40 HEAD AND NECK 



Some of the contents of the triangle which are now 

 displayed require further consideration. 



Vena Jugularis Externa. The external jugular vein is 

 superficial except in the terminal part of its extent. 



It commences on the surface of the sterno-mastoid, below 

 the lower end of the parotid gland, by the union of the 

 posterior auricular vein with a branch from the posterior 

 facial vein. After its formation it runs downwards and back- 

 wards, across the sterno-mastoid, to the upper and anterior 

 angle of the supraclavicular portion of the posterior triangle, 

 in which it pierces first the superficial layer and then the 

 second layer of the deep fascia, and it terminates in the 

 subclavian vein (Figs. 12, 15). 



As it crosses the sterno-mastoid it lies at first parallel with 

 but anterior to the trunk of the great auricular nerve, then 

 deep to the platysma, and whilst beneath the platysma it 

 crosses either superficial or deep to the nervus cutaneus colli 

 (Fig. 12). At the posterior border of the sterno-mastoid it 

 sometimes receives a vein called the posterior external jugular 

 vein> which descends across the upper part of the posterior 

 triangle from the occipital region. Between the two layers 

 of the deep fascia of the supraclavicular triangle it receives 

 the transverse cervical, the transverse scapular and the 

 anterior jugular veins, and it lies superficial to the lower 

 roots of the brachial plexus ; as it pierces the second layer 

 of deep fascia, it lies superficial to the third part of the sub- 

 clavian artery. 



Immediately above its termination it is provided with a 

 valve, consisting of two or three semilunar cusps. The 

 dissector should note that, as the vein pierces the deep fascia, 

 its wall is closely connected with the margin of the opening 

 through which it passes ; consequently when the fascia is 

 stretched the lumen of the vein is expanded. 



The Posterior Belly of the Omo-hyoid Muscle. The 

 posterior belly of the omo-hyoid muscle springs from the 

 upper border of the scapula and upper transverse scapular 

 ligament. It enters the posterior triangle, at its lower and 

 posterior angle ; runs upwards and forwards, at a variable 

 distance from the clavicle, to the posterior border of the 

 sterno-mastoid, and divides the posterior triangle into occipital 

 and subclavian or supraclavicular portions. Either immedi- 

 ately behind or under cover of the posterior border of the 



