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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
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. 1 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.

Relations.In the neck, the brachial plexus lies in the posterior triangle, being covered by the skin, Platysma, and deep fascia; it is crossed by the supraclavicular nerves, the inferior belly of the Omohyoideus, the external jugular vein, and the transverse cervical artery. It emerges between the Scaleni anterior and medius; its upper part lies above the third part of the subclavian artery, while the trunk formed by the union of the eighth cervical and first thoracic is placed behind the artery; the plexus next passes behind the clavicle, the Subclavius, and the transverse scapular vessels, and lies upon the first digitation of the Serratus anterior, and the Subscapularis. In the axilla it is placed lateral to the first portion of the axillary artery; it surrounds the second part of the artery, one cord lying medial to it, one lateral to it, and one behind it; at the lower part of the axilla it gives off its terminal branches to the upper limb.


FIG. 808– The right brachial plexus with its short branches, viewed from in front. The Sternomastoid and Trapezius muscles have been completely, the Omohyoid and Subclavius have been partially, removed; a piece has been sawed out of the clavicle; the Pectoralis muscles have been incised and reflected. (Spalteholz.) (See enlarged image)


Branches of Communication.—Close to their exit from the intervertebral foramina the fifth and sixth cervical nerves each receive a gray ramus communicans from the middle cervical ganglion of the sympathetic trunk, and the seventh and eighth cervical similar twigs from the inferior ganglion. The first thoracic nerve receives a gray ramus from, and contributes a white ramus to, the first thoracic ganglion.
Note 1.  The posterior division of the lower trunk is very much smaller than the others, and is frequently derived entirely from the eighth cervical nerve. [back]

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