OCCIPITAL BONE. 



61 



centres 



at It'rfX 



Ae Jfj>zc0e.s 



fej>a rale, 



/ for occipital 

 portion 



1 for each conJyloici 

 portion 



thick, at the ridges, protuberances, condyles, and anterior part of the basilar 

 process; whilst at the bottom of the fossse, especially the inferior, it is thin, semi- 

 transparent, and destitute of diploe. 



Development (fig. 24). The occipital bone has four centres of development ; 

 one for the posterior or occi- 

 pital part, which is formed in Fig. 24.-Development of Occipital Boue. 

 membrane ; one for the basilar 

 portion; and one for each 

 condyloid portion, which are 

 formed in cartilage. 



The centre for the occipi- 

 tal portion appears about the 

 tenth week of foetal life ; and 

 consists, according to Blandin 

 and Cruveilhier, of a small 

 oblong plate which appears 

 in the situation of the occi- 

 pital protuberance. 1 The 

 condyloid portions then 

 ossify, and lastly the basilar 

 portion. At birth, the bone 



consists of four parts, separate from one another, the occipital portion being fissured 

 in the direction above indicated. At about the fourth year, the occipital and the 

 two condyloid pieces join'; and about the sixth year the bone consists of a single 

 piece. At a later period between the eighteenth and twenty-fifth years, the 

 occipital and sphenoid become united, forming a single bone. 



Articulations. With six bones; two parietal, two temporal, sphenoid, and atlas. 



Attachment of Muscles. To the superior curved line are attached the Occipito- 

 frontalis, Trapezius, and Sterno-cleido-mastoid. To the space between the curved 

 lines, the Complexus, Splenius capitis, and Obliquus superior; to the inferior 

 curved line, and the space between it and the foramen magnum, the Rectus 

 posticus major and minor ; to the transverse process, the Rectus lateralis ; and to 

 the basilar process, the Recti antici majores and minores, and Superior Constrictor 

 of the pharynx. 



THE PARIETAL BOXES. 



The Parietal Bones (paries, a wall) form by their union the sides and roof of 

 the skull ; each bone is of an irregular quadrilateral form, and presents for ex- 

 amination two surfaces, four borders and four angles. 



Surfaces. The external surface (fig. 25) is convex, smooth, and marked about 

 its centre by an eminence, called the parietal eminence, which indicates the point 

 where ossification commenced. Crossing the centre of the bone in an arched 

 direction is a curved ridge, the temporal ridge, for the attachment of the temporal 

 fascia. Above this ridge, the surface of the bone is rough and porous, and covered 

 by the aponeurosis of the Occipito-frontalis.; below it the bone is smooth, forms 

 part of the temporal fossa, and affords attachment to the Temporal muscle. At 

 the back part of the superior border, close to the sagittal suture, is a small 

 foramen, the parietal foramen, which transmits a vein to the superior longitudinal 

 sinus. Its existence is not constant, and its size varies considerably. 



The internal surface (fig. 26), concave, presents eminences and depressions for 

 lodging the convolutions of the cerebrum, and numerous furrows for the ramifica- 

 tions of the meningeal arteries ; the latter run upwards and backwards from the 



1 BSclard considers this segment to have four centres of ossification, arranged in pairs, two 

 above, and two below the curved lines, and Meckel describes eight, four of which correspond in 

 situation with those above described : of the other four, two are placed in juxta-position, at the 

 upper angle of the bone, and the remaining two, one at each side, in the lateral angles. 



