OCCIPITAL BONE. 



curved line. In the middle of the ridge is a projection, called the ex- 

 ternal occipital protuberance ; and descending from it a small vertical 

 ridge, the spine. Above and below the superior curved line the sur- 

 face is rough, for the attachment of muscles. About three-quarters of 

 an inch below this line is another transverse ridge, the inferior curved 

 line, and, beneath the latter, the foramen magnum. On each side of 

 the foramen magnum, nearer to its anterior than its posterior segment, 

 and encroaching somewhat upon the opening, is an oblong articular 

 surface, the condyle, for articulation with the atlas. The condyles 

 approach towards each other anteriorly, and their articular surfaces 

 look downwards and outwards. Directly behind each condyle is an 

 irregular fossa, and a small opening the posterior condyloid foramen, for 

 the transmission of a vein to the lateral sinus. In front of the condyle 

 is the anterior condyloid foramen, for the hypoglossal nerve ; and on 

 the outer side of each condyle a projecting ridge, the transverse process, 

 excavated in front by a notch which forms part of the jugular fora- 

 men. In front of the foramen magnum is a thick square mass, the 

 basilar process, and in the centre of the basilar process a small tubercle 

 for the attachment of the superior and middle constrictor muscles of 

 the pharynx. 



Internal Surface. Upon the internal surface is a crucial ridge, 

 which divides the bone into four fossae ; the two superior or cerebral 

 fossae lodging the posterior lobes of the cerebrum ; and the two in- 

 ferior or cerebellar, the lateral lobes of the cerebellum. The superior 

 arm of the crucial ridge is grooved for the superior longitudinal sinus, 

 and gives attachment to the falx cerebri ; the inferior arm is sharp 

 and prominent, for the attachment of the falx cerebelli, and slightly 

 grooved, for the two occipital sinuses. The transverse ridge gives at- 

 tachment to the tentorium cerebelli, and is deeply grooved, for the 

 lateral sinuses. At the point of meeting of the four arms, is a pro- 

 jection, the internal occipital protuberance, which corresponds with the 

 similar process situated upon the external surface of the bone. The 

 convergence of the four grooves forms a slightly depressed fossa, upon 

 which rests the torcular Herophili. In the centre of the basilar por- 

 tion of the bone is the foramen magnum, oblong in form, and larger 

 behind than before, transmitting the spinal cord, spinal accessory 

 nerves, and vertebral arteries. Upon the lateral margins of the fora- 

 men magnum are two rough eminences, which give attachment to the 

 odontoid ligaments, and immediately above these the openings of the 

 anterior condyloid foramina. In front of the foramen magnum is the 

 basilar process, grooved on its surface, for supporting the medulla 

 oblongata, and along each lateral border, for the inferior petrosal si- 

 nuses. On each side of the foramen magnum is a groove, for the 

 termination of the lateral sinus ; a smooth surface, which forms part of 

 the jugular fossa ; and a projecting process which divides the two, and 

 is called the jugular eminence. Into the jugular fossa will be seen 

 opening the posterior condyloid foramen. 



The superior borders are very much serrated, and assist in forming 



