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.
When the common carotid has been tied the blood reaches the parts beyond from the branches of the carotid of the opposite side and from the subclavian artery of the same side. The branches of the external carotid anastomose across the median line. This is particularly the case with the superior thyroid and facial. The internal carotids communicate by means of the circle of Willis. From the subclavian the vertebral artery communicates by means of the basilar with the circle of Willis. The thyroid axis by its inferior thyroid branch communicates with the thyroid arteries of the opposite side. An ascending branch of the inferior thyroid as well as one from the transverse cervical, also from the thyroid axis, anastomose with branches of the princeps cervicis, which is a descending branch of the occipital.
Finally the superior intercostal, which, like the vertebral and thyroid axis, is a branch of the first portion of the subclavian, through its profunda cervicis branch anastomoses with a deep descending branch of the princeps cervicis (Fig. 172).
The internal carotid lies posterior and to the outer side of the external. It gives off no branches in the neck. Entering the skull through the carotid canal, in the apex of the petrous portion of the temporal bone and directly below and to the inner side of the Gasserian ganglion, it passes through the inner side of the cavernous sinus and at the anterior clinoid processes it bends up to divide into the anterior and middle cerebrals. Before its division it gives off the posterior communicating artery, the anterior choroid artery to supply the choroid plexus in the lateral ventricles, and the ophthalmic artery. The internal carotid artery in the neck is normally straight, but sometimes, particularly in elderly persons, it is tortuous. This may then be mistaken for aneurism. It lies about 2 cm. posterior and a little to the outer side of the tonsil. As the pharynx is the side of least resistance, when the vessel becomes tortuous it bulges into it, and on examination through the mouth a pulsating swelling can be distinctly seen in the pharynx just posterior to the tonsil. The finger introduced can feel the pulsations, and pressure on the carotid in the neck below causes the pulsations to cease. Thus the character of the pulsating swelling can be recognized. This artery is rarely ligated,. but if it is desired to do so it can readily be reached through an incision 6 or 7 cm. long behind the angle of the jaw. Aneurism or wounds may necessitate its ligation. At its commencement it is comparatively superficial, but as it ascends it gets quite deep, passing beneath the digastric and stylohyoid muscles. It should therefore be ligated below the angle of the jaw and not over 3 cm. from its origin at the upper border of the thyroid cartilage. It will be necessary to push the sternomastoid muscle posteriorly, as its anterior margin overlies the vessel. The internal jugular vein is to its outer side and between the two and posterior is the pneumogastric nerve. The sympathetic nerve lies behind it but is separated by a layer of fascia and is not liable to be caught up in passing the aneurism needle. The lingual, facial, and laryngeal veins may be encountered and are apt to cause trouble. They will have to be held aside or ligated and divided. The ascending pharyngeal artery may lie close to the internal carotid and care should be taken not to include it in the ligature. The needle is to be passed from without inward.
Fig. 172. - Collateral circulation after ligation of the common carotid artery.