Illustrated Encyclopedia of Human Anatomic Variation: Opus III: Nervous System: Plexuses
Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed
In one study of 175 plexuses, more than 60% received a communication from the fourth cervical nerve. Of those that did not receive a contribution from the fourth, almost 30% of the 175 plexuses studied received the entire anterior primary division of the fifth cervical. In the remaining 10%, a portion of the fifth cervical contributed to the cervical plexus.
The brachial plexus was bilaterally symmetrical in 61.9% of 63 cadavers.
Brachial plexuses receiving a branch from the fourth cervical nerve are more cephalic with reference to the vertebral column and have been designated as high or prefixed. Those receiving the largest part of the fifth cervical nerve are more caudal with reference to the vertebral column and have been designated as low or postfixed brachial plexuses.
The following variations in the organization or course of components of this plexus have been reported. The fifth cervical nerve may pass in front of or through the anterior scalene muscle.
The sixth cervical nerve may cross the anterior scalene. The seventh cervical nerve (as the middle trunk) may break up into three branches (instead of the usual two), one branch going to each of the three cords.
Occasionally the fibers of the posterior cord may arise from only the the seventh and eighth cervical nerves.
The first thoracic may fail to contribute to the posterior cord or may be the sole source of the medial cord.
Instead of the usual three cords, only two cords, a smaller and a larger, may be present. In such cases, the larger cord will replace either the medial and lateral or the medial and posterior cords.
Cases of a single cord have been reported.
A prefixed (high) type of brachial plexus exists in which the fourth cervical nerve contributes a fairly large branch, the dorsal branch of the first thoracic nerve is small, and there is no ascending branch from the second thoracic nerve. A postfixed (low) type of plexus is occasionally encountered in which the branch from the fourth cervical nerve is absent, the dorsal branch of the first thoracic is large, and the second thoracic nerve supplies a branch to the plexus.
In humans the brachial plexus is prefixed when compared to that in all primates below the anthropoids.
The variations in this plexus may be appreciated by examining the illustrations that accompanied this text. The memorization of a textbook drawing of the brachial plexus, for testing purposes, is without merit. It would be more meaningful to teach the significance of the plexus and its permutations.
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