 

 of the cartilage, and extending upwards towards the squamosal 

 bone and, downwards and inwards, towards the symphysis, 

 lavs the foundation for each ramus of the lower jaw. The 

 lower jaw, therefore, arises from membrane, and is not pre- 

 formed in cartilage. 



