84 SURGICAL ANATOMY OF THE 



which entering it obliquely, after inosculating in its sub- 

 stance with the second and third cervical nerves, passes 

 out behind its posterior border, and crosses the posterior 

 triangular space. The muscle is readily seen in action, 

 on rotating the head, or on bowing it upon the thorax, 

 when both muscles are used. 



Parts beneath the Sterno-cleido-mastoid. Immediately 

 behind the muscular fibres is the posterior layer of the 

 sheath ; and between it and the muscle lie a considerable 

 number of lymphatics, and some twigs of the sterno- 

 mastoid branches of the superior thyroid artery. 



The most convenient method of grouping the structures 

 which lie beneath the muscle is to divide it into three 

 portions, making the crossing of the omo-hyoid to sepa- 

 rate the middle and lower, and a line drawn backwards 

 from the angle of the jaw to separate its middle and 

 upper. Beneath the posterior layer of the sheath in the 

 upper third, from above downwards, the structures met 

 with are, the insertion of the splenius capitis, beneath 

 which are the attachments of the posterior belly of the 

 digastric and of the trachelo-mastoid, the posterior auricu- 

 lar and occipital vessels, the external border of the corn- 

 plexus, while still deeper are the attachments to the atlas, 

 of the obliqui and rectus lateralis muscles, the vertebral 

 artery, and suboccipital nerve. In the middle third, 

 passing obliquely into the under surface of the muscle, 

 is the spinal accessory nerve, which unites with some 

 filaments of the cervical plexus ; next appear a chain 

 of lymphatic glands (glandules concatenatce), and the 

 branches of origin of the superficial cervical plexus, the 

 descendens and cornmunicans noni nerves. Beneath 

 them lie the common, internal and external carotid arte- 

 ries and jugular vein, the hypoglossal nerve, the vagus, 



