74, 



THE SKULL AS A WHOLE. 



even simulate fracture. The osseous union of the supra- and exoccipitals, beginning in the 

 second or third, is completed in the fourth year ; that of the basi- and exoccipitals, beginning 

 in the third or fourth, is completed in the fifth or sixth year. The basioccipital is united to 

 the basisphenoid by intervening cartilage up to about the twentieth year, after which ossific 

 union begins and is completed in one or two years. 



The parietal bone begins to ossify in membrane in the seventh week. According to Toldt l 

 it has two nuclei, an upper and a lower, which speedily fuse into a single mass occupying the 

 position of the future parietal eminence. The radiating ossification extends in such a way as 

 to leave a notch or cleft at the upper part of the bone a little distance from the posterior 

 angle, giving rise to the sagittal fontanelle a space between the two bones, which gradually 

 becomes closed in during the latter half of foetal life. Traces of the fontanelle are often to be 

 recognized at the time of birth ; and the parietal foramina are remains of the interval. 2 In rare 

 cases a trans verse parietal Jissure persists. The parietal eminence is very conspicuous in the 

 young bone, and gives a marked character to the form of the skull for a number of years in 

 early life. 



The frontal bone is ossified from two nuclei which appear, one on each side above the orbital 

 arch, about the seventh week. At birth the bone consists of two separate lateral portions, 

 which meet in a vertical median suture during the first year. This frontal suture usually 



Fig. 75. FRONTAL BONE OP A FCETUS SHORTLY 



BEFORE BIRTH. (K. Quahl.) 



& b indicate the two portions of the bone, 

 in each of which the radiation of bony spicula 

 from the frontal eminence is seen. 



Fig. 76. SKULL OP A CHILD AT BIRTH, FROM 

 ABOVE. (Leishman.) ^ 



a, anterior fontanelle ; p, posterior fon- 

 tanelle ; b, b, parietal eminences ; for the lateral 

 fontanelles, see fig. 83, p. 82. 



becomes obliterated by osseous union taking place from below upwards, during the second 

 year, though not unfrequently it persists throughout life (p. 39). The frontal sinuses appear 

 about the seventh year, and continue to increase in size up to old age. 



Fontanelles. These are membranous intervals between the incomplete angles of the 

 parietal and neighbouring bones, in some of which movements of the soft wall of the cranium 

 may be observed in connection with variations in the state of the circulation and respiration. 

 They are six in number, two median, anterior and posterior, and four lateral. The anterior 

 fontanelle, situated between the antero-superior angles of the parietal bones and the superior 

 angles of the ununited halves of the frontal bone, is quadrangular in form, and remains open 

 for some time after birth. The posterior fontanelle. situated between the postero-superior 

 angles of the parietal bones and the superior angle of the occipital bone, is triangular in shape. 

 It is filled up before birth, but the edges of the bones, being united by membrane only, are 

 still freely moveable upon each other. The lateral fontanelles, small and of irregular form, 

 are situated at the inferior angles of the parietal bones. The fontanelles are gradually filled 

 up by the extension of ossification into the membrane which occupies them, thus completing 

 the angles of the bones and forming the sutures. The closure, especially of the posterior and 

 lateral, is often assisted by the development of Wormian bones in these situations. All traces 

 of these unossified spaces disappear before the age of four years. The sagittal fontanelle, 

 existing before birth, has been noticed above. 



The temporal bone in the later stages of fcetal life consists of three principal pieces, the 

 squamo-zygomatic, petro-mastoid or periotic, and tympanic. The sqiiamo-zygomatic is ossified 

 in membrane from a single nucleus, which appears in the lower part of the squamosal about 

 the seventh or eighth week, and extends upwards into the squamosal, and outwards into the 

 zygoma. From the hinder part of the squamosal a considerable postaudltory process grows 

 downwards below the supramastoid crest, separating the tympanic from the periotic, and 



1 "Lotos, Jahrbuch f. Naturw.," 1882. 



2 Broca, Bull. Soc. Anthrop. de Paris, 1875 ; Augier, " Rech. sur le developpement des parietaux 

 a la region sagiitale," Thess, Paris, 1875. 



