THE OCCIPITAL 



25 



The squamo-occipital is suucer-shaped, deeply concave on its cerebral, l)ut 

 convex on its external aspect. It consists of two parts which have different modes 

 of origin. The posterior surface is divided by a ridge, the superior nuchal line, 

 into a lozenge-shaped superior portion with a smooth surface and an inftiior louuh 

 j)ortion. The upi)er is the interparietal, and tlie lower the supra-occipital seg- 

 ment. The interparietal portion not infrec^uentlv persists as an indcpt iid^nt ossicle 

 (fig. 35). 



The supra-occipital is dividiMJ into two lateral lialves by a median vertical ridge 

 — the external occipital crest — which ascends from the middle of the i)osterior 

 margin of the foramen magnum, to terminate at the external occipital protuber- 

 ance, or inion, near the middle of the squamo-occipital. The protulxrance and 

 crest give attachment to the ligamentum nucha?. 



Each lateral half of the supra-occipital presents three pairs of transverse ridges, 

 the nuchal lines. Of these the superior is usually the least conspicuous, but 

 most curved; frequently it is absent; l)eginning at the external occipital protuljer- 

 ance, it curves outwards to the lateral angle. It affords attachment to the epi- 

 cranial aponeurosis and to few fibres of the occipito-fruntali-i muscle. 



Fio. 30. — The Occipital. (External view.) 



Trapezius INION OR EXTERNAL-OCCIPITAL PROTUBERANCE 



SUPERIOR NUCHAL LINE 



Occipito-frontalis 



MIDDLE NUCHAL LINE 



Sterno-mastoid 



Splenius capitis 



Rectus cap. post. min. 

 Rectus cap. post. maj. 



Superior oblique 



INFERIOR NUCHAL LINE 



Rectus capitis lateralis 



POSTERIOR CONDYLOID 

 FORAMEN 



JUGUUR PROCESS 



BA3I-0CC1PITAL 



The middle nuchal line (sometimes called the superior curved line) commences 

 a little distance Ix'low the protuberance, and curves outwards to end below the 

 lateral angle. In some cases the sui)erior and middle nuchal lines are confluent 

 in their outer thirds, and form a prominent ridge for the insertion of the sterno- 

 mastoid and splenius capitis muscles. When these muscles are well developed, there 

 is a fairly wide interval ])etween the lines. 



The inferior nuchal line begins near tlie middle of the crest and cur\-(\«^ down- 

 wards to the jugular ])rocess. 



Of the spaces deliminated by these lines, that between the superior and middle 

 is occupied ^yy the trapezius, and frequently by the sterno-mastoid and splenins capitis 

 muscles. The space lietween the middle and inferior receives the complexus and 

 superi(n- oblique, and the space between the inferior line and the foramen magnum 

 IS occupied V)y the rectus capitis posticus minor, and the rectus capitis postims vuijar 

 muscles. 



The cerebral surface is deeply concave, an<l divided by crucial ridges into 

 four fossae, of which the upi)er two accommodate the occipital l()bes of the cere- 

 brum, the lower i)air the cereljellar hemis])heres. The ridges intersect one another, 

 and at the point where they cross, an eminence, the internal occipital protuber- 



