74 



OS OCCIPITIS. 



common junction of the sinuses, the groove for the longitu- 

 dinal sinus divides into two grooves for the lateral sinuses ; 

 Fig. 17.* these form the horizontal 



limbs of the cross, and pro- 

 ceed towards the foramen 

 lacerum where the lateral 

 sinuses emerge from the 

 cavity of the cranium. The 

 lower limb of the cross is 

 formed by a spine which pro- 

 ceeds from the centre of the 

 bone to the great occipital fora- 

 men, and supports the falx of 

 the cerebellum. The internal 

 surface of the bone is divided 

 by the cross into four por- 

 tions, each of which is con- 

 siderably depressed so as to form fossae ; the two upper by the 

 posterior lobes of the cerebrum, and the lower by those of the 

 cerebellum. 



This circumstance occasions great inequality in the thickness 

 of the bone, as the depressed portions are extremely thin, while 

 the ridge adds greatly to the thickness, especially at the centre 

 of the cross, which is opposite to the great external protuber- 

 ance. 



Before the great occipital foramen is the cuneiform process, 

 which is thick and substantial ; it terminates by a broad 

 truncated extremity, that is articulated with the body of the 

 sphenoid bone. The internal surface of the cuneiform process 



* The internal surface of the occipital bone. 1. The left cerebral fossa. 

 2. The left cerebellar fossa. 3. The groove for the posterior part of the superior 

 longitudinal sinus. 4. The spine for the falx cerebelli, and groove for the 

 occipital sinuses. 5. The groove for the left lateral sinus. 6. The internal 

 occipital protuberance which lodges the torcular Herophili. 7. The foramen 

 magnum. 8. The basilar process, grooved for the medulla oblongata. 9. The 

 termination of the groove for the lateral sinus, bounded externally by the jugu- 

 lar eminence. 10. The jugular fossa this fossa is completed by the petrous 

 portion of the temporal bone. 11. The superior border. 12. The inferior border. 



13. The border which articulates with the petrous portion of the temporal bone. 



14. The anterior condyloid foramen. 



