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(d) Tracheal branches are distributed to the trachea ; they anastomose with branches of the superior thyreoid and with the bronchial arteries.

(e) An inferior laryngeal branch accompanies the recurrent nerve to the lower part of the larynx. It enters the larynx, at the lower border of the inferior constrictor, gives branches to its muscles and mucous membrane, and anastomoses with the laryngeal branch of the superior thyreoid artery.

() The ascending terminal branch supplies the posterior and lower part of the thyreoid gland, and anastomoses with branches of the superior thyreoid artery.

(9) The inferior terminal branch is distributed to the lower and medial part of the corresponding lobe of the thyreoid gland. It anastomoses with its fellow of the opposite side and with branches of the superior thyreoid artery.

(B) Arteria Transversa Colli. — The transverse cervical artery (Figs. 759 and 761) arises from the thyreo-cervical trunk and runs upwards and posteriorly across the posterior triangle of the neck to the anterior border of the trapezius; there it divides into a ramus ascendens (O.T. superficial cervical) and a ramus descendens (O.T. posterior scapular). It is very variable in size, and not infrequently the ramus descendens arises separately from the third part of the subclavian.

Immediately after its origin, under cover of the internal jugular vein, it crosses the scalenus anterior, lying superficial to the phrenic nerve and under cover of the sterno-mastoid muscle; on the left side it is also crossed, superficially, by the terminal part of the thoracic duct. Passing from beneath the sterno-mastoid, it enters the lower part of the posterior triangle of the neck, where it lies upon the trunks of the brachial plexus, and, as it runs upwards and backwards to its termination, it passes medial to the posterior belly of the omo-hyoid.

The ascending branch may be a separate vessel which springs from the thyreo-cervical trunk and takes the course described, whilst the descending branch arises from the third part of the subclavian artery and lies at a lower level. In such cases the upper of the two vessels is called the superficial cervical artery and the lower the transverse cervical artery. If the superficial cervical artery is absent it is replaced by the ascending branch of the transverse cervical.

Branches.—(a) Small muscular branches to the surrounding muscles.

(6) The ascending branch, usually a slender branch, passes beneath the trapezius; it sends branches upwards and downwards, superficial to the levator scapulæ and upon the splenius ; the ascending branches anastomose with the descending branch of the occipital artery, and the descending branches accompany the accessory nerve and anastomose with the descending branch and transverse scapular artery,

(C) The descending branch runs downwards, deep to the levator scapulæ and the rhomboid muscles, close to the vertebral border of the scapula. It runs parallel with, and a short distance away from, the dorsal scapular nerve (0.T. nerve to the rhomboid muscles), and it sends branches into the supraspinous, the infraspinous, and the subscapular fossæ, which anastomose with branches of the transverse scapular and subscapular arteries. It also sends branches backwards, through and between the rhomboid muscles, which anastomose with the branches of the ascending division of the transverse cervical and with the posterior branches of the intercostal arteries.

(C) Arteria Transversa Scapulæ.—The transverse scapular artery (O.T. suprascapular) springs from the thyreo-cervical trunk and terminates in the infraspinous fossa of the scapula. As a rule it is smaller than the transverse cervical artery.

Commencing behind the internal jugular vein, it crosses the scalenus anterior and phrenic nerve, and is covered superficially by the sterno-mastoid and the anterior jugular vein; on the left side it lies behind the termination of the thoracic duct also. Continuing, laterally, behind the clavicle, and crossing superficial to the third part of the subclavian artery and the cords of the brachial plexus, it reaches the scapular notch and passes over the superior transverse ligament. Then it descends, with the suprascapular nerve, through the supraspinous fossa and deep to the supraspinatus muscle, and after passing through the great scapular notch, deep to the inferior transverse ligament, it enters the infraspinous fossa, where it anastomoses with the circumflex scapular branch of the subscapular artery and with twigs of the descending branch of the transverse cervical artery.

Branches.-(a) Muscular, to the sterno-mastoid, the subclavius, and the muscles on the dorsum of the scapula.

(6) The medullary, a small branch to the clavicle.

(c) The suprasternal, to the sternal end of the clavicle and the sterno-clavicular joint.

(d) Acromial branches, which ramify over the acromion, anastomosing with the acromial branches of the thoraco-acromial and the posterior circumflex arteries.

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Fig. 765.—DISSECTION OF THE BACK OF THE SHOULDER AND ARM, showing the anastomosing vessels

on the dorsum of the scapula, and the posterior humeral circumflex and the profunda arteries.

(e) Articular, to the acromio-clavicular and shoulder-joints.

(f) The subscapular, which is given off as the artery passes over the superior transverse ligament. It passes down into the subscapular fossa, gives branches to the subscapularis, and it anastomoses with the branches of the subscapular artery and the descending branch of the transverse cervical artery.

(9) Supraspinous, which ramify in the supraspinous fossa, supplying the muscle, and anastomosing with the descending branch of the transverse cervical.

