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thoracic course, and, so far, its relations call for separate consideration; whilst in the rest of its course it passes upwards in the neck, like the right common carotid, and has almost similar relations.
Thoracic Portion of the Left Common Carotid.-The thoracic or mediastinal portion of the left common carotid artery extends from the upper aspect of the aortic arch, immediately posterior and to the left of the origin of the innominate artery, to the left sterno-clavicular articulation, where the cervical portion commences. It is from 25 to 37 mm. (1 or 1 inches) in length, and it runs upwards and slightly laterally through the upper part of the superior mediastinum. It lies on a more posterior plane than the innominate artery.
Relations. Posterior. The vessel is in contact posteriorly, and from below upwards, with the trachea, the left recurrent nerve, the oesophagus, and the thoracic duct; and the thoracic part of the left subclavian artery is a postero-lateral relation.
Anterior. The left innominate vein runs obliquely across the anterior aspect of the artery, upon which cardiac branches from the left vagus and sympathetic descend vertically. These structures, together with the remains of the thymus and the anterior margins of the left lung and pleura, separate the artery from the manubrium sterni, and from the origins of the sterno-hyoid and sterno-thyreoid muscles.
Medial. The innominate artery below, and the trachea above, are on the right side. Lateral. The left pleura, and, on a posterior plane, the left phrenic and vagus nerves and the left subclavian artery are on its left side.
Cervical Portion of the Left Common Carotid Artery. The cervical part of the left common carotid artery is about 85 mm. (three and a half inches) long; it extends from the left sterno-clavicular articulation to the level of the upper border of the thyreoid cartilage and the lower border of the third cervical vertebra, where it ends by dividing into the external and internal carotid arteries.
Course. It runs upwards, laterally, and backwards, through the muscular and in the lower portion of the carotid divisions of the anterior triangle of the neck. Below it is separated from its fellow of the opposite side by the trachea and the cesophagus, and above by the relatively wide pharynx.
Relations. It is enclosed, together with the internal jugular vein and the vagus nerve, in a sheath of deep cervical fascia-the carotid sheath.
Posterior. The longus colli and scalenus anterior, below, and the longus capitis, above, are separated from the posterior surface of the artery and its sheath by the prevertebral fascia and the sympathetic trunk. The vertebral artery and the thoracic duct are posterior to it at the level of the seventh cervical vertebra; the inferior thyreoid artery crosses behind it, either between it and the vertebral or between it and the transverse process of the sixth cervical vertebra, and the vagus nerve lies postero-lateral to it.
Superficial. The descendens branch of the hypoglossal nerve lies superficial to the artery, usually outside the sheath, but sometimes enclosed in it (Fig. 759). Opposite the sixth cervical vertebra the omo-hyoid muscle and the sterno-mastoid branch of the superior thyreoid artery cross superficial to the carotid artery, which is overlapped, above the omohyoid muscle, by the anterior border of the sterno-mastoid and by cervical lymph glands. It is frequently crossed, in that part of its extent, by the superior thyreoid vein (Figs. 759, 766). Below the omo-hyoid the artery is covered by the sterno-thyreoid, the sterno-hyoid, and the sterno-mastoid muscles, and it may be overlapped by the lateral lobe of the thyreoid gland; it is also crossed, deep to the muscles, by the middle thyreoid vein, whilst occasionally a communication between the common facial and anterior jugular veins descends anterior to the artery along the anterior border of the sterno-mastoid. Just above the sternum the anterior jugular vein is in front of the artery, but separated from it by the sterno-hyoid and sterno-thyreoid muscles.
Medial. The trachea and œsophagus, with the recurrent nerve in the angle between them, are medial to the lower part of the artery; the larynx and pharynx are medial to its upper part. The carotid gland or glomus caroticum lies on the medial side of the termination of the artery.
Lateral. The internal jugular vein occupies the lateral part of the carotid sheath. The vein lies not only to the lateral side of the artery, but also slightly in front of it, especially in the lower part of the neck.
Branches. As a rule no branches are given off from either of the common carotid
arteries, except the terminal branches and some minute twigs from each to the corresponding carotid sheath and glomus caroticum.
