Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/40

26 26 ANATOMY OF THE RAT

with the occipital condyles of the skull by means of two lateral concave surfaces, the superior articular foveae, one on each side. It bears posteriorly a pair of lateral ovoid surfaces, inferior articular foveae, for articulation with the epistropheus. A third articular surface connects these facets ventrally. In man, and presumably in the rat, the chief movement permitted between the superior articular foveae and the occipital condyles is a "nodding" of the head. In "shaking" the head the atlas revolves around the dens as a pivot. The elongated transverse process of the atlas is flattened dorsoventrally. Its dorsal surface slopes backward to the posterior end of the vertebra, where it joins a vertical plate. Between this plate and the inferior articular fovea is the posterior opening of the foramen transversarium, through which the vertebral artery passes. This foramen extends anterodorsally, and communicates with the dorsal surface of the atlas by an oval opening. The artery then turning medially traverses the atlantal foramen to the inner surface of the atlas.

The centrum of the epistropheus is flattened dorsoventrally and prolonged anteriorly by the dens. The dens articulate with the fovea dentis by the anterior articular facet on its ventral side. The cranial articular facets, one on each side of the epistropheus, impinge upon the inferior articular foveae of the atlas. The postzygapophyses, one toward the upper end of each pedicle, articulate behind with the prezygapophyses of the third cervical vertebrae. The laterally flattened neural spine is considerably higher than that of any other cervical vertebra. It extends forward over the atlas and backward over the third vertebra. The foramen transversarium pierces the base of the short transverse process.

In the fifth cervical vertebra the centrum and vertebral arch form in side view an obtuse angle with each other.