(h) Terminal branches ramify in the infraspinous fossa, and anastomose with the

circumflex scapular and with branches of the descending branch of the transverse cervical artery.

(3) Arteria Mammaria Interna.—The internal mammary artery (Figs. 757, 761) arises from the lower and anterior part of the subclavian, at the medial border of the scalenus anterior and immediately below the origin of the thyreo-cervical trunk. It terminates, behind the medial extremity of the sixth intercostal space, by dividing into the musculo-phrenic and the superior epigastric arteries.

The artery passes at first downwards, forwards, and medially, lying upon the pleura, and behind the innominate vein, the sternal extremity of the clavicle, and the cartilage of the first rib; it is crossed obliquely, from the lateral to the medial side, by the phrenic nerve, which usually passes anterior to it. From the cartilage of the first rib it descends vertically, about 12 mm. (half an inch) from the border of the sternum, and lies, in the upper part of its course, in front of the pleura, and, in the lower part, in front of the transversus thoracis muscle. It is covered anteriorly by the cartilages of the upper six ribs, the intervening intercostal muscles, and the terminal portions of the intercostal nerves; and it is accompanied by two venæ comites, which unite together above, and on its medial side, to form a single trunk which terminates in the innominate vein.

Branches.(a) The pericardiaco-phrenic (O.T. comes nervi phrenici) is a long slender branch which is given off from the upper part of the internal mammary. It accompanies the phrenic nerve, through the superior and middle mediastina, to the diaphragm, where it anastomoses with the inferior phrenic and musculo-phrenic arteries.

In its course downwards this branch gives off numerous small rami to the pleura and pericardium, which anastomose with offsets of the mediastinal and pericardial branches of the aorta and internal mammary arteries, and also with the bronchial arteries, forming the widemeshed subpleural plexus of Turner.

(6) Anterior mediastinal branches, small and numerous, pass to the areolar tissue of the anterior mediastinum and supply the areolar tissue and the sternum.

(c) Thymic.—Small twigs which supply the thymus.

(d) Bronchial.—One or several small branches which pass to the lower end of the trachea and to the bronchi.

(e) The intercostal are two in number in each of the upper six intercostal spaces. They pass laterally and, for a short distance, they lie either between the pleura and the internal intercostal muscles or between the transversus thoracis and the internal intercostal muscles; they then pierce the internal intercostal muscles, and ramify between them and the external intercostal muscles, anastomosing with the aortic and superior intercostal arteries and their collateral branches.

(f) The perforating branches, one in each of the upper six intercostal spaces, are small vessels which pass forwards, with the anterior branches of the thoracic nerves, piercing the internal intercostal muscle, the anterior intercostal membrane, and the pectoralis major. They terminate in the skin and subcutaneous tissue. They supply twigs to the sternum, and those in the third and fourth spaces, usually the largest of the series, give off branches to the mammary gland.

(9) The musculo-phrenic, or lateral terminal branch of the internal mammary artery, runs downwards and laterally, from the sixth intercostal space to the tenth costal cartilage. In the upper part of its course it lies upon the thoracic surface of the diaphragm, but it pierces the diaphragm about the level of the eighth costal cartilage, and terminates on its abdominal surface. Its branches are :

(i.) Muscular, which supply the diaphragm and anastomose with the superior and inferior phrenic arteries.

(ii.) Intercostal branches, two in each of the seventh, eighth, and ninth intercostal spaces ; they are distributed in the same manner as the corresponding branches of the internal mammary artery, and terminate by anastomosing with the aortic intercostals and their collateral branches.

(h) The superior epigastric, or medial terminal branch of the internal mammary artery, descends into the anterior wall of the abdomen. It leaves the thorax, between the sternal and costal origins of the diaphragm, and enters the sheath of the rectus abdominis muscle, lying first behind, and then in the substance of the muscle. It terminates by anastomosing with branches of the inferior epigastric artery. Its branches are :

(i.) Muscular, to the rectus, to the flat muscles of the abdominal wall, and to the diaphragm.

(ii.) Anterior Cutaneous. —Small branches which pierce the rectus abdominis muscle and the anterior portion of its sheath. They accompany the anterior terminal branches of the lower thoracic nerves, and terminate in the subcutaneous tissues and skin of the middle portion of the anterior abdominal wall.

(iii.) Xiphoid, a small branch which crosses the front of the xiphoid process to anastomose with its fellow of the opposite side. It supplies the adjacent muscles and skin.

(iv.) Hepatic branches of small size pass backwards in the falciform ligament to the liver, where they anastomose with branches of the hepatic artery.

(4) Truncus Costocervicalis.— The costo-cervical trunk (Fig. 761) springs from the posterior aspect of the second part of the subclavian artery on the right side and from the first part on the left side. It runs upwards and backwards, over the apex of the pleural sac, to the neck of the first rib, where it divides into superior intercostal and deep cervical branches.