The right common carotid artery, as already stated, differs as regards origin from the left common carotid. In length and general position it corresponds with the cervical portion of the left common carotid, and its relations also are very similar. Such differences as exist may be briefly summarised as follows:-The
FIG. 759.-DISSECTION OF THE CAROTID, SUBCLAVIAN, AND AXILLARY ARTERIES AND THEIR BRANCHES. Compare with Fig. 766, which is a drawing of the same body from a different point of view. The middle third of the clavicle has been removed and the subclavius muscle is turned downwards and medially.
internal jugular vein on both sides lies lateral to the artery; on the left side in the lower part of the neck it is also anterior to the artery, whilst on the right side the vein is separated from the lateral surface of the artery, at its lower end, by a wellmarked interval in which the vagus nerve appears. The thoracic duct does not come into relation with the right common carotid, and there is also a difference in the relations of the recurrent nerves to the arteries on the two sides. On the left side the nerve crosses posterior to the mediastinal part of the left artery, and lies medial to its cervical part, whilst the corresponding nerve on the right side
passes posterior to the lower part of the carotid artery in the neck to reach its. medial side, and the oesophagus has a less intimate relation with the right than with the left common carotid artery.
ARTERIA CAROTIS EXTERNA.
The external carotid artery (Figs. 759, 760) is the smaller of the two terminal branches of the common carotid; its length is about 62 mm. (2 inches). It extends from the upper border of the thyreoid cartilage to the back of the neck of the mandible, where it terminates by dividing into the superficial temporal and the internal maxillary arteries.
Course. It commences in the carotid triangle, passes upwards, medial to the posterior belly of the digastric and the stylo-hyoid muscles and the lower part of the postero-medial surface of the parotid gland, then it enters a groove in the medial border of the gland, through which it passes to the upper part of the antero-medial surface posterior to the neck of the mandible, where it terminates.
At its commencement it lies somewhat anterior and medial to the internal carotid artery, but it inclines posteriorly as it ascends, and thus becomes superficial to the internal carotid. Its course is indicated by a line drawn from the lobule of the ear to the posterior extremity of the greater cornu of the hyoid bone.
Relations. Posterior.-In the lower part of its extent it is in close relation with the internal carotid, and in the upper part of its course with the antero-medial surface of the parotid gland.
Medial. At its commencement the fibres of the inferior constrictor muscle are in contact with its medial side, but at a higher level the structures which intervene between it and the internal carotid-viz., the stylo-pharyngeus muscle, the tip of the styloid process, the stylo-glossus muscle, the glosso-pharyngeal nerve, and the pharyngeal branch of the vagus separate it from the wall of the pharynx; whilst medial both to it and to the internal carotid artery are the external and internal laryngeal branches of the superior laryngeal nerve.
Superficial. In the carotid triangle it is overlapped by the anterior border of the sterno-mastoid, and it is crossed, immediately below the level of its occipital branch, by the hypoglossal nerve. It is also crossed by the lingual and common facial veins, and sometimes by the superior thyreoid vein also. At the level of the angle of the mandible it passes under cover of the posterior belly of the digastric and the stylo-hyoid muscles, which separate it from the medial surface of the internal pterygoid muscle. As it emerges from under cover of the stylo-hyoid it enters a groove in the parotid gland, and as it lies in the gland the posterior facial vein (temporo-maxillary) descends superficial to the artery and both the artery and the vein are crossed, superficially, by the branches of the facial
Branches.-Eight branches arise from the external carotid artery; of these, three-the superior thyreoid, the lingual, and the external maxillary-spring from its anterior aspect in the carotid triangle; two arise from its posterior aspect, viz., the occipital and the posterior auricular, the former commencing below the posterior belly of the digastric and the latter above it; one from its medial side, viz., the ascending pharyngeal, which arises in the carotid triangle; and two from its termination, viz., the superficial temporal and the internal maxillary.
BRANCHES OF THE EXTERNAL CAROTID ARTERY.
(1) Arteria Thyreoidea Superior.-The superior thyreoid artery (Figs. 759 and 761) springs from the anterior aspect of the lower part of the external carotid artery, just below the tip of the greater cornu of the hyoid bone, and it terminates at the upper extremity of the corresponding lobe of the thyreoid gland by dividing into terminal branches.
Course. From its commencement, in the carotid triangle, the artery runs downwards and forwards to its termination.
Relations. Medially it is in relation with the inferior constrictor muscle and the external laryngeal branch of the superior laryngeal nerve.
