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THE OCCIPITAL BONE. 109 
cartilage which unites it to the sides and apex of the petrous part of the temporal 
bone. 
The foramen magnum, of oval shape, so disposed that its long axis lies in the 
sagittal plane, is of variable size and form. The plane of its outlet differs somewhat 
in individual skulls; in most instances it is directed downwards and slightly 
forwards. In front the condyles encroach upon it, and narrow to some extent its 
transverse diameter. To its margins are attached the ligaments which unite it 
with the atlas and axis. Through it pass the lower part of the medulla oblongata 
where it becomes continuous with the spinal cord, the two vertebral arteries, the 
spinal econ nerves, and the blood vessels of the meninges of the upper part of 
the cord. 
Connexions.—The occipital bone articulates with the two parietals in front and above, with 
the sphenoid in front and below, with the two temporals on either side, and with the atlas 
vertebra by means of its condyles. 
Architecture.—The squamous part displays thickenings in the position of the various ridges 
and crests, the stoutest part corresponding to the internal and external occipital protuberances, 
though it should be noted that the two protuberances do not necessarily coincide, the internal 
being, as a rule, placed at a higher level than the external. If the bone be held up to the light 
it will be at once apparent that it is much thinner where it forms the floor of the inferior fossee 
than in the upper part. The basilar portion consists of a spongy core surrounded by a more 
compact outer envelope, thickest on its lower surface. In the condyles the spongy tissue is 
arranged radially to their convex articular surfaces, the hypoglossal canal being surrounded by 
particularly dense and compact bone. , 
Variations.—The most striking of the many variations to which this bone is subject is the 
separation of the upper part of the occipital squama to form an independent bone—the inter- 
parietal bone, called also, from the frequency of its occurrence in Peruvian skulls, the os Ince. 
As will be seen below (see ossification), the occurrence of this anomaly is explained development- 
ally. In place of forming a single bone the interparietal is occasionally met with in two 
symmetrical halves, and instances have been recorded of its occurrence in three or even four 
pieces. In the latter cases the two anterior parts form the pre-interparietals. The articular 
surface of the condyles is sometimes divided into an anterior and posterior part. The so-called 
third occipital condyle is an outstanding process rising from the anterior border of the foramen 
magnum, the extremity of which articulates with the odontoid process of the axis. Springing 
from the under surface of the extremity of the jugular 
process, a rough or smooth elevated surface, or else a 
projecting process, the extremity of which may articu- 
late with the transverse process of the atlas, is some- 
times met with. This is the paroccipital or para- 
mastoid process. Numerous instances of fusion of the 
atlas with the occipital bone have been recorded. Many 
are, no doubt, pathological in their origin; others are 
associated with errors in development. — Interesting 
anomalies are these in which there is evidence of the 
intercalation of a new vertebral element between the 
atlas and occipital, constituting what is termed a 
proatlas. 
Ossification.—The major part of the bone 
ossifies in cartilage, the upper part of the squama 
(interparietal), alone developing in membrane. The 
basilar part begins to ossify about the sixth week 
of foetal life by the appearance of two centres, one 
in front of the other; the anterior, according to 
Albrecht, constitutes the basiotic, the posterior the 
basioccipital. These two centres—which there is 
some reason to believe—may themselves be formed 
by the fusion of pairs placed laterally, rapidly unite, 
so that the occurrence of one centre alone 1s 
frequently described. From this the fore part of Fie. 84.—Osstrication or OccrprraL BONE. 
the margin of the foramen magnum is formed, , pasilar cenitterh2ss Baoceipiial oh au Ossiele 
together with a portion of the anterior end of the — of Kerkring; d, Supra-occipital (from car- 
occipital condyle on either side. It helps also _ tilage); e, Fissure between supra-occipital 
Fomclose sup the front-of the anterior’ condylic 20d mterparietal; /, Interparietal (from 
canal. Union with the condylic parts is com- Eee Grasstney Peliyeer inher 
plete about the fourth or fifth year. Ankylosis , 
between the basiocvipital and the sphenoid takes place about the twenty-fifth year. 
The lateral, condylic, or exoccipital parts begin to ossify from a single centre about 