Branches.—a) Arteria Cervicalis Profunda—The deep cervical branch sometimes arises from the subclavian artery directly; but more commonly it springs from the costocervical trunk at the upper border of the neck of the first rib. It runs backwards, to the back of the neck, passing between the first thoracic and last cervical nerves, and between the transverse process of the last cervical vertebra and the neck of the first rib. In the back of the neck it ascends, between the semispinalis capitis (0.T. complexus) and the sernispinalis cervicis muscles and it terminates by anastomosing with the descending branch of the occipital artery. It anastomoses also with branches of the ascending cervical and vertebral arteries, supplies the adjacent muscles, and sends a spinal branch, through the intervertebral foramen between the last cervical and the first thoracic vertebræ, into the vertebral canal; this branch anastomoses with the spinal branches of the vertebral and intercostal arteries.

(6) Arteria Intercostalis Suprema.—The superior intercostal branch descends, anterior to the neck of the first rib, between the first thoracic nerve laterally and the first thoracic ganglion of the sympathetic trunk medially and, at the lower border of the neck of the rib, it gives off the posterior intercostal artery of the first space; then, after crossing anterior to the neck of the second rib, it becomes the posterior intercostal artery of the second intercostal space. The first two posterior intercostal arteries, which are respectively a branch and the continuation of the superior intercostal artery, run laterally, each in its own space, lying first between the pleura and the posterior intercostal membrane, and then between the internal and external intercostal muscles. Their branches terminate by anastomosing with anterior intercostal branches of the internal mammary artery. Each gives off muscular branches to the intercostal muscles, a nutrient branch to the rib below which it lies, and a collateral branch which runs along the lower border of the corresponding space.

ARTERIA AXILLARIS. The axillary artery, which lies in the axillary space, is the direct continuation of the subclavian artery, and it becomes the brachial artery.

It commences at the external border of the first rib, at the apex of the axillary space. It passes distally, with a lateral inclination, along the lateral wall of the space, i.e. to the medial side of the shoulder-joint and the humerus, to the lower border of the teres major, where it becomes the brachial artery. A line drawn from the middle of the clavicle to the medial border of the prominence of the coracobrachialis muscle, when the arm is abducted until it is at right angles with the side, indicates the position and direction of the artery.

The position and direction, however, and to a certain extent the relations of the axillary artery, are modified by changes in the position of the upper extremity. With the arm hanging by the side the axillary artery describes a curve with the concavity directed downwards and medially, and the vein is to its medial side. When the arm is at right angles with the side, the axillary artery is almost straight; it lies closer to the lateral wall of the axilla, and the vein overlaps it antero-medially. When the arm is raised above the level of the shoulder the axillary artery is curved over the head of the humerus, with the convexity of the curve below, and the vein lies still more in front of it.

For descriptive purposes the artery is divided into three parts: the first part lies above, the second behind, and the third part below the pectoralis minor.

Though it is the usual custom to describe three parts of the axillary artery, -a division which is of practical interest in so far as it emphasises the fact that the axillary artery is surgically accessible above the pectoralis minor,—it is to be

noted that the upper border of the pectoralis minor is frequently exactly opposite the external border of the first rib, at the point where the axillary artery begins. In the strict sense, therefore, no part of the artery is above the pectoralis minor.

Relations of the First Part-Posterior.— The first part of the artery is enclosed, together with the vein and the cords of the brachial plexus, in a prolongation of the cervical fascia known as the axillary sheath. Posterior to the sheath are the upper serration of the serratus anterior, the contents of the first intercostal space, and the long thoracic nerve, the latter descending vertically between the artery and the serratus

A. transvers3 colli, ramus descendens
V. jugularis externa

A. carotis communis M. omohyoideus

V. jugularis interna | M. sternothyreoideus A. transversa scapulæ

Nn. thoracales anteriores! pracoacromialis et rami deltoideus et pectoralis !

A. et V., axillaris

M. deltoideus
N. musculocutanæus

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M, pectoralis minor
M. pectoralis major

M. latissimus dorsi

M. serratus anterior
M. obliquus externus abdominis

Fig. 766.—THE AXILLARY ARTERY AND ITS BRANCHES.
Note.—The middle third of the clavicle has been removed ; and the arm has been slightly abducted and

rotated laterally. Parts of the pectoralis major and minor have been removed ; the positions of the lower border of the pectoralis major and the upper and lower borders of the pectoralis minor are indicated by broken black lines. Compare with Fig. 759, which represents a dissection of the same body from a different point of view.

anterior ; whilst, within the sheath, the medial anterior thoracic nerve and the medial cord of the brachial plexus lie behind the artery. Anterior.—It is covered in front by the costo-coracoid membrane. The membrane intervenes between the artery and the cephalic vein, the branches of the lateral anterior thoracic nerve, the branches of the thoracoacromial artery with their accompanying veins, and the clavicular part of the pectoralis major muscle, superficial to which are the deep fascia, the platysma, the supra-clavicular branches of the cervical plexus, and the superficial fascia and the skin. Posterior to the costo-coracoid membrane the artery is crossed by a loop of communication between the lateral and medial anterior thoracic nerves.

Lateral.Above and to the lateral side are

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