The posterior primary divisions of the spinal nerves spring from the trunks immediately outside the intervertebral foramina, and they pass dorsalward between the adjacent transverse processes. With the exceptions of the first and second cervical nerves they are smaller than the corresponding anterior primary divisions, which in these nerves is smaller from the fact that a large portion of them go over into the hypoglossal or cervical loop. The posterior primary divi- sions, after passing between the transverse processes into the region of the back, divide into medial and lateral branches. This division, however, does not occur in the cases of the first cervical, the last two sacral, and the coccygeal nerves.
The posterior primary divisions of the upper four sacral nerves escape from the vertebral canal by passing through the posterior sacral foramina; those of the fifth sacral nerve pass out through the hiatus sacralis between the posterior sacro-coccygeal ligaments.
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The posterior primary division of the first cervical or suboccipital nerve springs from the trunk, between the vertebral artery and the posterior arch of the atlas, passes dorsalward into the sub-occipital triangle, and breaks up into branches which supply the superior oblique, the inferior oblique, and the major rectus capitis posterior muscles, which form the lateral boundaries of the triangle.
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