The brachial plexus is formed by five roots, the union of the anterior divisions of the lower four cervical nerves and the greater part of the anterior division of the first thoracic nerve; the fourth cervical usually gives a branch to the fifth cervical, and the first thoracic frequently receives one from the second thoracic.
The plexus extends from the lower part of the side of the neck to the axilla.
The nerves which form it are nearly equal in size, but their mode of communication is subject to some variation. The following is, however, the most constant arrangement.
This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from http://www.bartleby.com/107/).
- The fifth and sixth cervical unite soon after their exit from the intervertebral foramina to form a trunk.
- The eighth cervical and first thoracic also unite to form one trunk, while the seventh cervical runs out alone.
- Three trunks—upper, middle, and lower—are thus formed, and, as they pass beneath the clavicle, each splits into an anterior and a posterior division.
- The anterior divisions of the upper and middle trunks unite to form a cord, which is situated on the lateral side of the second part of the axillary artery, and is called the lateral cord or fasciculus of the plexus.
- The anterior division of the lower trunk passes down on the medial side of the axillary artery, and forms the medial cord or fasciculus of the brachial plexus.
- The posterior divisions of all three trunks unite to form the posterior cord or fasciculus of the plexus, which is situated behind the second portion of the axillary artery.