Tin: srin:.\oin 



29 



Ligaments : — 



Ligamentum nuchse. 



Capsular. 



Posterior occii)ito-atlantal. 



Anterior occipito-atlantal. 



Oblique occipito-atlantal. 



Suspensory ligament. 



Cheek ligaments. 



Vertical slip of the crucial. 



Posterior common ligament of spine. 



Anterior common ligament of sjjine. 



The til)rous bag of the })harvnx 



Blood-supply. — The occipital bone receives branches from the occipital, pos- 

 terior auiicular, middle meningeal, vertebral and ascending pharyngeal arteries. 



Development. — The interparietal portion of the occipital is a menil)rane-bone, 

 and arises usually by two and occasionally by four centres about the twelfth week; 

 these nuclei rapidh^ become confluent and fuse with the supra-occi{)ital portion 

 about the fifteenth Aveek. Occasionally this fusion fails. The centres for the rest 

 of the bone are dt'i)osited in cartilage. The nucleus for the basi-occii)ital appears 



Fig. 35. — The Occii-rrAL with a Separate Interparietal. 





INTER PA RliETAL. 



r 



W' ' SUPRAQCCIPiTAL 



in the tenth week, and is quickly followed by a nucleus for each ex-occipital; the 

 su|)ra-occipital ossifies from two laterally disposed nuclei, Avhich <juickly coalesce 

 and fuse with the interparietal portion near the situation of the future occi})ital 

 protuberance. For many weeks two deep lateral fissures separate the interparietal 

 and supra-occipital portions, and a membranous space, extending from the centre 

 of the squamo-occipital to the foramen magnum, partially separates the lateral 

 portions of the supra-occipital. This space becomes occupied by a sjMcule of bone, 

 and is of interest, because through it hernia of the brain and its membranes, known 

 as occii)ital meningocele or encephalocele, occurs. 



At birth the occipital consists of four parts: the squamo-occipital. tAvo ex-occip- 

 itals, and the l)asi-occipital, united V)v strips of cartilage. The ex-occipitals and 

 scjuamo-occipital fuse together about the fifth year, and unite with the basi-occi}>ital 

 before the seventh year. The posterior tAvo-thirds of each occipital condyle belongs 

 to the ex-occipitals, and the anterior third to the basi-occipital (fig. 33). 



Not infrequently the interparietal portion remains separate throughout life, and 

 may even be represented by numerous detached ossicles or Wormian l)ones. By 

 the tAventy-fifth year the basi-occii)ital is firmly ankylosed to the si)henoid. 



THE SPHENOID 



The sphenoid forms a large part of the base of the skull in the region of the 

 anterior and middle fossae. It is very irregular in shape, and is best descrilied as 

 consisting of a l)ody, two pairs of wings, and two ])airs of processes. 



