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Each posterior intercostal vein is provided with valves, both at its termination and along its course, which prevent the blood flowing towards the anterior aspect of the thoracic wall. Its tributaries are derived from the adjacent muscles and bones, and a short distance from its termination it receives a posterior tributary which passes to it between the transverse processes of the vertebræ. This posterior vessel is formed by the union of small veins which issue from the muscles of the back, from the anterior and posterior spinal plexuses which lie respectively in front of the bodies and behind the arches of the vertebræ, and by venous channels which issue through the intervertebral foramina; the latter vessels commence in the vertebral canal, where they are connected with the anterior and posterior spinal veins.

VENE ANONYMÆ.

The innominate veins (Figs. 756 and 757) are two in number, right and left. They return blood from the head and neck, the upper extremities, the upper part of the posterior wall of the thorax, the anterior wall of the thorax, and the upper part of the anterior wall of the abdomen. Each innominate vein commences behind the medial end of the clavicle of the corresponding side, and is formed by the union of the internal jugular and subclavian veins; the two innominate veins terminate by uniting together, at the lower border of the first costal cartilage of the right side, to form the superior vena cava. To reach that point the left vein has to pass from left to right behind the manubrium sterni, and it is therefore about three times as long as the right vein. The innominate veins do not possess valves.

The right innominate vein is a little more than 25 mm. (1 inch) in length. It descends almost vertically to the lower border of the first costal cartilage, and terminates in the superior vena cava.

Relations. It is in relation, anteriorly, with the medial end of the clavicle and the sterno-hyoid and sterno-thyreoid muscles. It partly overlaps the innominate artery, which lies to its left side, and it is in front of the internal mammary artery, the right vagus nerve, and the upper end of the right pleural sac. The phrenic nerve and the accompanying vessels run along its right side, and intervene between it and the right pleural sac.

Tributaries. In addition to the veins by the union of which it is formed, the right innominate vein receives the right vertebral and internal mammary veins, the first right posterior intercostal vein, and sometimes the right inferior thyreoid vein. The right lymphatic duct also opens into it.

The left innominate vein passes from left to right, with a slight obliquity downwards, behind the upper part of the manubrium sterni, to the lower border of the first right costal cartilage, where it terminates in the superior vena cava. It is about 60 to 75 mm. (3 inches) long.

Relations. It is covered anteriorly, in the greater part of its extent, by the sternohyoid and sterno-thyreoid muscles, but at its right extremity it is slightly overlapped by the right pleura, and in the median plane the remains of the thymus intervene between it and the posterior surface of the sternum. It rests, posteriorly, upon the left pleura, the left internal mammary artery, the left subclavian artery, the left phrenic, and the left vagus nerves, the left superior cardiac branch of the sympathetic, the inferior cervical branch of the left vagus, the left common carotid artery, the trachea, and the innominate artery.

Its lower border is in relation with the arch of the aorta, and on its upper border it receives the inferior thyreoid vein of one or both sides.

Tributaries. It receives the vertebral, internal mammary, inferior thyreoid, superior intercostal veins of its own side, the first left posterior intercostal vein, and some pericardial, thymic, anterior bronchial, and anterior mediastinal veins. Sometimes the right inferior thyreoid vein joins it, but not uncommonly that vessel terminates in the right innominate vein or in the commencement of the superior vena cava.

The thoracic duct opens into it just at the angle of junction of the internal jugular and subclavian veins.

Venæ Mammariæ Internæ The Internal Mammary Veins.- Each internal mammary artery is accompanied by venæ comites; they commence by the union of the venæ comites of the superior epigastric and musculo-phrenic arteries, between the sixth costal cartilage and the transversus thoracis; and at the upper part of the thorax

they fuse into a single vessel which enters the superior mediastinum and ends in the innominate vein of the same side.

The tributaries of the internal mammary veins are (a) The venæ comites of the superior epigastric and musculo-phrenic arteries, which in their turn receive tributaries which correspond with the branches of the arteries they accompany. (b) Six anterior perforating veins which accompany the corresponding arteries, one lying in each of the upper six intercostal spaces. (c) Twelve anterior intercostal veins from the upper six intercostal spaces, two veins lying in each space with the corresponding branches of the internal mammary artery. (d) Small and irregular pleural, muscular, mediastinal, and sternal veins.

The internal mammary veins are provided with numerous valves which prevent the blood from flowing downwards.

Venæ Epigastrica Superiores-The Superior Epigastric Veins. The venæ comites of the superior epigastric artery receive tributaries from the substance of the rectus abdominis, the sheath of the muscle, and the superjacent skin and fascia; they pass, with the artery, between the sternal and costal origins of the diaphragm, and terminate in the internal mammary veins.

