The Arterial Circulation

Before assessing the branches of the external carotid artery (ECA) and the course of the internal carotid artery (ICA) and the vertebral artery, it is well to review the carotid sheath (as covered in Chap. 2 ). The sheath is a vertically disposed neurovascular bundle of the neck lying on each side of the median airway and foodway extending from the root of the neck to the base of the skull. It lies on the sympathetic chain and the prevertetbral muscles where it contains the internal jugular vein (IJV) with the common carotid artery (CCA) situated medial with Cranial X placed behind and between the main artery and vein. The carotid bifurcation is typically located at the upper border of the thyroid notch where it divides into an ICA which inclines upwards and forwards lying on the surface of the pharynx to get to the carotid canal higher up and running together with the internal carotid nerve (a bundle of sympathetic fibres derived from the superior cervical ganglion). The bifurcation level is normally at the C3/4 vertebral junction (also at the level of the greater horn of the hyoid bone) which acts as a landmark. This bifurcation level is of critical surgical importance in the procedure of carotid endarterectomy or in the vascular approach to excision of a carotid body tumour (chemodectoma) that develops from the chemoreceptor cells at the bifurcation. If the bifurcation is higher, extra room may be achieved by dividing the posterior belly of the digastric muscle, ligating the occipital and posterior auricular arteries coming from the back of the ECA below and above the muscle respectively and by blindly fracturing the styloid process as it lies deep to the vessels. The need to perform these additional procedures in order to gain access to a high carotid bifurcation may also be required for a level III carotid vascular injury.