Extracranial Arterial System

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The ascending aorta arises from the left ventricle. The aortic arch gives rise to several important branches. The first branch of the aorta is normally the innominate artery, which is also referred to as the brachiocephalic trunk. Shortly after its origin, the innominate artery divides into the right subclavian and right common carotid arteries. The right vertebral artery arises from the right subclavian artery, courses posteromedially, and then enters the foramen transversarium of C6. The right common carotid artery bifurcates into the internal and external carotid arteries in the midcervical region, usually at C3 or C4, about 3 cm below the angle of the mandible. However, the bifurcation may be as rostral as C1 or as caudal as T2.

The next branch of the aorta is the left common carotid artery, which bifurcates into the internal and external carotid arteries at the same level as the right common carotid artery (.Fig, 2.2.-1). The last branch of the aortic arch is the left subclavian artery. The left vertebral artery is the first branch of the left subclavian artery. This branch usually arises from the superior portion of the artery and then courses cephalad to enter the foramen transversarium of C6. It is reported that the left vertebral artery is as large or larger in caliber than the right vertebral artery in approximately 75 percent of cases. y


The external carotid artery arises from the common carotid artery in the midcervical region. The proximal portion of this artery lies anteromedial to the internal carotid artery, but as it ascends, it courses posteromedially to supply structures of the face. The external carotid artery has nine major branches ( .Fig, 22-2 ). Several of these branches, particularly the maxillary artery and its branches, the ascending pharyngeal artery, and the facial artery, can be important sources of collateral blood flow in the event of internal carotid artery occlusion. The order in which these branches arise and the selected important structures they supply are as follows:

1. Superior thyroid artery, which supplies the larynx and portions of the thyroid.

2. Ascending pharyngeal artery, which supplies portions of the meninges, middle ear, lower cranial nerves, and the upper cervical nerves.

3. Lingual artery, which supplies the tongue and pharynx.

4. Facial artery, which supplies the face, palate, and pharynx.

5. Occipital artery, which supplies musculocutaneous structures of the scalp and neck.

6. Posterior auricular artery, which supplies the scalp, tympanic cavity, pinna, and the parotid gland.

7. Maxillary artery, which is the larger terminal branch that has three major portions, each with its own branches. The most important branches to remember are the middle meningeal artery, which can be lacerated in head trauma and produce an epidural hematoma, and the anterior deep temporal artery, which can be a source of collateral blood flow through the ophthalmic artery when the internal carotid artery is occluded.

8. Transverse facial artery, which with the facial artery supplies the buccal area.

9. Superficial temporal artery, which is the smaller terminal branch that supplies the anterior two thirds of the scalp and portions of the face. VERTEBRAL EXTRACRANIAL PORTION AND ANTERIOR SPINAL ARTERY

The vertebral artery is divided into four segments. y The first or V1 segment is the portion from its origin at the

Figure 22-1 Branches of the aortic arch. (© 1996 Indiana University Medical Illustration Dept.)

Figure 22-2 Major branches of the external carotid artery. (© 1996 Indiana University Medical Illustration Dept.)

subclavian artery to its entry into the costotransverse foramen of C6 or C5. The second or V2 segment travels between C6 and C2, entirely within the transverse foramina. The third or V3 segment takes a tortuous course between C2 to the suboccipital triangle between the atlas and the occiput, where it is covered by the atlanto-occipital membrane. The fourth segment or V4 is the intracranial portion, after it has pierced the dura mater to enter the foramen magnum to join the opposite vertebral artery at the medullopontine level.

The anterior spinal artery is formed by the anastomosis of the two branches of the vertebral arteries. This artery extends from the olivary nucleus to the conus medullaris and supplies the ventral surface of the medulla and the anterior two thirds of the spinal cord.

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