The occipital bone (os occipitis; Germ. Occi pital-knochen, Hinterhaupts-knochen,) is situ ated behind the sphenoid, and forms the pos terior part of the base of the cranium and the contiguous projection of the occiput. Its figure is that of a lozenge with its anterior anole truncated, and is so curved as to be generally concave on one surface and convex on the other. The inferior and anterior half of it is situated between the two temporal bones ; the superior and posterior half is be tween the posterior margins of the twO pa rietal.
At its anterior part it is pierced by a large elliptical foramen (the foramen magnum), through which there pass, from the skull,. the medulla spinalis and its membranes, the sinus venosus and the spinal arteries ; and, into the skull, the vertebral arteries, the posterior me ningeal arteries, and the 1101'11S accessorius.
On the cerebral surface the internal crucial spine divides it into four fossx, the two supe rior of which are the l'ossee cerebri for the pos terior lobes of the cerebrum, the two inferior, the Jossw cerebelli, for the hemispheres of the cerebellum ; the former being marked by the convolution of the brain, they are not so smooth as those Nvhich 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 lingum 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 nuch 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 miens 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 condyloid 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 proccssus 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 condyloideumposticum) through which a vein of the scalp communicates with the terminal portion of the lateral sinus.