OCCIPITAL BONE. 59 



OCCIPITAL BONE. This bone is situated at the posterior part and base 

 of the cranium. It is trapezoid in figure, and divisible into two surfaces, 

 four borders, and four angles. 



External Surface. Crossing the middle of the bone transversely, from 

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

 line. In the middle of the ridge is a 

 projection, called the external occipital Fig. 22* 



protuberance ; and descending from it a 

 small vertical ridge, the spine. Above 

 and below the superior curved line the 

 surface is rough, for the attachment of 

 muscles. About three-quarters of an 

 inch below this line is another trans- 

 verse ridge, the inferior curved line, 

 and beneath the latter the foramen mag- 

 num. On each side of the foramen 

 magnum, nearer to its anterior than 

 its posterior segment, and encroaching 

 somewhat upon the opening, is an ob- 

 long articular surface, the condyle, for 

 articulation with the atlas. The con- 

 dyles approach towards each other an- 

 teriorly, and their articular surfaces look dow r nwards 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 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 fossse ; the two superior or cerebral fossae lodging 

 the posterior lobes of the cerebrum ; and the two inferior 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 corre 

 spends 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 



* The external surface of the occipital bone. 1. The superior curved line. 2. The 

 external occipital protuberance. 3. The spine. 4. The inferior curved line. 5. The 

 foramen magnum. 6. The condyle of the right side. 7. The posterior condyloid fo&sa, 

 in which the posterior condyloid foramen is found. 8. The anterior condyloid 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 o r 

 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. 



