208 Anatomy of Skeleton 



and the Digastric, then beyond the Trachelomastoid, and then, beyond the level of the posterior 

 border of the Sternomastoid, is only covered by Splenius capitis, and makes for the interval between 

 this muscle and the Trapezius. It lies on fibres of Complexus at this interval, then crosses the bone 

 between the upper curved line and linea suprema, to pass on to the Occipito-frontalis. Before 

 reaching the Complexus it gives off its princeps cervicis branch, which thus lies over the suboccipital 

 muscles and can divide into deep and superficial branches in relation with the Complexus. 



The supraoccipital or part of the bone above the superior curved line is covered 

 by the occipito-frontalis, which is attached to the bone a little distance above the 

 upper curved line on the so-called linea suprema ; the central part of the line is usually 

 better marked because the aponeurosis is fastened here, the muscle fibres arising 

 further out. 



The bone between the linea suprema and linea superior is crossed by the occipital 

 artery as it runs up to lie on the aponeurosis after emerging from between Trapezius 

 and Splenius (Fig. 171) : this part of the bone is sometimes prominently bulged, being 

 then known as the torus occipitalis transversus. 



The blunt prominence of the external protuberance is easily felt from the surface, 

 and the bone can be easily palpated through the movable scalp above this level : 

 the apex of the lambdoid suture is about 2\ inches above the prominence. Below 

 this level, however, the bone is deeply covered by the thick muscles of the neck and 

 cannot be felt. As might be expected from the absence of connection between the 

 factors responsible for them, the outer and inner prominences do not by any means 

 necessarily correspond in position or level on the bone. This is a fact to be borne 

 in mind when using the outer projection for purposes of locating the deep structures : 

 thus, for example, the lateral sinus may have the surface relations seen in Fig. 168, or 

 may be higher or lower compared with the upper curved line and outer protuberance ; 

 as a rule the groove is well above the level of the superior line. 



The front ends of the condyles are in a line joining the middle of the two outer 

 meatuses, but on a lower level. The middle of the foramen magnum is very little 

 behind a line joining the tips of the mastoid processes, but above its level. Observe, 

 moreover, that in the skull in the ordinary position the lower surface of the condyles 

 is about on a level with the hard palate. 



Development. 



The basi-, ex-, and post- occipital are laid down in cartilage, but the supraoccipital 

 is a membrane bone. The basi-occiput begins to ossify first, quickly followed by the 

 post-occipital, in the sixth to seventh week, while the centres for the exoccipital appear 

 about the eighth week. Centres for the basi-occiput are paired, fusing rapidly to 

 form one : in addition an anterior centre has been described, looked on by some as an 

 epiphysis-en the bone. Each exoccipital has a single centre. For the cartilaginous 

 part of the post-occipital two or four centres are described, while for the upper mem- 

 branous part four centres appear to be the rule, appearing first about the beginning 

 of the third month. Of the four membranous centres the two lower are termed the 

 interparietal and the smaller upper ones the preinterparietals. At birth the bone is 

 in four pieces (see Fig. 214). The tabular and exoccipital parts unite in the third year, 

 and the basal portion joins a year or two later. Looking at the figure it is seen that the 

 exoccipitals form the greatest part of the margin of the foramen, the other elements 

 of the bone only contributing a small piece each in front and behind : the hinder 

 end of the margin may, however, have a separate small centre constituting the " ossicle 

 of Kerckring." 



