The anterior primary divisions of the upper four cervical nerves unite to form the cervical plexus, and each receives a communicating branch from the superior cervical sympathetic ganglion. The anterior divisions of the lower four cervical nerves are joined by the greater part of the first thoracic nerve and they unite to form the brachial plexus. The fifth and sixth cervical nerves receive communicating branches from the middle cervical sympathetic ganglion, and the seventh and eighth from the inferior cervical ganglion, while the first thoracic nerve is always connected with the first thoracic sympathetic ganglion by a grey ramus and in most cases also by a white ramus communicans.
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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.