OSSIFICATION OF THE TEMPORAL BONE. 69 



the posterior inferior angles of the parietal bones. The fontaneUes ai-e gradually 



Fig. 58. — Skull of a Child at Birth, seek Fig. 58. 



FROM ABOVE (from Leisbman). ^ 



a, superior fontanelle ; p, posterior fontanelle ; 

 h, h, parietal eininences ; for the lateral fontanelles, 

 see fig. 65, p. 77. 



filled up by the extension of ossification 

 into the membranes which occupy them 

 from neighbourmg bones, thus completing the 

 angles of the bones and forming the sutures. 

 The closure. especiaUj"- of the posterior and 

 lateral, is further assisted by the frequent 

 development of "Wonnian bones in these situa- 

 tions. All traces of these unossified spaces 

 disappear before the age of four years. 



The temporal bone in respect of its state of ossification in the later stages 

 of foetal life, consists of thi-ee pieces, the squamo-zygomatic. petro-mastoid, 

 and tympanic. The .squamo-zygomatic is ossified fi-om a single nucleus, 

 which appears in the lower part of the squamosal about the seventh or eighth 



Fig. 59. — Separate Parts op the Temporal Bone Fig. 59. 



OF A Child at Birth (R, Quain). 



rt, squamo-zygomatic ; h, the tympanic bone, ^ifftf ' . "^ 



forming an imxierfect ring, open superiorly ; c, the 

 petro-mastoid part ; c being placed on the mastoid 

 part : part of the cavity shown in shade is the 



tympanum. 



week. From this point ossification extends 

 upwards into the squamosal, and outwards into 

 the zygoma. Dui'ing the third month an osseous 

 nucleus appears in the lower part of the external 

 membranous wall of the tympanum, and 

 extends upwards as a slender arch, forming 

 the tf/mjmnic rhifj, an imperfect ch-cle. open 

 superiorly, and which encloses the tympanic 

 membrane. Before bu-th the extremities of this 

 ring become luiited with the squamo-zygomatic. 



Petro-mastoid or periotic. The ossification of this part of the temporal bone 

 does not begin so soon as that of the other parts. It is only towards the end of 

 the fifth month and in the course of the sixth (according to Kolliker and Huxley) 

 that the bony parts of the internal car begin to appear in the cartilage which 

 precedes them. These parts arise, at least in the case of the labyrinth, from 

 numerous minute bony deiiosits ; some, however, soon coalescing. The fii-st 

 osseous nuclei appear at the end of the fifth month on the anterior surface of 

 the petro-mastoid over the cochlea, and shortly afterwards one or two nuclei 

 appear on the posterior surface in connection with the formation of the semi- 

 ciicular canals. These different nuclei soon unite, and ossification extends out- 

 wards into the mastoid portion. According to Huxley the principal ossific centres 

 come to be disi^osed so as to form three portions, which may be distinguished 

 by the foUo^^dng names applied to them respectively by him, viz. : 1, Proof ic, 

 including most of the labyrinth and upper part of the petrous bone, together 

 with part of the mastoid ; 2, Oj)i.'<uif'u', comprehendmg mainly the lower jaart of 

 the petrous bone, with the fenestra rotunda and half of the fenestra ovalis ; ^'^^^Vcv,'^^ 

 and 8, Ujjiotic, coiTesponding to the lower part of the mastoid. 



At biith the petro-mastoid is separated from the squamosal by a thin plate of 

 intervenmg cartilage, bony union taking place dui'ing the fii'st year ; the mastoid 

 l^ortion also is quite flat, the glenoid fossa shallow, the articular eminence scarcely 



