THE NECK. 445 



across the upper part of the trachea. Its lobes encroach 

 upon the inferior triangles, which are bounded on each side 

 above by the omo-hyoid, posteriorly by the sterno-mastoid, 

 and anteriorly by the median line. Its floor is formed by 

 the scalenus anticus and the longus colli muscles. It con- 

 tains, beneath the cover of the sterno-mastoid, the common 

 carotid artery, internal jugular vein, the pneumogastric 

 and phrenic nerves, the trachea, and thyroid gland. The 

 posterior triangles are: the superior, or suboccipital, and 

 the inferior, or subclavian. The superior triangle is 

 bounded in front by the posterior border of the sterno- 

 mastoid, below by the posterior belly of the omo-hyoid, 

 and posteriorly by the trapezius. Its floor is formed by the 

 scalenus anticus, scalenus posticus, levator anguli scapulaB, 

 and the splenius muscles. It is crossed by the spinal 

 accessory nerve, the transversalis colli artery and vein, 

 and contains branches of the cervical plexus, lymphatics, 

 and adipose tissue. The subclavian triangle is bounded 

 below by the clavicle, anteriorly and above by the sterno- 

 mastoid, posteriorly and above by the posterior belly of 

 the omo-hyoid. Its floor is formed by the scalenus 

 medius, scalenus anticus, and scalenus posticus, the first 

 rib, and some of the fibres of the serratus magnus. It 

 contains the third part of the subclavian artery, which 

 curves downward beneath the middle of the clavicle into 

 the axilla. The subclavian vein is in front of the artery, 

 but at a lower level. The brachial plexus lies on the 

 scalenus medius muscle, above the subclavian artery. A 

 quantity of fat and some lymphatics are also found in 

 this triangle. 



AVhen the sterno-mastoid muscle is removed the 

 deeper structures of the neck are exposed. The most 

 important are: the carotid artery, internal jugular vein, 

 subclavian artery and vein ; the pneumogastric. phrenic, 



