This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Both these arteries are affected at times with aneurisms, necessitating their ligation. Ligation of the main trunks or their branches is also required in various operations on the head, as in removal of the Gasserian ganglion or maxilla, or excision of the tongue, thyroid gland, etc. The communication between the arteries on the two sides of the body is quite free, as also is that between the arteries above and those lower down. For this reason bleeding from the distal end of a cut artery will be almost as free as from its proximal end. The various branches of the external carotid anastomose across the median line of the body. The vertebrals communicate above through the basilar. The internal carotids communicate through the anterior cerebral and anterior communicating and with the basilar through the posterior communicating and posterior cerebral. Between the parts above and those below we have the superior thyroid anastomosing with the inferior thyroid branch of the thyroid axis from the subclavian artery. The princeps cervicis, a branch of the occipital, anastomoses with the ascending cervical branch of the inferior thyroid, the transverse cervical of the thyroid axis, and the profunda cervicis from the superior intercostal. These free communications enable the surgeon to ligate to any extent without incurring the risk of gangrene. The line of the carotid arteries is from a point midway between the mastoid process and the angle of the jaw to the sternoclavicular articulation. At the upper border of the thyroid cartilage the common carotid divides into the internal and external carotids; this is opposite the fifth cervical vertebra.
This lies on the longus colli muscle and a small portion of the rectus capitis anticus, which separate the artery from the transverse processes of the vertebrae. The artery can be compressed against the vertebrae and its pulsations stopped by pressing backward and slightly inward. It is superficial in the upper portion of its course but becomes deeper as it approaches the chest. The anterior tubercle of the transverse process of the sixth cervical vertebra is called Chassaignac's tubercle. It is about opposite the cricoid cartilage. It is one of the guides to the artery. The omohyoid muscle crosses the artery opposite the cricoid cartilage and just above it is the site of election for ligation.
In making the incision, which should be 5 or 6 cm. long, it should be laid along the anterior edge of the sternomastoid muscle with its middle opposite to or a little above the level of the cricoid cartilage. This incision may be a little anterior to the direct line of the artery as given from midway between the angle of the jaw and mastoid process to the sternoclavicular articulation. This is because the muscle bulges forward and overlaps and hides the artery. The artery is beneath its edge. On cutting through the superficial fascia and platysma the deep fascia is reached, some small veins perhaps being divided in so doing. The deep fascia is divided along the edge of the sternomastoid muscle, which is then pulled outward. Beneath it and running obliquely across the lower portion of the wound is the omohyoid muscle. It is recognized by the direction of its fibres, they being more or less transverse or oblique. Sometimes a small artery, the sternomastoid branch of the superior thyroid, crosses the common carotid just above the omohyoid muscle. The artery is also crossed by veins. The lingual, superior, and middle thyroid veins all pass over it to enter the internal jugular. The middle thyroid vein may be above or just below the omohyoid muscle. These vessels all pass transversely across the artery and beneath the deep fascia. The artery lies in a separate sheath to the inner side of the jugular vein. In the living body it is to be recognized by its pulsations. The vein being filled with blood may overlap the artery. Veins are readily emptied of their blood by pressure on the parts during the operation; hence if the vein happens to be collapsed it may not be recognized and is liable to be wounded. Therefore in examining for the artery see that the pressure from the retractors or other sources does not obstruct the flow of blood through the jugular vein. Running down on the anterior surface of the artery is the descendens hypoglossi nerve. If seen it should be pushed aside. It supplies the sternohyoid, sternothyroid, and both bellies of the omohyoid muscles. The pneumogastric nerve lies posteriorly, between the artery and the vein. Care will be necessary to avoid including it in the ligature. The ligature is to be carried from the outer to the inner side, the needle being passed between the vein and the artery. Ligation of the Common Carotid Artery Below the Omohyoid Muscle. - The artery below the omohyoid muscle becomes deeper and less accessible. The sternomastoid muscle overlaps it and is less easily displaced. The sternohyoid and sternothyroid muscles likewise tend to encroach on it and have to be drawn inward. The internal jugular vein and carotid artery diverge as they descend, so that at the level of the sternoclavicular joint they are separated 2.5 cm. In this interval the first portion of the subclavian artery shows itself. The anterior jugular vein will probably be encountered along the edge of the sternomastoid muscle, and near the omohyoid muscle the artery will be crossed by the middle thyroid vein. Still lower it may be that the inferior thyroid will be encountered. Posterior to the carotid artery is the inferior thyroid artery, coming from the thyroid axis and going to the thyroid gland, and winding around from posteriorly to the inner side is the recurrent laryngeal nerve. The ligating needle is to be passed from without inward.
Fig. 171. - Ligation of the common carotid artery.