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Show 158 ON THE STHUCTUHE OF THE SKUIJlJ. the ali sphenoids and orbito-sph noid , add th ms l ves by a similar obliteration of the pri1nitivo s pnrations. By addition of bony matter to it free n1aro'in, n1 r p ·ially to that of its lower part, the tympanic bone be om converted into the gutter-like exteTnal auditory m atus. Th pioti grows out, inferiorly, into th mastoid pro e s. 'rh \ cavity brn ath the bony ar h in which th snp •rior v rtical s "lni h· ·ular rannJ is lodged, at first filled only by a plng f dura Inat r,- becomes oLliterated by bon . B Fig. G3.- The hal>i- phenoitl and pr sph noid, with th or!Jito- phcnoids of a human sku ll nt hirth.-A, view (1 from aho1e; I , from the r ight sid' ; ', f rom h low. T.S., tnbel'culum sellre ; L. ., L ingula >plumoidalis ; a, ba ·i-prcsphenoidal synchondrosis. The basi- phenoid aequir lartr r dim n ion in proportion to th lingulm sphenoidales, and th • po t rior clinoid processes, at first rartilaginou , L cmn n11l t I y ~ ifi d. ~rhe bones of Bertin unite with the uncl r- urfa · f th pr ]>h0noicl, and the latt r becmn almo ·t ulit rat cl, or ·onv rtccl into a mere vertical lamina of b n , by th xt n ion of tho olfactory eha1nb rs backward t gi v ri. to the phenoiclal sinuses. The lat ral masse of tho .. tlnn id b cOJn an hylosed with the lamina perpendieula1·is, and ~ nn one bono-tho ethmoid of human anatmny. 'riiE DEVELOPMENT OF THE HUMAN SKULJJ. 159 Of the facial bones, the premaxilla is d veloped within that part of the naso-frontal process which forms the ant rior boundary of the mouth. The maxilla, the palatine, and pt rygoid bones are produced within the maxillary process-th 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 downward 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 tho :first arch becomes modified into an upper portion, the future inc·us, and a lower portion, articulated with this, the future malleus, from which the rest of the cartilage is continued, as "l\ieckol' cartilage,'' along the inner side of the visceral arch (Fig. o4). The incudal and n1alleal portions of the cartilao-e are at 0 ' first, proportionally very largo, 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 vi. ceral arch is close to the outer and front part of the periotic cap ule, so the incus and malleus have a corresponding position, and the tympanic bone, which is developed around the circum£ renee of the modified first v~sceral cleft (which becomes converted into the auditory meatus, the tympanurn, and the Eustachian tube), necessarily lies outside them, so that Meckel' cartilage .Pa~ses between the tympanic bone and the peri tic capsule, In Its course from the malleus forwards and downwards. In front, the tympanic circl t n1arks the limit of it os~ification. .so far, it constitutes the processus gracilis (Pg., ~Ig. G4), while, beyond this point, it eventually becomes obliterated. Very early, however, ossification takes place in the membrane. of the first visceral arch, adjacent t the Iniddl of the cartilage, and extending upwards toward the squamo al bone and, downwards and inwards, towards the symphysi.: lays t~e foundation for each ramus of the lower jaw. ~rh~ lower J~W, th~refore, arises from membrane, and i not preformed In cartilage. |