Branches of arch of aorta
1st Innominate divides into right subclavian and right common carotid
2nd left common carotid
3rd left subclavian
Common carotid arteries
The common carotid arteries differ on the right and left sides with respect to their origins. On the right, the common carotid arises from the brachiocephalic artery as it passes behind the sternoclavicular joint. On the left, the common carotid artery comes directly from the arch of the aorta in the superior mediastinum. The right common carotid has, therefore, only a cervical part whereas the left common carotid has cervical and thoracic parts. Following a similar course on both sides, the common carotid artery ascends, diverging laterally from behind the sternoclavicular joint to the level of the upper border of the thyroid cartilage of the larynx (C3–4 junction), where it divides into external and internal carotid arteries. This bifurcation can sometimes be at a higher level. The artery may be compressed against the prominent transverse process of the sixth cervical vertebra (Chassaignac's tubercle), and above this level it is superficial and its pulsation can be easily felt.
In 12% of cases the right common carotid artery arises above the level of the sternoclavicular joint, or it may be a separate branch from the aorta. The left common carotid artery varies in origin more than the right and may arise with the brachiocephalic artery. Division of the common carotid may occur higher, near the level of the hyoid bone, or, more rarely, at a lower level alongside the larynx. Very rarely it ascends without division, so that either the external or internal carotid is absent, or it may be replaced by separate external and internal carotid arteries which arise directly from the aorta, on one side, or bilaterally.
Although the common carotid artery usually has no branches, it may occasionally give rise to the vertebral, superior thyroid, superior laryngeal, ascending pharyngeal, inferior thyroid or occipital arteries.
External carotid artery
The external carotid artery begins lateral to the upper border of the thyroid cartilage, level with the intervertebral disc between the third and fourth cervical vertebrae. A little curved and with a gentle spiral, it first ascends slightly forwards and then inclines backwards and a little laterally, to pass midway between the tip of the mastoid process and the angle of the mandible. Here, in the substance of the parotid gland behind the neck of the mandible, it divides into its terminal branches, the superficial temporal and maxillary arteries. As it ascends, it gives off several large branches, and diminishes rapidly in calibre. In children the external carotid is smaller than the internal carotid, but in adults the two are of almost equal size. At its origin, it is in the carotid triangle and lies anteromedial to the internal carotid artery. It later becomes anterior, then lateral, to the internal carotid as it ascends. At mandibular levels the styloid process and its attached structures intervene between the vessels: the internal carotid is deep, and the external carotid superficial, to the styloid process. A fingertip placed in the carotid triangle perceives a powerful arterial pulsation, which represents the termination of the common carotid, the origins of external and internal carotids and the stems of the initial branches of the external carotid.
The external carotid artery has eight named branches distributed to the head and neck. The superior thyroid, lingual and facial arteries arise from its anterior surface, the occipital and posterior auricular arteries arise from its posterior surface and the ascending pharyngeal artery arises from its medial surface. The maxillary and superficial temporal arteries are its terminal branches within the parotid gland.
Superior thyroid artery
The superior thyroid artery is the first branch of the external carotid artery, and arises from the anterior surface of the external carotid just below the level of the greater cornu of the hyoid bone. It descends along the lateral border of thyrohyoid to reach the apex of the lobe of the thyroid gland. Lying medially are the inferior constrictor muscle and the external laryngeal nerve: the nerve is often posteromedial, and therefore at risk when the artery is being ligatured. Occasionally it may issue directly from the common carotid.
Ascending pharyngeal artery
The ascending pharyngeal artery is the smallest branch of the external carotid. It is a long, slender vessel which arises from the medial (deep) surface of the external carotid artery near the origin of that artery. It ascends between the internal carotid artery and the pharynx to the base of the cranium. The ascending pharyngeal artery is crossed by styloglossus and stylopharyngeus, and longus capitis lies posterior to it. It gives off numerous small branches to supply longus capitis and longus colli, the sympathetic trunk, the hypoglossal, glossopharyngeal and vagus nerves and some of the cervical lymph nodes. It anastomoses with the ascending palatine branch of the facial artery and the ascending cervical branch of the vertebral artery. Its named branches are the pharyngeal, inferior tympanic and meningeal arteries.
The lingual artery provides the chief blood supply to the tongue and the floor of the mouth. It arises anteromedially from the external carotid artery opposite the tip of the greater cornu of the hyoid bone, between the superior thyroid and facial arteries. It often arises with the facial or, less often, with the superior thyroid artery. It may be replaced by a ramus of the maxillary artery. Ascending medially at first, it loops down and forwards, passes medial to the posterior border of hyoglossus and then runs horizontally forwards deep to it. The lingual artery next ascends again almost vertically, and courses sinuously forwards on the inferior surface of the tongue as far as its tip.
