The common or primitive carotid arteries of the right and left sides of the body (art. carotis dextra et sinistra) are nearly similar in their course and position, whilst they are in the neck ; but they differ materially in their mode of origin, and consequently in their length, and position at their commencement. On the right side the carotid artery commences at the root of the neck behind the sterno-clavicular articulation, at the place of bifurcation of the innominate artery, whilst the carotid of the left side arises, within the thorax, from the highest part of the arch of the aorta, very near the origin of the innominate artery. The left carotid is therefore longer than the right, and ii is at first placed deeply within the thorax.

In consequence of this difference, it is convenient to describe, at first, the thoracic portion of the left carotid, or that part which intervenes between the arch of the aorta and the sterno-clavicular articulation, — after completing which the same description will suffice for both vessels.

Whilst within the thorax, the left carotid ascends obliquely behind, and at some little distance from the upper piece of the sternum and the muscles (sterno-hyoid and sterno-thyroid) connected with that part of the bone; it is covered by the remains of the thymus gland, and is crossed by the left innominate vein. This part of the artery lies in front of the trachea; and the esophagus (which, at the root of the neck, deviates a little to the left side) and the thoracic duct, are also behind it. The left carotid artery here lies between the innominate and the subclavian arteries, and the vagus nerve is to its outer side.

In the neck the common carotid artery, of either side, reaches from behind the sterno-clavicular articulation to the level of the upper border of the thyroid cartilage, where it divides into two great branches, of which one, 6, is distributed to the cranium and face, and the other, c, to the brain and the eye. These divisions have, from their destination, been named respectively the external and internal carotid arteries.

The oblique course taken by the common carotid artery along the side of the neck, is indicated by a line drawn from the sterno-clavicular articulation to a point midway between the angle of the jaw and the mastoid process of the temporal bone. At the root of the neck, the arteries of both sides are separated from each other only by a narrow interval, corresponding with the width of the trachea ; but, as they ascend, the two vessels are separated by a much larger interval, — corresponding with the breadth of the pharynx and larynx. The appearance which the carotid arteries have of being placed further back at the upper than at the lower part of the neck, is owing to the projection of the larynx forward in the former situation.

In considering the position of the common carotid artery with regard to the adjacent structures, it is first to be observed that this vessel is enclosed, together with the internal jugular vein and the vagus nerve, in a common membranous investment or sheath, derived from or continuous with the deep cervical fascia. Separated by means of this sheath from all the surrounding parts, except the vein and nerve just mentioned, the carotid 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 different layers of fascia between and beneath the muscles; and it is crossed opposite the lower margin of the cricoid cartilage, or nearly in this situation, by the omo- hyoid muscle. From this last-named point upwards to its bifurcation, the sheath of the vessel is covered only by the common integument, the platysma and fascia; and this part of the artery lies in a triangular space bounded by the sterno-mastoid, the omo-hyoid, and the digastric muscles.

Behind, the artery is supported by the transverse processes of the cervical vertebrae, from which, however, it is separated by a thin stratum of muscular fibres, — parts of the longus colli and scalenus anticus muscles. From the muscles, as well as from the inferior thyroid artery, and the nerves which rest on those muscles, the vessel is separated by its sheath. In consequence of the contiguity of the bones, the circulation through the carotid artery may be commanded by pressure directed backwards against the vertebral column.

On the inner side the vessel is in juxtaposition (the sheath only intervening) with the trachea below, and with the thyroid body (which often overlaps the artery), the larynx and the pharynx higher up. Along its outer side are placed the internal jugular vein and the vagus nerve.

Veins

The internal jugular vein, as already mentioned, lies along the outer side of the artery, and is enclosed in the same sheath, but with a thin partition intervening. The vein is close to the artery at the upper part of the neck ; but in approaching the thorax, and on the right side, it inclines outwards, and the two vessels are separated by a triangular interval, across the base of which lies the subclavian artery. A different arrangement obtains on the left side, the vein being usually nearer to the artery, and occasionally even in front of it, at the lower part of the neck. The inclination of the veins in both these cases is in the same direction — towards the right side — and it may be accounted for by their destination to the right side of the heart. Their tendency to the right side has necessarily the effect of approximating one to the artery which it accompanies, and removing the other from it.

