The common carotid arteries extend in the neck from the sterno-clavicular articulation to the upper border of the thyroid cartilage, a distance of about 3½ inches. In the lower part of their course they are placed deeply, and about ¾ inch apart, while in the upper part they are superficial, and about 2 inches apart. On the right side the vessel arises as a terminal branch of the innominate, and is in front and to the right of the trachea ; while on the left it is given off from the arch of the aorta, and is more deeply placed, being situated to the side of the oesophagus. The vessel is enclosed in the carotid sheath, which is triangular in section, and is derived from the deep cervical fascia. In the sheath and accompanying the artery lies the internal jugular vein, situated to the outside of the artery, and the vagus nerve behind and between the artery and vein. Within the sheath, and closely associated with the vein, a number of small lymphatic glands frequently exist, which are often enlarged in tubercular disease. Outside the sheath, on its anterior surface, the descendens noni nerve runs downwards and inwards, and numerous lymphatic glands lie in this position, while posterior to the sheath the cervical sympathetic runs. Other posterior relations are the inferior thyroid and vertebra] arteries, prevertebral fascia and muscles, the transverse processes of the cervical vertebrae, and, on the right side, the recurrent laryngeal nerve. On the left side the nerve is internal. Anteriorly, the sheath is crossed by the omohyoid muscle and a branch of the superior thyroid artery, at the level of the sixth cervical. The artery is generally ligatured above this point, as it is more superficial, being overlapped by the sterno-mastoid, while below this point it is covered by the sterno-hyoid, thyroid, and mastoid muscles, the lateral lobe of the thyroid, and thyroid veins. This point, therefore, is an important one surgically, and is defined deeply by the tubercle on the transverse process of the sixth cervical (Chassaignac's tubercle), which forms a reliable guide to the artery in ligaturing, and against which the artery may be compressed. The common carotid is sometimes affected by aneurism, which occurs generally at the bifurcation, where there is normally a dilatation, or at the root of the neck. The pressure exerted on the surrounding structures by the tumour may give rise to spasm of the larynx, and of the diaphragm, contracted pupil (sympathetic), oedema, and lividity of face and arm, and anaemia of the brain. Ligature of the common carotid may be performed for aneurism affecting the vessel itself, for aneurism of the innominate (Wardrop's operation), for wounds of the vessel, and profuse haemorrhage from the neck or throat (sarcoma, etc.). Where possible the artery should be ligatured above the omo-hyoid (seat of election). The line of the artery is from the sterno-clavicular articulation to a point midway between the angle of the jaw and the mastoid process, and a 3-inch incision is made along this line, with its centre opposite the cricoid. The deep lascia is divided, the sterno-mastoid drawn back, the omo-hyoid drawn down, the sheath cleared, opened on its inner side, and the needle passed from without inwards to avoid the vein, the vagus also being avoided. The subsequent anastomosis is between the superior and inferior thyroid vessels, branches of the two external carotids, a branch of the occipital of the carotid, and the superior intercostal of the subclavian ; but is frequently not sufficient, notwithstanding the circle of Willis, to keep the brain sufficiently supplied with blood, syncope occurring at the time of the operation, or cerebral softening some days later.
The external carotid is the smaller terminal branch, and at its commencement is in front of, and to the inside of, the internal. It curves forwards as it ascends, and divides within the parotid recess of the lower jaw into the temporal and internal maxillary arteries. It is about 2½ inches long ; passes beneath the digastric and stylo-hyoid muscles ; is separated from the internal carotid by the stylo-glossus and -pharyngeus muscles, stylo-hyoid ligament, and glossopharyngeal nerve and pharyngeal branch of the vagus. Ligature of the external is less serious than that of the common carotid, and may be performed as a preliminary to operations on parotid tumours. The artery is generally ligatured in its first part, which is below the digastric, and is about 1 inch long. An incision is made in the line of the artery from the angle of the jaw to the upper border of the thyroid cartilage through the skin, superficial and deep fascia, the latter being very strong at this level, and drawing the sterno-mastoid toward the angle of the jaw. The sterno-mastoid muscle and the parotid gland are now retracted, and the digastric tendon exposed. The hypoglossal nerve, giving off its descending branch, the laryngeal and facial veins, and one or two lymphatic glands, lie in front of the vessel ; while the internal laryngeal nerve lies behind, and the submaxillary gland and great cornu of the hyoid internally, the latter forming a valuable guide to the vessel. The sheath is opened at the level of the hyoid cornu, and the needle passed from without inwards, the internal laryngeal nerve being avoided. The circulation is maintained by the branches from either side (facial, occipital, etc.) ; anastomosis between facial and branches of the ophthalmic (internal carotid), and between a branch of the superior intercostal and of the occipital.
The internal carotid is the larger branch, and extends within the cranium to the anterior clinoid process, where it divides into anterior and middle cerebrals. In the carotid triangle, where it might be ligatured, it is deeper and posterior to the external, and has the same relations to the internal jugular vein and vagus nerve as the common carotid. It is crossed by the occipital and posterior auricular arteries and hypoglossal nerve, and is separated from the external carotid by the stylo-glossus and pharyngeus muscles, stylohyoid ligament, glosso-pharyngeal, and pharyngeal branch of the vagus nerves. The internal jugular vein begins in the jugular fossa of the petrous as a bulb or dilatation at the junction of the lateral and inferior petrosal sinuses. At its exit from the skull it is separated from the internal carotid artery by the ninth, tenth, and eleventh nerves, and it joins the subclavian vein behind the sterno-clavicular articulation to form the innominate. The internal jugular vein is occasionally in danger of wounding in operations for extensive tubercular disease of the glands of the neck. During inspiration the vessel is collapsed, and looks like connective tissue, while during expiration it may become greatly distended. In such cases the danger from haemorrhage is not so great as that from air embolism, air being sucked in through the wound in inspiration, and leading to embolism of the coronary arteries, etc. The connection of the vein with tubercular glands has already been referred to. It is frequently affected by thrombosis extending from the sigmoid, and becomes tender and palpable as a thickened cord in the neck. Where, as generally happens, the clot is septic, infection is very apt to be carried to the lungs, or sometimes to other parts of the body.