The suprahyoid artery is a small branch which runs along the upper border of the hyoid bone to anastomose with the contralateral artery.
Dorsal lingual arteries
The facial artery arises anteriorly from the external carotid in the carotid triangle, above the lingual artery and immediately above the greater cornu of the hyoid bone. In the neck, at its origin, it is covered only by the skin, platysma, fasciae and often by the hypoglossal nerve. It runs up and forwards, deep to digastric and stylohyoid. At first on the middle pharyngeal constrictor, it may reach the lateral surface of styloglossus, separated there from the palatine tonsil only by this muscle and the lingual fibres of the superior constrictor. Medial to the mandibular ramus it arches upwards and grooves the posterior aspect of the submandibular gland. It then turns down and descends to the lower border of the mandible in a lateral groove on the submandibular gland, between the gland and medial pterygoid. Reaching the surface of the mandible, the facial artery curves round its inferior border, anterior to masseter, to enter the face: its further course is described on page 490. The artery is very sinuous throughout its extent. In the neck this may be so that the artery is able to adapt to the movements of the pharynx during deglutition, and similarly on the face, so that the artery can adapt to movements of the mandible, lips and cheeks. Facial artery pulsation is most palpable where the artery crosses the mandibular base, and again near the corner of the mouth. Its branches in the neck are the ascending palatine, tonsillar, submental and glandular arteries.
The occipital artery arises posteriorly from the external carotid artery, approximately 2 cm from its origin. At its origin, the artery is crossed superficially by the hypoglossal nerve, which winds round it from behind. The artery next passes backwards, up and deep to the posterior belly of digastric, and crosses the internal carotid artery, internal jugular vein, hypoglossal, vagus and accessory nerves. Between the transverse process of the atlas and the mastoid process, the occipital artery reaches the lateral border of rectus capitis lateralis. It then runs in the occipital groove of the temporal bone, medial to the mastoid process and attachments of sternocleidomastoid, splenius capitis, longissimus capitis and digastric, and lies successively on rectus capitis lateralis, obliquus superior and semispinalis capitis. Finally, accompanied by the greater occipital nerve, it turns upwards to pierce the investing layer of the deep cervical fascia connecting the cranial attachments of trapezius and sternocleidomastoid, and ascends tortuously in the dense superficial fascia of the scalp where it divides into many branches.
The occipital artery has two main branches (upper and lower) to the upper part of sternocleidomastoid in the neck. The lower branch arises near the origin of the occipital artery, and may sometimes arise directly from the external carotid artery. It descends backwards over the hypoglossal nerve and internal jugular vein, enters sternocleidomastoid and anastomoses with the sternocleidomastoid branch of the superior thyroid artery. The upper branch arises as the occipital artery crosses the accessory nerve, and runs down and backwards superficial to the internal jugular vein. It enters the deep surface of sternocleidomastoid with the accessory nerve.
Posterior auricular artery
The posterior auricular artery is a small vessel which branches posteriorly from the external carotid just above digastric and stylohyoid. It ascends between the parotid gland and the styloid process to the groove between the auricular cartilage and mastoid process, and divides into auricular and occipital branches. In the neck, it provides branches to supply digastric, stylohyoid, sternocleidomastoid and the parotid gland. It also gives origin to the stylomastoid artery – described as an indirect branch of the posterior auricular artery in about a third of subjects – which enters the stylomastoid foramen to supply the facial nerve, tympanic cavity, mastoid antrum air cells and semicircular canals. In the young, its posterior tympanic ramus forms a circular anastomosis with the anterior tympanic branch of the maxillary artery.
Internal carotid artery
The internal carotid artery supplies most of the ipsilateral cerebral hemisphere, eye and accessory organs, and forehead and, in part, the nose. From its origin at the carotid bifurcation (where it usually has a carotid sinus), it ascends in front of the transverse processes of the upper three cervical vertebrae to the inferior aperture of the carotid canal in the petrous part of the temporal bone. Here it enters the cranial cavity and turns anteriorly through the cavernous sinus in the carotid groove on the side of the body of the sphenoid bone. It terminates below the anterior perforated substance by division into the anterior and middle cerebral arteries. It may be divided conveniently into cervical, petrous, cavernous and cerebral parts.
Carotid sinus and carotid body
The common carotid artery shows two specialized organs near its bifurcation, the carotid sinus and the carotid body. They relay information concerning the pressure and chemical composition of the arterial blood respectively, and are innervated principally by carotid branch(es) of the glossopharyngeal nerve, with small contributions from the cervical sympathetic trunk and the vagus nerve.
