OCCIPITAL BONE. 31 



External Surface. Crossing the middle of the bone transversely, 

 from one lateral angle to the other, is a prominent ridge, the superior 

 semicircular ridge. In the middle of the ridge is a projection, called 

 the occipital protuberance ; and descending from it a small vertical 

 ridge, the spine. Above and below the superior semicircular ridge the 

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

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

 semicircular ridge, 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 , l and on each side of each condyle a pro- 

 jecting ridge, the transverse process, excavated in front by a notch 

 which forms part of the jugular foramen. 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 fossa3 ; 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 attachment to the tentorium cerebelli, and is deeply 

 grooved, for the lateral sinuses. At the point of meeting of the four 

 arms, is a projection, the internal occipital protuberance, which cor- 

 responds 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 portion 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 foramen magnum are two rough eminences, 

 which give attachment to the odontoid ligaments, and immediately 

 above these the openings of the anterior condyloid foramina. In 



The foramen magnum. 6. The condyle of the right side. 7. The posterior con- 

 dyloid fossa, in which the posterior condyloid foramen is found. 8. The anterior con- 

 dyloid foramen concealed by the margin of the condyle. 9. The transverse process ; this 

 process upon the internal surface of the bone forms the jugular eminence. 10. The 

 notch in front of the jugular eminence which forms part of the jugular foramen. 11. 

 The basilar process. 12, 12. The rough projections into which the odontoid ligaments 

 are inserted. 



