732 



CRANIUM. 



spine divides it into lour fossae, the two supe- 

 rior of which are l\\e fossa cere.br i for the pos- 

 terior lobes of the cerebrum, the two inferior, 

 the J'ossee cerebelli, for the hemispheres of the 

 cerebellum ; the former being marked by the 

 convolution of the brain, they are not so smooth 

 as those which lodge the cerebellum. The 

 lower limb of the crucial spine is prominent, 

 and arises by a bifid root from the margin of 

 the foramen magnum; the upper limb is 

 grooved for the reception of the longitudinal 

 sinus, and to its borders the septum cerebri is 

 attached ; this groove is mostly directed to one 

 side or the other, and generally to the right ; 

 to the lateral limbs the tentorium is fixed, and 

 the grooves which are on them contain the 

 lateral sinuses. At the point where the trans- 

 verse bisects the vertical portion of the crucial 

 spine, it is very prominent, is called the inter- 

 nal occipital protuberance, and marks the situ- 

 ation of the torcular Herophili. 



In front of the foramen magnum, ascending 

 obliquely towards the sphenoid bone, and nar- 

 rowing in its ascent, is the upper surface of 

 the basilar process, which is concave from side 

 to side for the lodgement of the pons Varolii 

 and medulla oblongata, and exhibits on each 

 margin a depression (the sulcus basilaris) for 

 the basilar or inferior petrosal sinus. 



On either side of the foramen magnum is a 

 groove which advances from without inwards, 

 and from behind forwards, and lodges the ter- 

 mination of the lateral sinus. The anterior 

 extremity of this groove turns downwards and 

 forms a large notch (the fossa jugularis), which 

 is bounded on the outer side by a strong rough 

 process (the processus jugularis), and on the 

 inner side by a smooth oval eminence which 

 is situated between it and the sulcus basilaris, 

 and below which is the orifice of the foramen 

 condyloideum anticum for the passage of the 

 motor linguae nerve. 



The external convex surface, in that part 

 which is behind the foramen magnum, is di- 

 vided into an inferior rough, and a superior 

 smooth, triangular portion. The division be- 

 tween the two is marked by a curved line 

 (the superior occipital ridge), which abuts on 

 the petrous masses of the temporal bones, 

 and exhibits in its centre the tuberose process, 

 or the external occipital tubercle, to which the 

 ligamentum nuchae is attached. From the 

 ridge next to the tubercle the occipito-frontalis 

 and trapezius muscles arise, and, still more 

 outwardly, the splenius capitis and the sterno- 

 cleido-mastoideus are attached. From the tu- 

 bercle to the foramen magnum extends a 

 longitudinal spine, which is bisected in its 

 middle by a second curved line (the inferior 

 occipital ridge), and constitutes, thereby, the 

 external crucial spine. On each side of the 

 spine and between the two ridges, there is a 

 considerable rough depression for the attach- 

 ment of the complexus, and, to the outer side 

 of it, one which is smoother, for the trachelo- 

 mastoideus. Between the inferior ridge and 

 the foramen magnum, there are on either side 

 of the longitudinal spine, indications of the 

 attachment, in succession, of the recti capitis 



postici minores et majores and of the obliquus 

 capitis superior. On the outer side of this region 

 is the sulcus occipitalis, which runs backwards 

 and upwards between the surfaces of attach- 

 ment of the trachelo-mastoideus and the com- 

 plexus, and is formed by the occipital artery. 



Underneath the anterior half of the margin 

 of the foramen magnum are the condijloid pro- 

 cesses, two elongated articulating eminences, 

 convex in both directions, wider in the middle 

 than at either end, inclined from above down- 

 wards, from behind forwards, and from with- 

 out inwards, and having their internal edges 

 below the level of the external. On the inner 

 side of each process is a rough surface for the 

 attachment of the odontoid ligament ; on the 

 outer side is a ridge (the processus lateralis) 

 which ends in the jugular process, and gives 

 insertion to the rectus capitis lateralis; ante- 

 riorly is the anterior orifice of the anterior 

 condyloid foramen ; and posteriorly there is a 

 depression in which is sometimes seen a fora- 

 men (foramen condyloideum posticum) through 

 which a vein of the scalp communicates with 

 the terminal portion of the lateral sinus. 



In front of the foramen magnum is the 

 under surface of the basilar process, which, 

 by reason of the superior thickness of its an- 

 terior extremity, is not so oblique as it appears 

 on its upper surface. There is a slight tuber- 

 cle on the middle line to which is fixed the 

 middle constrictor of the pharynx, and behind 

 it, on both sides, a transverse line for the su- 

 perior constrictor, between which and the 

 foramen are depressions caused by the recti 

 capitis antici majores et minores. 



The superior angle of this bone is applied 

 on the junction of the two parietal, and the 

 serrated borders which extend from it to the 

 lateral angles are articulated to the posterior 

 borders of the same bones. The upper angle 

 itself and more than half of the borders pro- 

 ceeding from it, overlap the parietals, but in 

 the remainder of their extent the latter bones 

 overlap the occipital ; in each case the arrange- 

 ment being the same as that which exists be- 

 tween the parietal and the frontal bones. 



From the lateral angles to the jugular pro- 

 cesses, a rough but not denticulated border 

 articulates it to the posterior border of the 

 mastoid portion of the temporal bone. Im- 

 mediately in front of the jugular process is the 

 fossa jugularis, which forms, in common with 

 the temporal bone, the foramen jugulare or 

 foramen lacerum posticum in basi cranii, 

 through which emerge the jugular vein, the 

 pneumo-gastric, glosso-pharyngeal, and spinal 

 accessory nerves. The rest of the border from 

 the fossa jugularis to the anterior angle is in 

 apposition with the petrous portion of the 

 temporal bone, but the quantity of cartilage 

 between them is too large to admit of there 

 being any fixed articulation at this part. 



The anterior angle itself is truncated and 

 presents a rough quadrilateral surface, which 

 articulates, and, indeed, consolidates itself, at 

 an early period of life, with the basilar process 

 and body of the sphenoid bone. This union 

 is so complete and so similar to the union 



