Common carotid artery and its bifurcation in the carotid triangle
The common carotid artery bifurcates (splits in two) into the internal and external carotid arteries inside the carotid triangle, a subdivision of the anterior triangle of the neck.
Remember
Think of the anterior triangle as the route between the head and the thorax and the posterior triangle as the route between the thorax and the arm. The common carotid artery and its bifurcation lie in the anterior triangle, within the carotid triangle.
Memory aid
A fork in the road: one branch goes inwards to the brain, the other outwards to the face and scalp
Picture the bifurcation as a fork in the road, with the internal carotid artery taking the inward branch and the external carotid artery the outward one.
The sternocleidomastoid (the large muscle that runs obliquely over the side of the neck) divides each side of the neck into an anterior triangle in front of it and a posterior triangle behind it.
Memory aid: the anterior triangle serves the head. It holds structures passing between the head and the thorax: the common carotid artery, the internal jugular vein, which drains the head and neck, and cranial nerves such as the vagus nerve and the hypoglossal nerve (cranial nerve XII).
It also contains a smaller subdivision, the carotid triangle. Its upper border is the posterior belly of digastric (a muscle that attaches to the skull behind the ear), its lateral border is sternocleidomastoid, and its lower border is the superior belly of omohyoid (a muscle attaching to the hyoid, the U-shaped bone at the front of the neck).
The common carotid artery runs up through the carotid triangle and ends in its bifurcation. There it divides into the internal carotid artery, which supplies the brain and orbit (the eye socket), and the external carotid artery, which supplies the face, scalp and neck. The split usually lies near the level of the fourth neck vertebra (C4), about the level of the upper edge of the thyroid cartilage (the main cartilage in the voice box), although its height varies between people.
Many of the carotid triangle's vessels and nerves lie fairly close to the surface, so surgeons use it to reach the carotid arteries, and the carotid pulse can be felt there.
The posterior triangle lies behind sternocleidomastoid and holds structures passing between the thorax and the upper limb, such as the accessory nerve (the eleventh cranial nerve), the subclavian artery and vein, the external jugular vein and the trunks of the brachial plexus (the nerve network that supplies the arm).
Key facts
- Anterior triangle
- holds structures passing between the head and the thorax, including the common carotid artery, the internal jugular vein and several cranial nerves.
- Anterior triangle borders
- the lower border of the mandible above, the anterior border of sternocleidomastoid at the side and the midline of the neck.
- Carotid triangle borders
- the posterior belly of digastric above, the superior belly of omohyoid below and sternocleidomastoid at the side.
- Bifurcation level
- usually around C4, although the height varies between people.
- Carotid sinus
- lies close to the bifurcation and is a swelling of the artery containing baroreceptors (sensors that detect blood pressure).
- Carotid pulse
- a strong pulse just medial to the front border of sternocleidomastoid is the common carotid artery in the carotid triangle.
Exam traps
- The subclavian artery is a large artery of the neck, but it is not an anterior triangle structure, because its distal part passes through the posterior triangle towards the upper limb.
- The external jugular vein does not belong in the anterior triangle, because it runs down into the posterior triangle and drains into the subclavian vein; the jugular vein that belongs deep in the anterior triangle, beside the carotid artery, is the internal jugular vein.
- The accessory nerve (cranial nerve XI) is classically a posterior triangle structure, not an anterior triangle one: it passes between sternocleidomastoid and trapezius near the surface, so posterior triangle operations put it at risk.
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