The carotid sinus usually appears as a dilation of the lower end of the internal carotid, and functions as a baroreceptor.
The carotid body is a reddish-brown, oval structure, 5–7 mm in height and 2.5–4 mm in width. It lies either posterior to the carotid bifurcation or between its branches, and is attached to, or sometimes partly embedded in, their adventitia. Occasionally it takes the form of a group of separate nodules. Aberrant miniature carotid bodies, microstructurally similar but with diameters of 600 μm or less, may appear in the adventitia and adipose tissue near the carotid sinus.
The carotid body is surrounded by a fibrous capsule from which septa divide the enclosed tissue into lobules. Each lobule contains glomus (type I) cells which are separated from an extensive network of fenestrated sinusoids by sustentacular (type II) cells. Glomus cells store a number of peptides, particularly enkephalins, bombesin and neurotensin, and amines including dopamine, serotonin, adrenaline (epinephrine) and noradrenaline (norepinephrine), and are therefore regarded as paraneurones. Unmyelinated axons lie in a collagenous matrix between the sustentacular cells and the sinusoidal endothelium, and many synapse on the glomus cells. They are visceral afferents which travel in the carotid sinus nerve to join the glossopharyngeal nerve. Preganglionic sympathetic axons and fibres from the carotid sinus synapse on parasympathetic and sympathetic ganglion cells, which lie either in isolation or in small groups near the surface of each carotid body. Postganglionic axons travel to local blood vessels: the parasympathetic efferent fibres are probably vasodilatory and the sympathetic ones are vasoconstrictor.
The carotid body receives a rich blood supply from branches of the adjacent external carotid artery, which is consistent with its role as an arterial chemoreceptor. When stimulated by hypoxia, hypercapnia or increased hydrogen ion concentration (low pH) in the blood flowing through it, it elicits reflex increases in the rate and volume of ventilation via connections with brain stem respiratory centres. The bodies are most prominent in children and normally involute in older age, when they are infiltrated by lymphocytes and fibrous tissue. Individuals with chronic hypoxia, or who live at high altitude or suffer from lung disease, may have enlarged carotid bodies as a result of hyperplasia.
Other small bodies, resembling carotid bodies, and also considered to be chemoreceptors, occur near the arteries of the fourth and sixth pharyngeal arches and hence are found near the aortic arch, ligamentum arteriosum and right subclavian artery, and are supplied by the vagus nerve.
The right subclavian artery arises from the brachiocephalic trunk, the left from the aortic arch. For description, each is divided into a first part, from its origin to the medial border of scalenus anterior, a second part behind this muscle and a third part from the lateral margin of scalenus anterior to the outer border of the first rib, where the artery becomes the axillary artery. Each subclavian artery arches over the cervical pleura and pulmonary apex. Their first parts differ, whereas the second and third parts are almost identical.
Parts of the subclavian arteries
First part of right subclavian artery
The right subclavian artery branches from the brachiocephalic trunk behind the upper border of the right sternoclavicular joint, and passes superolaterally to the medial margin of scalenus anterior. It usually ascends 2 cm above the clavicle, but this varies.
First part of left subclavian artery
The first part of the left subclavian artery springs from the aortic arch, behind the left common carotid, level with the disc between the third and fourth thoracic vertebrae. It ascends into the neck, then arches laterally to the medial border of scalenus anterior.
Second part of subclavian artery
The second part of the subclavian artery lies behind scalenus anterior; it is short and the highest part of the vessel.
Third part of subclavian artery
The third part of the subclavian artery descends laterally from the lateral margin of scalenus anterior to the outer border of the first rib, where it becomes the axillary artery. It is the most superficial part of the artery and lies partly in the supraclavicular triangle, where its pulsations may be felt and it may be compressed. The third part of the subclavian artery is the most accessible segment of the artery. Since the line of the posterior border of sternocleidomastoid approximates to the (deeper) lateral border of scalenus anterior, the artery can be felt in the anteroinferior angle of the posterior triangle. It can only be effectively compressed against the first rib: with the shoulder depressed, pressure is exerted down, back and medially in the angle between sternocleidomastoid and the clavicle. The palpable trunks of the brachial plexus may be injected with local anaesthetic allowing major surgical procedures to the arm.
The vertebral artery arises from the superoposterior aspect of the first part of the subclavian artery. It passes through the foramina in the transverse processes of all of the cervical vertebrae except the seventh, curves medially behind the lateral mass of the atlas and enters the cranium via the foramen magnum. At the lower pontine border it joins its fellow to form the basilar artery. Occasionally it may enter the cervical vertebral column via the fourth, fifth or seventh cervical vertebra.
Reference: Gray's Anatomy, 40th Edition, By Susan Standring PhD DSc FKC