974 THE NERVOUS SYSTEM 



the spinal nerves. The anterior primar}^ 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 divi- 

 sion 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 divi- 

 sion cannot be clearly distinguished either in the upper cervical nerves, or in the 

 coccj^geal nerve. 



1. CERVICAL NERVES 



The anterior primary divisions of the upper four cervical nerves unite to form 

 the cervical plexus, ancl each receives a communicating branch from the superior 

 cervical sympathetic ganglion. The anterior divisions of the lower four cervical 

 nerves are joined bj^ the greater part of the first thoracic nerve and they unite to 

 form the brachial plexus (figs. 751, 754, 755). The fifth and sixth cervical nerves 

 receive communicating branches from the middle cervical sympathetic ganglion, 

 and the seventh and eighth from the inferior cervical ganglion, while the first 

 thoracic nerve is always connected with the first thoracic sympathetic gang- 

 lion by a grey ramus (figs. 751, 786) and in most cases also by a white ramus 

 communicans. 



THE CERVICAL PLEXUS 



The cervical plexus (figs. 751, 752) is formed by the anterior primary divisions 

 of the upper four cervical nerves which constitute the roots of the plexus. It 

 lies in the upper part of the side of the neck, under cover of the sterno-mastoid, 

 and upon the levator scapulae and the scalenus medius. It is a looped plexus, 

 consisting of three loops. 



A large part of the anterior primary division of the first cervical nerve is given 

 to the hj'poglossal or cervical loop; the remainder passes to the cervical plexus and 

 in doing so it runs lateralward on the posterior arch of the atlas beneath the verte- 

 bral artery, then it turns forward, between the vertebral artery and the outer side 

 of the upper articular process of the atlas, and finally it descends, in front of the 

 transverse process of the atlas, and unites with the upper branch of the second 

 nerve, forming with it the first loop of the plexus. It gives branches to the rectus 

 capitis lateralis, longus capitis (major rectus capitis anterior), and to the rectus 

 capitis anterior (minor). The division communicates with the ganglion of the 

 trunk of the vagus and with the superior cervical ganghon of the sympathetic 

 system (fig. 752). From the first loop of the plexus, two branches of the division 

 pass over into the sheath of the hypoglossal nerve and descend with it to contrib- 

 ute to the hypoglossal loop [ansa hypoglossi] or better, the cervical loop. The 

 fibres entering the sheath of the hypoglossus, after giving a few twigs to the 

 genio-hyoid and thyreo-hyoid muscles, leave the sheath as the descendens cer- 

 vicalis {hypoglossi) and this latter joins the communicans cervicalis, (the portion 

 of the loop from the second and third cervical nerves) and thus completes the 

 cervical or hypoglossal loop. 



This loop usually may be found between the sheaths of the sterno-mastoid muscle and the 

 carotid artery, superficial to the internal jugular vein; sometimes it may lie in the carotid 

 sheath between the carotid artery and the internal jugular vein; rarely it may lie dorsal to both 

 the artery and vein. (Sometimes it is relatively long, descending toward the sternum below the 

 level <jf flie thyreoid cartilage; again it is quite short and occurs near the level of the hyoid 

 bone. The descendens cervicalis (hypoglossi) parts comjiany with the hypoglossal nerve at 

 the level at which the; nerve curves around the occipital artery. It runs downward and slightly 

 medialward on the sheaths of the great vessels and occasionally within the sheath of one of 

 them. 



The second cervical nerve (anterior primary division) passes behind the upper 

 articular ])roc<'.ss of the axis and the verte})ral artery, and between the inter- 

 transverse muscles extending from the first to tlu^ second cervical vertebrae, to 

 the interval between tlie scalenus medius and tlu; longus capitis (rectus capitis 

 anterior major), where it divides into two parts. The u])])er ])art ascends and 

 unites with tlie first nerve to form the first loop of the plexus, and the lower branch 

 passes downward and dorsal ward and joins the upper branch of the third nerve in 



