TH£ 60A?£^ OF THE HEAD 6l 



transverse process of the atlas, and is homologous with it. Pos- 

 teriorly it is continuous with the tabulcir portion, and anteriorly it 

 presents the jugular notch, which, with the jugular fossa of the 

 petrous portion of the temporal, forms the jugular foramen. 

 Superiorly it presents a short, but deep and \\-ide, groove for a portion 

 of the lateral venous sinus just before it leaves by the jugular fora- 

 men. This groove may be pierced by a posterior condylar fora- 

 men. Interiorly it gives attachment to the rectus capitis lateralis, 

 and ma}' send downwards a projection towards the transverse 

 process of the atlas, which represents the paramastoid process of com- 

 parative anatomy. Externally the jugular process articulates with 

 the jugular facet on the petrous portion of the temporal by synchon- 

 drosis up to the twenty-fifth year, after which ankylosis takes place. 



The foramen magnum is situated at the lower and anterior part 

 of the bone, and is oval, its long axis extending from before back- 

 wards. The inferior margin, in front of the condyles, gives attach- 

 ment to the anterior occipito-atlantal ligament, and, behind them, to 

 the posterior occipito-atlantal ligament. The foramen transmits 

 the medulla oblongata and its membranes, the spinal accessory 

 nerves, the vertebral arteries, the anterior and posterior spinal 

 arteries, and parts of the cerebellar amygdalae. 



The chief blood-supply of the bone is derived from the occipital 

 and posterior auricular arteries. 



Articulations. — Superiorly with the parietals, laterally with the 

 temporals (mastoid and petrous portions), anteriorly with the 

 sphenoid, and inferiorly with the atlas, and in rare cases with the 

 odontoid process of the axis. 



Structure. — The occipital, being a tabular bone, is composed of 

 two tables of compact bone, with cancellated tissue, called diploe, 

 between them. 



Varieties. — (i) There may be a minute foramen piercing the external 

 occipital protuberance for an emissary vein, which passes between the intra- 

 cranial torcular HeropMli and one of the tributaries of the extracranial 

 occipital vein. (2) The upper di\-ision of the tabular portion mav be separate, 

 representing the interparietal bone of comparative anatomy, and it may be in 

 one piece, or in two or more. (3) The semilunar area between the highest 

 and superior curved hnes may be prominent, constituting the torus occipitalis 

 transversus. (4) The anterior condylar foramen may be double on its cranial 

 aspect. (5) There may be a third occipital condyle on the anterior margin 

 of the foramen magnum. (6) There may be a paramastoid process on the 

 under aspect of the jugular process. (7) The condyle may be di\'ided into 

 two parts, anterior and posterior. (8) There may be an intrajugular process 

 on the front of the jugular notch, which mav extend as far as the petrous 

 portion of the temporal. 



Ossification. — The bone is developed in four parts. The tabular portion 

 usually ossifies from four centres, which appear around the internal occipital 

 protuberance about the eighth week of intra-uterine life. Two are deposited 

 in cartilage, one for each cerebellar fossa, which soon fuse and give rise to 

 the lower or supra-occipital division. The other two are deposited in membrane, 

 one in each cerebral fossa, which also soon fuse and give rise to the upper 

 or interparietal division. Indeed, as a general rule, all four ultimately blend. 

 There may, however, be two other centres for the interparietal portion, placed 



