CAROTID ARTERY, (human anatomy,) (arteria carotis ; Gr. stateaPng; Fr. carotide ; Germ. die Carotis, Kopfindsader ;) the great artery which on each side distributes blood to the different parts of the. head. The term carotid, derived from ;mpg, sopor, appears to have been first applied to the arteries of the head by the ancients from a supposition that a, state of drowsiness or deep sleep depended on compression or some other affection of these vessels exercising an influence over the circula tion of the blood in its passage through them to the brain : in accordance with the same opinion they have been also called arteria soporyeree.
The carotid arteries consist of-1st, the pri mitive carotids, of which the right arises from the arteria innominata, while the left comes directly froni the arch of the aorta ; 2d, the ex ternal carotid ; and. 3d, the internal carotid : these last two vesseis on each side being pro duced by the bifurcation of the primitive ca rotid.
Both primitive carotids are of equal size according to Ilichat, Boyer, and Cloquet ; nei ther Meckel nor Tiedemann make any remark as to a difference in their size, while, according to Soemmerring, the right is one-twenty-fifth larger than the left in the majority of in stances.
The origin of the right carotid from the arteria innominata is opposite the right sterno clavicular articulation. The left carotid arises from the transverse portion of the arch of the aorta behind the first bone of the sternum, on a plane with the centre of the junction of the cartilages of the first pair of ribs with that bone in front, and corresponding with the superior edge of the second thoracic vertebra posteriorly; owing to this difference in their origins, the left primitive carotid is from one inch to one inch and a quarter longer than the right, and is con tained within the thorax in the commencement of its course; it may therefore be divided into a thoracic and a cervical portion.
The thoracic portion of the left primitive carotid, by which I mean that portion which extends from the origin of the artery to a point on a level with the stemo-clavicular articulation, has the following relations :—anteriorly it is covered by the left vena innominata the remains of the thymus gland, some loose cellular tissue, and occasionally a few lymphatic glands ; in front of these the origins of the sterno-thyroid and sterno-hyoid muscles separate it from the sternum ; posteriorly it rests on the cesophagus, left recurrent nerve, the origin of the left sub clavian artery, the left par vagum, the thoracic duct, and some loose cellular tissue, in addition to which the longus colli is interposed between it and the front of the spinal column ; on its right side it is bounded by the trachea, and on its left by the phrenic nerve and the mediasti nal portion of the left pleura, which gives a loose covering to a small portion of its surface, against which the internal side of the apex of the left lung is applied.
The right primitive carotid and the cervical portion of the left are of equal length, and have similar relations : at first, in the lower part of the neck these vessels of opposite sides are only separated by the breadth of the trachea : as they ascend, however, they diverge from each other, and are separated by the larynx and thyroid body : in their ascent they seem to pass backwards, owing to the prominence of the larynx forwards, but in reality they cannot re cede, as they are closely applied to the front of the spinal column • they are not contorted in their course, nor do they furnish any branch until they arrive as high as the superior margin of the larynx, where each bifurcates by dividing into the external and the internal carotids.
Relations qf the trunk qf the Primitive Carotid.—.Anteriorly the primitive carotid is covered by the three following layers of mus cles from the sterno-clavicular articulation to the level of the cricoid cartilage ; 1st, the pla tysma myoides, beneath which lies the superfi cial layer of the cervical fascia; 2d, the sternal portion of the sterno-cleido-rnastoid ; and 3d, by the sterno-hyoid, sterno-thyroid, and the omo-hyoid, which latter muscle crosses the sheath of the artery, having its internal edge connected with the outer edge of the sterno thyroid by a dense fascia, a part of the deep layer of the cervical fascia, which is firmly con nected to the posterior margin of the clavicle inferiorly: between the lower part of the sterno mastoid and the front of the artery there is an interval of about an inch on the left side, and something less on the right, in consequence of the origin of the right carotid being so much more anterior on that side ; this interval is filled by cellular and adipose tissue, some large veins, one or more of the sub-clavicular branches of the cervical plexus, and occasionally a few lymphatic glands ; at the level of the cricoid cartilage the sterno-mastoid passes backward, and the omo-hyoid coming from beneath, it passes forwards to its insertion into the os hyoides. Above the crossing of these two muscles the carotid has no muscular covering, except the platysma, from which it is separated by cellular membrane, several veins from the thyroid body and larynx, and some lymphatic glands ; the nervus descendens noni also lies in front of the primitive carotid at its upper por tion, being found sometimes vvithin, sometimes outside, and occasionally embedded in the sub stance of the wall of its sheath ; the thyroid body also generally overlaps the carotid by its outer edge.