THE COMMON CAROTID ARTERIES. 391 



The common carotid artery is enclosed, together with the internal jugular vein 

 and the pneumo-gastric nerve, in a common sheath, which is continuous with the 

 deep cervical fascia (fig. 276, p. 297) ; the nerve, artery and vein have each, how- 

 ever, a separate investment of connective tissue within the sheath. The artery is 

 deeply placed at the lower part of the neck, but is comparatively superficial towards 

 its upper end. It is covered below by the sterno-mastoid, sterno-hyoid, and sterno- 

 thyroid muscles, in addition to the platysma and the layers of fascia between and 

 beneath the muscles ; and it is crossed opposite or near the lower margin of the 

 cricoid cartilage by the omo-hyoid muscle. From this point upwards to its bifurca- 

 tion, the vessel is covered by the common integument, the platysma and fascia, and 

 in the natural condition of the parts also by the sterno-mastoid ; but in the dissected 

 subject, in consequence of the retraction of this muscle when the fascia is removed, 

 the upper portion of the artery is exposed in a triangular space, the sides of which 

 are formed by the posterior belly of the digastric, the anterior belly of the omo- 

 hyoid, and the sterno-mastoid muscles, and which is known as the carotid triangle. 

 In this space the artery is crossed by the small sterno-mastoid branch of the superior 

 thyroid artery. 



Posteriorly, the artery rests against the longus colli and scalenus anticus muscles, 

 which intervene between it and the transverse processes of the vertebrae. The 

 inferior thyroid artery crosses behind the carotid sheath. 



Internally, the vessel is in relation with the trachea, the thyroid body (which 

 commonly overlaps the artery), the larynx, the oesophagus, and the pharynx. On 

 the inner side of the point of division of the artery, and closely united to its wall, is 

 placed the small vascular body known as the carotid gland (see Vol. I, p. 371). 



Relation to veins. The internal jugular vein is close to the artery at the upper 

 part of the neck, but, on approaching the thorax, the two are separated on the right 

 side by an angular interval, in which the commencement of the subclavian artery 

 and the pneumo-gastric nerve are exposed ; on the left side, the vein is usually 

 nearer to the artery, and may even overlap it at the lower part of the neck. 



Crossing over the upper part of the common carotid artery to join the jugular 

 vein, is the superior thyroid vein, often double, and occasionally forming a sort of 

 plexus over the artery. A middle thyroid vein frequently crosses the artery about 

 half-way up the neck ; and the anterior jugular vein, as it turns outwards under the 

 sterno-mastoid, crosses the lower part of the artery, but is separated from it by the 

 sterno-hyoid and sterno- thyroid muscles. There is also in many cases a communicat- 

 ing branch, sometimes of large size, between the facial and anterior jugular veins, 

 which descends obliquely over the front of the artery, lying along the anterior border 

 of the sterno-mastoid. 



Relation to nerves. The descending branch of the hypoglossal nerve passes down 

 on the surface of the artery, crossing it very gradually from the outer to the inner 

 side ; and this nerve, together with the branches of the cervical nerves which join 

 it, may be placed either within or on the front of the carotid sheath. The pneumo- 

 gastric nerve lies within the sheath of the vessels between the artery and vein pos- 

 teriorly. The sympathetic nerve is placed along the back of the sheath, between it 

 and the prevertebral muscles, and the recurrent laryngeal nerve crosses upwards and 

 inwards behind the lower part of the sheath. 



The common carotid artery usually gives off no branch, except minute twigs to 

 its areolar sheath and to the carotid gland, and therefore continues of equal size 

 throughout its course until close to its bifurcation, where a slight dilatation is 

 observable. 1 



1 According to the measurements of Stahel the artery is largest at its commencement ; it diminishes 

 slightly to the middle of its length, and again enlarges towards the distal end (Arch. f. Anat., 1886). 



