SUPERFICIAL CERVICAL REGION. 41 



purposes of surgical description ; thus, the sterno-mastoid 

 passing obliquely across the neck, that portion bounded by 

 the median line in front, the jaw above and the sterno- 

 mastoid behind, is called the great anterior triangle; that 

 bounded by the sterno-mastoid in front, the clavicle below 

 and the trapezius posteriorly, the great posterior triangle. 

 These triangles are each subdivided by the omo-hyoid 

 muscle; the hyoid portion divides the anterior triangle, the 

 space below it being called the inferior carotid triangle; 

 that above it being again divided by the digastricus muscle 

 into the superior carotid triangle and the submaxillary tri- 

 angle. The scapular portion divides the posterior triangle 

 into two smaller spaces, that above the belly of the muscle 

 being called the occipital triangle and that below it the 

 subclavian triangle. 



The sterno-mastoid muscle may now be divided and its two ends 

 reflected. 



In reflecting the lower half of the sterno-mastoid muscle, 

 its outer border will be seen to correspond with the outer 

 border of the scalenus anticus muscle. As the position of 

 the last-named muscle at its insertion to the first rib is of 

 importance in connection with the operation of ligature 

 of the subclavian arteiy, this relation is a valuable land- 

 mark to recognize. 



In reflecting the upper half of the sterno-mastoid muscle, 

 its posterior surface should be examined for the spinal 

 accessory nerve; this nerve is one of the eighth pair of 

 cranial nerves (p. 37); after emerging from the foramen 

 lacerum posterius, it becomes connected by a branch of 

 considerable size with the pneumogastric nerve, and then 

 continues onward to perforate the sterno-mastoid muscle at 

 its under surface, after, passing through which, and being 

 joined by branches of the cervical plexus in the occipital 

 triangle, it is distributed to the trapezius muscle. 



It is presumed that the carotid artery has not yet been 

 exposed, but that it still remains covered with its sheath. 

 Its sheath is a portion of the deep cervical fascia which 

 invests it and also the internal jugular vein. Upon this 

 sheath may be seen a small nerve called the descendens 

 noni; it is a branch from the hypoglossal, one of the cra- 

 nial nerves, which crosses the neck in a transverse direc- 

 tion just above the hyoid bone. The descendens noni forms 

 a loop with a deep branch from the second or third nerve 



4* 



