Sternocleidomastoid is not supplied by:
**Core Concept**
The sternocleidomastoid (SCM) muscle is a paired muscle in the neck that plays a crucial role in movements of the head and neck. The SCM muscle receives its blood supply from branches of the sternocleidomastoid artery, which is a branch of the costocervical trunk.
**Why the Correct Answer is Right**
The SCM muscle receives its blood supply from the sternocleidomastoid artery, which arises from the costocervical trunk. This artery supplies the muscle with a rich network of blood vessels, allowing for its growth and maintenance. The SCM muscle does not receive its blood supply from the external carotid artery or its branches, which are responsible for supplying other structures in the neck.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the SCM muscle does receive its blood supply from the sternocleidomastoid artery, which is a branch of the costocervical trunk.
**Option B:** Incorrect because the SCM muscle does not receive its blood supply from the external carotid artery, which is responsible for supplying other structures in the neck.
**Option C:** Incorrect because the SCM muscle does not receive its blood supply from the vertebral artery, which is responsible for supplying the posterior circulation of the brain.
**Clinical Pearl / High-Yield Fact**
It is essential to remember that the SCM muscle receives its blood supply from the sternocleidomastoid artery, which arises from the costocervical trunk. This is a key anatomical detail that is often tested in medical exams.
**Correct Answer: B. The SCM muscle is not supplied by the external carotid artery.**