Crossing over the upper part of the common carotid artery to join with the jugular vein, will be found two or more superior thyroid veins. These veins vary in number, and they occasionally form a sort of plexus over the artery. Another vein, likewise from the thyroid body (middle thyroid vein), not unfrequently crosses over the artery near its middle.

The anterior jugular vein, after descending along the front of the neck, usually near the middle line, turns outwards at the lower part of the neck, under the sterno-mastoid muscle, and thus crosses the artery. This vein is generally of small size. Occasionally, however, the vein is rather large, and is placed nearly over the carotid artery along the neck.

Nerves

The descending branch of the ninth cerebral nerve (descendens noni) usually rests on the fore part of the sheath of the carotid artery (together with the branches of cervical nerves which join it) and crosses it from the outer to the inner side. In some in- stances the branch of the ninth nerve descends within the sheath, and between the artery and vein. The vagus nerve, as already mentioned, lies within the sheath of the vessels between the artery and vein. This nerve was in one case observed to descend over the artery. The sympathetic nerve descends along the back of the sheath, be- tween it and the rectus anticus muscle; and the recurrent laryngeal nerve crosses inwards behind the sheath.

The common carotid artery usually gives off no branch, and there- fore continues of equal size in its whole length, except at its bifurca- tion, where an enlargement is observable.

Peculiarities of the common carotid arteries

Origin

The peculiarities affecting the origin of the common carotids of the two sides must necessarily be considered separately.

The right carotid artery, instead of forming one of the divisions of the innominate trunk, occasionally arises directly from the aorta, or in conjunction with the left carotid. When it arises from the aorta, it is usually the first vessel from the arch; 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 of course with the point of bifurcation of the innominate artery. A change from the usual position (on a level with the upper border of the clavicle) was found in the proportion of about one case in eight and a half; and it was found to occur more frequently above than below that point.

The left carotid artery varies in its origin much more frequently than the right. In the greater number of its deviations from the ordinary place of origin, this artery arises from, or in conjunction with, the innominate artery; and in those cases in which the right subclavian is a separate offset from the aorta, the two carotids most frequently arise by a common trunk.

The left carotid may be said to have a tendency towards the right side. — Ordinarily placed nearer to the innominate than to the left subclavian, it not unfrequently unites with that (the innominate) artery, as already stated; and it has been observed, very rarely, however, to precede the other branches which arise from the transverse part of the aortic arch. On the other hand, the combination of the left carotid with the left subclavian, has been very seldom met with, except in cases of general transposition of the viscera.

Place of division

The deviations from the usual place of division of the com- mon carotids of both sides of the neck, into the external and internal carotids, may be considered together. The place of division often varies somewhat from the point mentioned as the usual one (the level of the upper margin of the thyroid cartilage), and it more frequently tends upwards than in the opposite direction. The arteries often bifurcate opposite the os hyoides, and, 'occasionally, much higher than that bone. On the contrary, the bifurcation from time to time occurs about the middle of the larynx, and (but with much less frequency) opposite the lower margin of the cricoid cartilage. Instances are recorded of the common carotid dividing at a still lower point. One was observed by Morgagni, in which the carotid artery, measuring one inch and a half in length, divided at the root of the neck (" De Sedibus et Causis morborum," c. Epist. 29, Art. 20).

The common carotid artery has, as a very rare occurrence, been found to ascend in the neck without dividing into its usual terminal branches, — the internal carotid artery being altogether wanting.

In two cases the common carotid artery was altogether wanting — the external and internal carotids arising directly from the arch of the aorta. This peculiarity occurred on both sides in one of the cases referred to, and on one side in the other (Dr. Power's case — in "The Arteries," c. by R. Quain, page 101).

Occasional branches from common carotid

Though, in the ordinary condition, no branches arise from the common carotid, this is not always the case ; for it not unfrequently gives origin to the superior thyroid artery, and, though very seldom, to a laryngeal, or an inferior thyroid branch. Lastly, the vertebral artery has in a few instances been observed to come from the common carotid.

From Quain's anatomy.