THE NECK 125 



medulla and efferent fibres to the blood-vessels, sweat-glands, 

 etc., of the head, neck, and upper limb. 



The anterior ramus (primary division) of each spinal nerve 

 receives from the sympathetic a grey ramus communicant, in 

 which afferent fibres are conveyed to the central nervous system. 

 Efferent fibres from the spinal medulla to the sympathetic are 

 contained in white rami communicantes , which are only present 

 in certain regions ; they connect the anterior rami of T. i or 

 T. 2 L. i or L. 2, and those of S. 2 and 3 or S. 3 and 4 to the 

 corresponding sympathetic ganglia. As the cervical ganglia 

 of the sympathetic trunk receive no white rami communicantes, 

 the efferent fibres which they distribute leave the spinal medulla 

 in the highest white ramus communicans, and then ascend 

 through the sympathetic trunk into the cervical region (Fig. 40). 

 They are, therefore, involved in complete transverse lesions of 

 the spinal medulla, above the level of the first thoracic segment. 

 Such lesions are accompanied by paralysis of the dilatator 

 pupillae muscle, which is supplied by fibres which leave the 

 superior cervical ganglion of the sympathetic trunk and enter 

 the skull with the internal carotid artery. As a result of this 

 paralysis, the pupil on the affected side is small and does not 

 react to light. 



The Posterior Triangle of the Neck. The upper part 

 of the posterior triangle of the neck is exposed during the re- 

 moval of the postero-superior group of the deep cervical lymph 

 glands ; its lower part is exposed (i) in operations on the postero- 

 inferior group of lymph glands, (2) in operations on the brachial 

 plexus, (3) in ligature of the third part of the subclavian artery, 

 and (4) in the removal of a cervical rib. The base of the triangle 

 is formed by the middle third of the clavicle, and its anterior 

 and posterior boundaries by the posterior border of the sterno- 

 mastoid and the anterior border of the trapezius respectively. 

 The layers of fascia which form the roof and cover the floor of 

 the triangle are described on pp. in, 112. Between them a 

 varying amount of loose fat is found, in which lie the accessory 

 nerve, the descending branches of the cervical plexus, and the 

 termination of the external jugular vein. 



The Accessory Nerve emerges from the sterno-mastoid 

 at the junction of the upper and middle thirds of its posterior 

 border, and runs obliquely downwards and laterally across the 

 posterior triangle. About two inches above the clavicle, or on 

 a level with the seventh cervical spine, it disappears under cover 



