124 



OSTEOLOGY. 



sagittal plane, is of variable size and form. The plane of its outlet differs somewhat 

 in individual skulls; in most instances it is directed inferiorly and slightly 

 forwards. Anteriorly the condyles encroach upon it, and narrow to some extent its 

 transverse diameter. To its margins are attached the ligaments which unite it 

 with the atlas and epistropheus. Through it pass the lower part of the medulla 

 oblongata where it becomes continuous with the spinal medulla, the two vertebral 

 arteries, the accessory nerves, and the blood-vessels of the meninges of the superior 

 part of the spinal medulla. 



Connexions. The occipital bone articulates with the two parietals in front and above, with 

 the sphenoid in front and below, with the two temporals on either side, and with the atlas 

 by means of its condyles. 



Ossification. The major part of the bone ossifies in cartilage, the upper part of the 

 squamous part (interparietal) alone developing in membrane. The basilar part begins to 

 ossify about the sixth week of foetal life by the appearance of two centres, one in front of 

 the other ; the anterior, according to Albrecht, constitutes the basiotic, the posterior the 

 basi-occipital. These two centres which there is some reason to believe may themselves 

 be formed by the fusion of pairs placed laterally rapidly unite, so that the occurrence of 

 one centre alone is frequently described. From this the anterior part of the margin of the 

 foramen magnum is formed, together with a portion of the anterior end of the occipital 

 condyle on either side. It helps also to close up the front of the hypoglossal canal. 

 Union with the condylic parts is complete about the fourth or fifth year. Ankylosis 

 between the basi-occipital and the sphenoid takes place about the twenty-fifth year. 



The lateral, condylic, or exoccipital parts begin to ossify from a single centre about 

 the end of the second month of foetal life. The notch for the hypoglossal canal appears 

 about the third month. From this centre is formed the posterior three-fourths of the 

 occipital condyle. The exoccipital is usually completely fused with the squamous part 

 by the third year or earlier. 



As already noted, the squamous part consists of two parts the one above the occipital 



crest, the other below it; the former develops in 

 membrane, the latter in cartilage. In a three-months 

 foetus this difference is very characteristic. The 

 cartilaginous part (supra-occipital) begins to ossify 

 from two centres (four according to Mall) about the 

 sixth or seventh week, which rapidly join to form an 

 elongated strip placed transversely in the region of 

 the occipital protuberance. The centres for the 

 superior part (interparietal) appear later. According 

 to Maggi (Arch. Ital. Biol. tome 26, fas. 2, p. 301), 

 they are four in number, of which two placed on 

 either side of the median plane appear about the 

 second month. The other pair, placed laterally, are 

 seen about the third month ; fusion between these 

 takes place early, but their disposition and arrange- 

 ment explain the anomalies to which this part of the 

 bone is subject. The medial pair may persist as 

 separate ossicles, or fuse to form the pre-interparietals, 

 whilst the lateral pair may remain independent of the 

 supra-occipital as a single or double interparietal 

 bone, the former, owing to the frequency of its 

 occurrence in Peruvian skulls, being sometimes 

 called the "os Incce." Union between the supra- 

 FIG. 135,-OssiFicATioN OF THE OCCIPITAL occipital and the i nt erparietal elements occurs about 



a, Basilar centre; b, Exoccipital; c, Ossicle the third r f Urth m nth ' bu * evidence of their 



of Kerkring ; d, Supra- occipital (from car- separation is frequently met with even in the adult 



tilage) ; e, Fissure between supra-occipital by the persistence of a transverse suture running 



and interparietal ; /, Interparietal (from inwards from each lateral angle of the squamous part, 



m arietlLs ne) 5 9t Fi * SUre betW6en intei " or ' as above mentioned > th ere may be an os Incse. 



The supra -occipital forms a small part of the 



median part of the posterior border of the foramen magnum, though here a small inde- 

 pendent centre, known as the ossicle of Kerkring, is occasionally met with. Other 

 independent centres are sometimes seen between the supra-occipital and the exoccipitals. 



