Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/29

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Having examined the general topography of the skull, the structure of each bone should be studied in greater detail.

This bone first appears in the mammalian embryo as four centers of ossification in the chondrocranium (cartilaginous cranium). One of these centers is dorsal (supraoccipital), one ventral (basioccipital), and two are lateral (exoccipitals) to the foramen magnum. These four bones fuse in mammals to form the occipital. The supra- and ex-occipital regions in the rat are the posterior wall of the cranium. The basioccipital is the floor of the cerebellar fossa.

The basioccipital region is trapezoidal in shape, broader behind than in front. Anteriorly it joins the basisphenoid bone by a suture at right angles to the long axis of the skull. Posteriorly it forms the ventral boundary of the foramen magnum. Laterally it comes in contact with, but is not fused to, the tympanic bulla and the petrosal bone. Note the two marked depressions on the ventral surface of the bone which are separated from one another by a median ridge. The inner surface of the basioccipital, where it comes in contact with the brain, is concave.

The occipital condyles and the triangular jugular processes previously mentioned, are parts of the exoccipital regions. The condyles bound the foramen magnum ventrolaterally. They articulate with the atlas, or first neck vertebra. This articulation permits the head to be raised