Posteriorly the carotid is bounded by the longus colli and rectus capitis anticus major, which separate it from the anterior surface of the spinal column; the cervical cord of the sympathetic nerve and its superior and middle cardiac branches are closely connected to the posterior part of its sheath; the vertebral artery and vein are behind it at its lower part; and higher up it crosses the inferior thyroid artery at a point corresponding to that at which it is covered in front by the omo-hyoideus ; some times the inferior thyroid crosses over the ca rotid : the arteria cervicalis ascendens often lies behind the carotid towards the upper part of the neck ; moreover, the recurrent nerve on the right side, in its course from behind the sub clavian artery to the side of the trachea, passes behind the origin of the right carotid. From the relations of the primitive carotid posteriorly, it is evident that it can be most effectually compressed against the front uf the spinal co lumn, but to continue such pressure for any length of time would obviously be followed by injurious effects, from the lesion to which the nerves behind the sheath of the vessel would be thus subjected.
Externally the carotid artery is bounded by the internal jugular vein and the pneumo-gas tric nerve, both of which are contained within its sheath ; the vein when distended advances in front of it and partly conceals it ; the nerve lies in the posterior part of the sheath, behind and between the artery and vein, more closely attached to the latter vessel ; on the left side the internal jugular vein lies closer to the ca rotid, in front of which it passes at the lower part of the neck in its course to the vena inno minata ; on the right side the jugular vein is separated from the carotid inferiorly by a small intervening space, principally occupied by cel lular tissue, in consequence of the vein of this side descending to join the commencement of the superior cava in a perpendicular course further from the mesial line than the point at which the carotid is given off from the arteria innominata.
Internally the carotid is bounded by the trachea at its lower part; higher up by the thyroid body and the inferior constrictor of the pharynx, by which it is separated from the cricoid and thyroid cartilages; the recurrent nerve also lies on its inner side, but separated from it by a quantity of loose cellular tissue ; in addition to the foregoing relations, the left carotid lies in contact with the cesopliagus.
The varieties to which the origins of the carotid arteries are subject are the following : 1. the right carotid sometimes arises separately from the aorta ; this variety occurs when there are four large trunks arising from the arch of the aorta, of which the right carotid is the first, and the right subclavian tbe last in order ; 2. sometimes the arteria innorninata gives origin to the left carotid, in addition to the right ca rotid and right subclavian, in which case the left carotid has to cross in front of the lower part of the trachea to enter upon its cervical course ; 3. the right and left carotids some
times spring from a common trunk, which arises from the arch of the aorta between the right and left subclavian arteries ; in this variety as well as in the preceding, the situation of the carotids in front of the trachea exposes them to the danger of being wounded in the operation of tracheotomy, in performing wnich the sur geon should always be prepared to meet with the existence of such irregularities of distribu tion: 4. the left carotid sometimes arises from a left arteria innominata, which also gives off the left subclavian. (See AORTA.) The bifurcation of the primitive carotid most frequently occurs opposite the superior margin of the thyroid cartilage, in front of the third cervical vertebra; it may, however, take place above or below that point. It sometimes bifur cates opposite the cornu of the os hyoides, or, which rarely happens, behind the angle of the lower jaw ; in cases where the bifurcation is higher than usual, the primitive carotid often furnishes some of the branches ordinarily arising from the external carotid. The high bifurcation is an approximation to that condi tion of the carotid in which no bifurcation takes place, but where the primitive carotid, after having given all the bmnches which the external carotid usually supplies,' enters the cranium and becomes the internal carotid. 2. The primitive carotid sometimes bifurcates lower down in the neck than usual. I have seen such a bifurcation occurring on both sides in an old female subject, as low as the inferior border of the thyroid body.
The bifurcation of the carotid has the same relation to the larynx at all periods of life : it is more distant from the angle of the jaw in the infant than in the adult; the depth of the lower jaw in the former being much less, owing to the non-development of the roots of the teeth and alveolar processes : in old persons who have lost their teeth, and whose alveolar processes have been absorbed, the jaw being in the edentulous condition, the angle of the jaw is carried forward and thus removed farther from the bifurcation of the carotid. By de pressing the head the angle of the jaw is brought nearer to the bifurcation ; while the distance between these parts may be consi derably increased by throwing the head back wards.
The bifurcation of the primitive carotid gives origin to the external and internal carotids; the former of these supplies the larynx, thyroid body, pharynx, throat, face, and external parts of the head; the latter is distributed to the brain and the internal parts of the organs of hearing and vision. These two vessels lie close together at their origins. The internal is at first more superficial and more external in situation than the external, but becomes the more deeply situated of the two as they ascend. They are nearly of equal size in the adult when the bifurcation occurs at the usual place; while in the infant the internal is larger than the external.