35S AETEEIES OF THE HEAD AND NECK. 



within the sheath. The artery is deeply placed at the lower part of 

 the neck, but is comparatively supei'ficial towards its upper end. It 

 is covered below by the sterno-mastoid, sterno-hyoid, and sterno-thyroid 

 muscles, in addition to the platysina and the layers of fascia between 

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

 marg'in of the cricoid cartilage by the omo-hyoid muscle. From this 

 point upwards to its bifurcation, the vessel is covered by the sterno- 

 mastoid, by the platysma and fascia, and by the common integument ; 

 and lies in a triangular space bounded by the sterno-mastoid, the omo- 

 hyoid, and the digastric muscles. 



Posteriorly, the artery is supported by the cervical vertebra?, the 

 longus colli and rectus capitis anticus muscles intervening. Hence the 

 flow of blood through it may be commanded by pressure directed back- 

 wards against the vertebral column. The inferior thyroid artery crosses 

 behind the carotid sheath. 



On the inner side the vessel is in contact with the trachea, the 

 thyroid body (which often overlaps the artery), the larynx, and the 

 pharynx. Along its outer side are placed the internal jugular vein 

 and the vagus nerve. 



Relation fo veins. — The internal jugular vein is close to the artery 

 at the upper part of the neck, but, in approaching the thorax, the veins 

 of both sides incline to the right, and hence that of the right side is 

 separated from the artery by an angular interval, while that of the left 

 side approaches the artery, and even lies in front of it at the lower part 

 of the neck. 



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

 with the jugular vein, are two or more superior thi/roid veins, which 

 occasionally form a sort of plexus over the artery. A middle th]jroiil 

 vein not unfrequently crosses the artery about half way up the neck. 



The anterior jugular vein, where it turns outwards under the sterno- 

 mastoid muscle to join the subclavian, crosses the lower part of the- 

 artery. This vein is generally of small size, but occasionally is rather 

 large, and is placed nearly over the carotid artery along the neck. 



Relation to nerves. — The descending branch of the hypoglossal 

 nerve, descendens noni, usually rests, together with the branches of 

 cervical nerves which join it, on the fore part of the sheath of the 

 carotid artery, and crosses it from the outer to the inner side : in some 

 instances this branch descends within the sheath between the carotid 

 artery and jugular vein. The vagus nerve lies within the sheath of the 

 vessels between the artery and vein posteriorly : it was in one case 

 observed to descend over the artery. The sgmpatJwtic nerve is placed 

 along the back of the sheath, between it and the vertebral muscles, and 

 the recurrent largngeal nerve crosses inwards behind the lower part of 

 the sheath. 



The common carotid artery usually gives off no branch, and therefore 

 continues of equal size in its whole length, except at its bifurcation, 

 where a slight enlargement is observable. 



Varieties. — Orir/iu. — The rir/Jif rnrotid arteiy occasionally arises directly from 

 the aorta, or in conjunction -with the left carotid. A\Tien it arises from the aorta, 

 it is usually the first vessel from the arch, the subclavian being displaced ; but it 

 has been found to occupy the second place, — the right subclavian, or, very rarely, 

 the left carotid being the first. 



The place at which the right carotid artery commences varies with the point 



