

THE NECK 115 



becomes indefinite. The posterior wall of the sheath is con- 

 nected to the prevertebral fascia by some loose connective tissue, 

 which, however, does not prevent pus from spreading laterally 

 between them ; its anterior wall fuses with the pretracheal 

 fascia. 



Relations of the Carotid Sheath. The carotid sheath is so 

 frequently exposed in operations on the neck that its relations 

 require careful consideration. At its lower end it is covered 

 by the sterno-hyoid and the sterno-thyreoid muscles and, more 

 superficially, by the sternal head of the sterno-mastoid. Between 

 these two muscular layers the anterior jugular vein runs laterally, 

 just above the clavicle, and hence it runs the risk of injury when 

 these muscles are separated. Opposite the cricoid cartilage, 

 the sheath is crossed obliquely by the anterior belly of the 

 omo-hyoid (Fig. 38). (The intermediate tendon of the muscle 

 exactly overlies the internal jugular vein.) Above that level, 

 the carotid sheath is overlapped by the anterior border of the 

 sterno-mastoid, but, just below the mandible, it appears partially 

 from under cover of the muscle. Above the level of the hyoid 

 bone the great vessels pass deep to the stylo-hyoid and the 

 posterior belly of the digastric. Numerous tributaries of the 

 internal jugular vein (p. 118) cross the medial part of the sheath 

 to reach their destination. Opposite the hyoid bone the sheath 

 is crossed transversely by the hypoglossal nerve (p. 124) and, 

 at the upper border of the omo-hyoid, by the branch from the 

 superior thyreoid artery to the sterno-mastoid. 



Posteriorly, the sheath rests on the longus colli in its lower 

 part, and it forms the roof of the deep triangle of the neck (p. 138). 

 Superiorly, it lies on the transverse processes of the cervical 

 vertebrse and on the muscles which arise from them, viz. the 

 scalenus anterior and the longus capitis (rectus cap. antic, major). 

 The sheath extends laterally so as to overlap the phrenic nerve 

 as it lies on the scalenus anterior. 



On its medial side the carotid sheath is closely applied to the 

 trachea, the oesophagus, and the recurrent (laryngeal) nerve ; 

 the posterior border of the lateral lobe of the thyreoid gland is 

 an intimate relation and may overlap the sheath, while, at a 

 higher level, the inferior and middle constrictors of the pharynx 

 and the thyreo-hyoid membrane lie to its medial side. 



Laterally, the sheath is related to the deep surface of the 

 sterno-mastoid, save where the depressors of the hyoid bone 

 intervene. 



Sa 