Superficially it is covered, at its origin, by the anterior border of the sterno-mastoid;
afterwards, for a short distance, by fascia, platysma, and skin, and in the lower part of its extent by the omo-hyoid, the sterno-hyoid, and the sterno-thyreoid muscles, and it is overlapped by an accompanying vein.
Branches.-(1) In the carotid triangle-(a) A hyoid branch runs along the lower border of the greater cornu of the hyoid bone, under cover of the thyreo-hyoid muscle, to anastomose with its fellow of the opposite side and with the hyoid branch of the lingual artery. It supplies the thyreo-hyoid muscle and membrane.
(6) The superior laryngeal branch runs forwards, deep to the thyreo-hyoid muscle. It pierces the thyreoid hyo-membrane, in company with the internal laryngeal nerve, supplies the muscles, ligaments, and mucous membrane of the larynx, and anastomoses with its fellow of the opposite side, with branches of the crico-thyreoid artery, and with the terminal branches of the inferior thyreoid artery.
(c) The sternocleidomastoid branch passes downwards and posteriorly, along the upper border of the anterior belly of the omo-hyoid muscle and across the common carotid artery, to the deep surface of the sterno-mastoid muscle. It anastomoses, in the sternomastoid, with branches of the occipital and transverse scapular arteries.
(2) In the muscular triangle-(d) A crico-thyreoid branch passes anteriorly, either superficial or deep to the sterno-thyreoid. It crosses the crico-thyreoid muscle to anastomose, in front of the crico-thyreoid ligament, with its fellow of the opposite side, and, by branches which perforate the crico-thyreoid ligament, with laryngeal branches of the superior and inferior thyreoid arteries. It supplies the adjacent muscles and membrane.
(e) The terminal branches are anterior, medial, and lateral.
The anterior terminal branch descends along the anterior border of the corresponding lobe of the thyreoid gland, and the upper border of the thyreoid isthmus, to anastomose with its fellow of the opposite side. The medial branch is the largest; it is distributed to the medial surface of the lobe. The lateral branch, which ramifies in the lateral surface of the corresponding lobe, is the smallest. All three terminal branches supply glandular branches to the thyreoid gland. They anastomose with each other and with branches from the inferior thyreoid artery.
(2) Arteria Lingualis.-The lingual artery (Figs. 759 and 761) springs from the anterior aspect of the external carotid, opposite the tip of the greater cornu of the hyoid bone, and terminates, as the arteria profunda linguæ (O.T. ranine artery), which ends beneath the tip of the tongue, where it anastomoses with its fellow of the opposite side.
Course. Whilst in the carotid triangle, the first part of the artery forms a loop with the convexity upwards. The second part passes forwards, medial to the hyoglossus muscle, immediately above the greater cornu of the hyoid bone, to the anterior border of the hyo-glossus, where it gives off a sublingual branch and becomes the arteria profunda linguæ. The profunda linguæ artery passes obliquely forwards and upwards, under cover of the anterior border of the hyo-glossus, and then turns directly forwards on the under surface of the tongue to the tip, lying between the inferior lingualis laterally and the genio-glossus medially.
Relations. The first part of the lingual artery is crossed superficially by the hypoglossal nerve, and is covered by skin, fascia, and the platysma; it rests medially against the middle constrictor of the pharynx. The second part is deeper. It lies between the middle constrictor medially and the hyo-glossus laterally, and is separated by the latter from the hypoglossal nerve, the vena comitans hypoglossi, and the lower part of the submaxillary gland. The profunda artery of the tongue ascends almost vertically, parallel with and medial to the anterior fibres of the hyo-glossus, which are covered by the mylohyoid, and between the hyoglossus and the genio-glossus; then it runs forwards between the inferior lingualis and the genio-glossus muscles, and is covered, on its lower surface, by the mucous membrane of the tongue. Thus, at its termination, near the frenulum linguæ, it is comparatively superficial.
Branches. (a) The ramus hyoideus, a small branch which arises in the carotid triangle and runs along the upper border of the greater cornu of the hyoid bone. It anastomoses with its fellow of the opposite side and with the hyoid branch of the superior thyreoid artery.
