The anterior primary divisions of the spinal nerves are larger than the posterior primary divisions, and each is joined near its origin by a grey ramus communicans from the sympathetic gangUated cord. Beginning with the first or second thoracic nerve and ending with the second or third lumber nerve, each anterior division sends to the gangliated cord a white ramus communicans. The same is true of the second and third or of the third and fourth sacral nerves. These white rami are appropriately designated the visceral branches of the spinal nerves. The anterior primary divisions of the cervical, lumbar, sacral, and coccygeal nerves unite with one another to form plexuses, but the anterior primary divisions of the thoracic nerves, except the first and last, remain separate, pursue independent courses, and each divides, in a typical manner, into a lateral and an anterior or ventral branch. The separation of the anterior primary division into lateral and anterior branches is not confined to the thoracic nerves; it occurs also in the lower cervical, the lumbar, and the sacral nerves, but such a division cannot be clearly distinguished either in the upper cervical nerves, or in the coccygeal nerve.
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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