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 brachial plexus is formed by the anterior primary divisions of the four lower cervical nerves and the greater part of that of the first thoracic nerve. It is usually joined by small twigs from the fourth cervical and second thoracic nerves.
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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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