(b) The dorsalis linguæ is a branch of moderate size which arises from the second part of the artery and is not uncommonly double. It ascends, between the hyo-glossus and the genio-glossus, to the dorsum of the tongue, where it branches and anastomoses with its fellow of the opposite side around the foramen cæcum. It supplies the posterior part of
the tongue as far back as the epiglottis, and sends branches, posteriorly, to the palatine tonsil which anastomose with the tonsillar twigs of the ascending palatine branch of the external maxillary and with the ascending pharyngeal artery.
(c) A sublingual branch arises at the lower part of the anterior border of the hyoglossus muscle and runs anteriorly and upwards, between the mylo-hyoid and the genioglossus, to the sublingual gland, which it supplies; it also supplies the mylo-hyoid, the genio-glossus, and the genio-hyoid muscles. It anastomoses with its fellow of the opposite side, with the arteria profunda by a branch which it sends along the frenulum linguæ, and, through the mylo-hyoid muscle, with the submental branch of the external maxillary.
(3) Arteria Maxillaris Externa (O.T. Facial).—The external maxillary artery (Fig. 759) arises from the front of the external carotid, immediately above the lingual. It ends at the angle of the mouth, where it becomes the angular artery.
Course.-It commences in the carotid triangle, immediately above the lingual, and passes upwards to the angle of the mandible, on the lateral surface of the middle constrictor muscle. Still ascending, it lies between the posterior belly of the digastric and the stylo-hyoid muscles laterally, and the superior constrictor medially, and it is separated from the palatine tonsil by the superior constrictor. When it reaches the upper border of the stylo-hyoid it enters a groove in the posterior part of the submaxillary gland and runs downwards and anteriorly, between the lateral surface of the gland and the internal pterygoid muscle, to the posterior end of the lower border of the body of the mandible. There it pierces the deep cervical fascia, turns round the inferior border of the mandible, at the anterior border of the masseter, enters the face and continues upwards and forwards to its termination.
Relations. In the carotid triangle the artery is comparatively superficial, except just at its origin, which is overlapped by the anterior fibres of the sterno-mastoid muscle. As it ascends it is in relation, on the medial side, with the middle and superior constrictor muscles, and, as already stated, the superior constrictor separates it from the palatine tonsil. Its relations between the point where it passes medial to the posterior belly of the digastric and the point where it turns round the lower border of the mandible have been given in the description of its course.
After turning round the lower border of the body of the mandible, which it grooves slightly, the artery becomes more superficial than in any other part of its course, being covered only by platysma, fascia, and skin. At that point the anterior facial vein is immediately posterior to the artery, lying on the surface of the masseter. In the face the artery lies between the platysma, the risorius, the zygomaticus major, and the infraorbital section of the quadratus labii superioris (O.T. levator labii superioris), which, with skin and fascia, are superficial to it, and the buccinator and the musculus caninus (O.T. levator anguli oris), which are deeper. The termination of the artery is in the substance of the quadratus labii superioris.
The anterior facial vein, though still posterior to the artery in the face, runs a somewhat straighter course, and is situated at some little distance from it.
Branches.-Four named branches are given off in the neck, and several in the face.
In the Neck. (a) The ascending palatine branch (Fig. 761) is a small artery which arises from the external maxillary under cover of the posterior belly of the digastric. It ascends, and, after passing between the stylo-glossus and the stylo-pharyngeus muscles, reaches the apex of the petrous portion of the temporal bone, where it turns downwards, accompanying the levator veli palatini muscle, pierces the pharyngeal aponeurosis, and enters the soft palate.
It supplies the lateral wall of the upper part of the pharynx, the soft palate, the palatine tonsil, and the auditory (O.T. Eustachian) tube, and it anastomoses with the tonsillar branch of the external maxillary, the dorsalis linguæ, the descending palatine branch of the internal maxillary, and with the ascending pharyngeal artery, which sometimes replaces it.
(6) The tonsillar branch, a small artery which arises close to the ascending palatine. It passes upwards between the internal pterygoid and the stylo-glossus, pierces the superior constrictor, and terminates in the palatine tonsil. It supplies the middle and superior constrictor muscles, and it anastomoses with the dorsalis linguæ, with the ascending palatine branch, and with the ascending pharyngeal artery.
(c) The submaxillary or glandular branch is frequently represented by two or three small twigs which pass directly into the submaxillary gland.