52 



THE SKELETON 



Speaking generally, this division corresponds to the four separate parts of which the bone 

 consists at the time of birth (fig. 69), known as the basi-occipital, supra-occipital, and ex- 

 occipital. In early life these parts fuse together, the lines of junction of the supra-occipital and 

 ex-occipitals extending lateralward from the posterior margin of the foramen magnum, and 

 those of the ex-occipitals and basi-occipital passing through the condyles near their anterior 

 extremities. It must be noted, however, that the upper portion of the squamous part represents 

 an additional bone, the interparietal. 



The squamous part [squama occipitalis] (supra-occipital and interparietal) 

 presents on its convex posterior surface, and midway between the superior angle 

 and the posterior margin of the foramen magnum, a prominent tubercle known 

 as the external occipital protuberance, from which a vertical ridge — the external 

 occipital crest — runs downward and forward as far as the foramen. The pro- 

 tuberance and crest give attachment to the ligamentum nuchae. 



Fig. 65. — The Occipital. (External view.) 



External occipital protuberance 



Trapezius 

 Semispinalis capitis 



Occipitalis 



Area covered by 

 —' scalp 



Linea suprema 



Rectus capitis anterior 

 Longus capitis 



Condyle 



Attachment of superior constrictor of 

 pharynx to pharyngeal tubercle 



Arching lateralward on each side from the external occipital protuberance 

 toward the lateral angle of the bone is a semicircular ridge, the superior nuchal 

 line [linea nucha superior], which divides the surface into two parts — an upper 

 [planum occipitale] and a lower [planum nuchale]. Above this line, a second less 

 distinctly marked ridge — the highest nuchal line [linea nuchse suprema] — is 

 usually seen. It is the most curved of the three lines on this surface and gives 

 attachment to the epicranial aproneuosis and to a few fibres of the occipitalis 

 muscle. Betw(!('n th(! superior and highest curved lines is a narrow crescentic 

 area in which the bone; is smoother and denser than the rest of the surface, whilst 

 the part of the bone above tiie iin(;a supr(;ma is convex and covered by the scalp. 



The lower part of the surface is very uneven and subdivided into an upper 

 and a lower area by the inferior nuchal line, which runs laterally from the middle 

 of the crest to the jugular process. 



