GREAT VESSELS AND NERVES OF NECK 217 



crosses the phrenic nerve posterior to the internal jugular vein. 

 At the medial border of the scalenus anterior the thyreo- 

 cervical trunk is posterior to it, and, at a lower level, the 

 first part of the subclavian artery and the dome of the pleura. 



The right vein is usually the larger of the two; and as 

 they approach the root of the neck both veins incline slightly 

 to the right, with the result that, on the right side, the lower 

 part of the vein is separated from the common carotid artery 

 by a small triangular interval bounded below by the subclavian 

 artery, whilst on the left side the vein overlaps the anterior 

 aspect of the common carotid artery. 



Tributaries. Immediately below its commencement the 

 internal jugular vein is joined by the inferior petrosal sinus, 

 and then, successively, by offsets from the pharyngeal plexus, 

 by the lingual veins, the common facial vein, and the superior 

 and middle thyreoid veins. In some cases it is joined near 

 its upper end by a vena comitans which runs with the occipital 

 artery; and, occasionally, near its lower end, it receives the 

 lymph trunks which usually open into the commencement of 

 the innominate vein. 



Dissection. Slit open the lower part of the vein and examine 

 the valve which lies close to its extremity. It consists of two 

 or three semilunar flaps which prevent regurgitation of blood 

 from the innominate vein into the internal jugular. 



Nervi G-losso-pharyngeus, Vagus et Accessorius. After 

 the removal of the brain the glosso-pharyngeal, vagus, and 

 accessory nerves were seen leaving the cranial cavity, 

 through the middle compartment of the jugular foramen, in 

 the interval between the commencement of the internal 

 jugular vein postero-laterally and the inferior petrosal sinus 

 antero-medially (p. in, and Fig. 81, p. 218). The dissector 

 should again examine the interior of the cranial cavity and 

 refresh his memory as to the manner in which the nerves 

 enter the foramen. The glosso-pharyngeal occupies the most 

 anterior position, and it is cut off from the others by a 

 separate, tube-like sheath of dura mater. The accessory is 

 placed posterior to the vagus, and both are included within 

 the same sheath of dura mater. They therefore traverse 

 the foramen in close contact with each other. Reaching 

 the exterior of the skull, the three become associated with 

 the hypoglossal nerve ; and the four nerves lie, for a short 

 distance, in the interval between the internal jugular vein 



