BRACHIAL PLEXUS. 637 



joined with the third, and supplies the Complexus, Splenius, and Trachelo- 

 mastoid muscles. 



The internal branches, the larger, are distributed differently in the upper and 

 lower part of the neck. Those derived from the third, fourth, and fifth nerves 

 pass between the Semispinalis and Complexus muscles, and having reached the 

 spinous processes, perforate the aponeurosis of the Splenius and Trapezius, and 

 are continued outwards to the integument over the Trapezius; whilst those 

 derived from the three lowest cervical nerves are the smallest, and are placed 

 beneath the Semispinalis, which they supply, and do not furnish any cutaneous 

 filaments. These internal branches supply the Complexus, Semispinalis Colli, 

 Interspinales, and Multifidus Spina3. 



The posterior branches of the three first cervical nerves require a separate 

 description. 



The posterior branch of the first cervical nerve (suboccipital) is larger than the 

 anterior, and escapes from the spinal canal between the occipital bone and the 

 posterior arch of the atlas, lying behind the vertebral artery. It enters the 

 triangular space formed by the Eectus Posticus Major, the Obliquus Superior, 

 and Obliquus Inferior, and supplies the Eecti and Obliqui muscles, and the 

 Complexus. From the branch which supplies the Inferior Oblique a filament 

 is given off', which joins the second cervical nerve. This nerve also occasionally 

 gives off a cutaneous filament, which accompanies the occipital artery, and 

 communicates with the occipitalis major and minor nerves. 



The posterior division of the first cervical has no branch analogous to the 

 external branch of the other posterior cervical nerves. 



The posterior branch of the second cervical nerve is three or four times greater 

 than the anterior branch, and the largest of all the posterior cervical nerves. 

 It emerges from the spinal canal between the posterior arch of the atlas and 

 lamina of the axis, below the Inferior Oblique. It supplies this muscle, and 

 receives a communicating filament from the first cervical. It then divides into 

 an external and an internal branch. 



The internal branch, called, from its size and distribution, the occipitalis major, 

 ascends obliquely inwards between the Inferior Oblique and Complexus, and 

 pierces the latter muscle and the Trapezius near their attachments to the cranium. 

 It is now joined by a filament from the third cervical nerve, and ascending on 

 the back part of the head with the occipital artery, divides into two branches, 

 which supply the integument of the scalp as far forwards as the vertex, com- 

 municating with the occipitalis minor. It gives off an auricular branch to the 

 back part of the ear, and muscular branches to the Complexus. 



The posterior branch of the third cervical is smaller than the preceding, but 

 larger than the fourth ; it differs from the posterior branches of the other cervi- 

 cal nerves in its supplying an additional filament to the integument of the 

 occiput. This occipital branch arises from the internal or cutaneous branch 

 beneath the Trapezius ; it pierces that muscle, and supplies the skin on the 

 lower and back part of the head. It lies to the inner side of the occipitalis 

 major, with which it is connected. 



The internal branches of the posterior divisions of the first three cervical 

 nerves are occasionally joined beneath the Complexus by communicating 

 branches. This communication is described by Cruveilhier as the posterior 

 cervical plexus. 



THE BRACHIAL PLEXUS. (Fig. 349.) 



The Brachial Plexus is formed by the union of the anterior branches of the 

 four lower cervical and first dorsal nerves. It extends from the lower part of 

 the side of the neck to the axilla. It is very broad, and presents little of a 

 plexiform arrangement at its commencement, is narrow opposite the clavicle, 

 becomes broad, and forms a more dense interlacement in the axilla, and divides 

 opposite the coracoid process into numerous branches for the supply of the 