Musculo-phrenic Veins.-The venæ comites of the musculo-phrenic artery commence in the abdomen, pass through the diaphragm with the musculo-phrenic artery, and terminate in the internal mammary veins. They receive as tributaries the anterior intercostal veins of the seventh, eighth, and ninth intercostal spaces, and small venules from the substance of the diaphragm and walls of the abdomen.

Venæ Vertebrales-The Vertebral Veins correspond only to the extra-cranial parts of the vertebral arteries. Each commences by the union of offsets from the intraspinal venous plexuses, and, issuing from the vertebral canal, passes across the posterior arch of the atlas, with the vertebral artery, to the foramen in the transverse process of the atlas. In the foramina in the cervical transverse processes, a plexus of venous channels surrounds the artery. At the lower part of the neck efferents from the plexus unite to form a single trunk which issues from the foramen in the transverse process of the sixth cervical vertebra, and descends, in the interval between the longus colli and scalenus anterior muscles, to terminate in the upper and posterior part of the innominate vein; at its termination there is a uni- or bi-cuspidate valve.

Relations. In the first part of its course the vein lies in the sub-occipital triangle. The second, plexiform portion, is in the canal formed by the foramina in the transverse processes of the cervical vertebræ, and, with the artery, which it surrounds, lies anterior to the trunks of the cervical spinal nerves. The third part, in the root of the neck, is between the longus colli and scalenus anterior muscles, in front of the first part of the vertebral artery, and behind the internal jugular vein.

Tributaries. In addition to the offsets from the intraspinal venous plexuses by the union of which it is formed, each vertebral vein receives the following tributaries :-(a) Small vessels which issue from the muscles, ligaments, and bones of the deeper parts of the neck, and the lower and posterior part of the head. (6) Offsets from the intraspinal venous plexuses which pass out of the vertebral canal by the intervertebral foramina. (c) The ascending cervical vein, a vessel which is formed by the union of tributaries which issue from a venous plexus on the anterior aspects of the bodies and roots of the transverse processes of the cervical vertebræ. This vessel accompanies the ascending cervical artery, and terminates in the lower part of the vertebral vein, immediately after the latter has issued from the foramen in the sixth cervical transverse process. (d) The deep cervical vein; this commences in the sub-occipital triangle from a venous plexus with which the vertebral and occipital veins communicate. It descends, posterior to the transverse processes of the cervical vertebræ, in company with the profunda cervicis artery, turns forwards at the root of the neck, between the transverse processes of the sixth and seventh cervical vertebræ or between the latter and the neck of the first rib, and opens into the vertebral vein. It receives blood from the muscles, ligaments, and bones of the back of the neck. (e) The posterior intercostal vein from the first intercostal space sometimes opens into the vertebral vein.

Occasionally the venous plexus around the vertebral artery ends below in two terminal trunks, anterior and posterior, instead of one. In those cases the second terminal vessel lies behind the lower part of the vertebral artery, passes through the foramen in the transverse process of the seventh cervical vertebra, and turns forwards on the lateral side

of the artery to join the anterior trunk, thus forming a common terminal vein which ends in the usual manner.

Venæ Thyreoidea Inferiores.-Each inferior thyreoid vein commences by the union of tributaries which issue from the isthmus and the corresponding lobe of the thyreoid gland. The two veins descend, along the front of the trachea, into the superior mediastinum, where the right inferior thyreoid vein terminates either in the right innominate vein or in the junction of the two innominate veins, and the left in the upper border of the left innominate vein; or the two veins unite to form a single trunk, which ends, usually, in the left innominate vein, but, occasionally, in the right. As they descend in the neck the inferior thyreoid veins anastomose together, and sometimes the anastomoses are so frequent that a venous plexus is formed in front of the lower cervical portion of the trachea.

VEINS OF THE HEAD AND NECK.

Vena Jugularis Interna (Figs. 756, 759, 787, 800 and 801).—Each internal jugular vein commences, in the posterior compartment of the jugular foramen, as the direct continuation of the transverse sinus, and terminates, behind the medial part of the clavicle, by uniting with the subclavian vein of the same side to form the innominate vein.

Its commencement, which is dilated, forms the superior bulb of the jugular vein. In the upper part of the neck it lies postero-lateral to the internal carotid artery and the last four cerebral nerves. As it descends it accompanies first the internal and then the common carotid artery. It inclines forwards as it descends, and gradually passes from its original position, behind and to the lateral side of the internal carotid artery, until it lies more completely to the lateral side of the internal and common carotid arteries, and, indeed, somewhat overlaps the latter anteriorly. This is more especially the case on the left side, for both internal jugular veins trend slightly towards the right as they descend; consequently, at the root of the neck, the right vein is separated from the right common carotid artery by a small interval filled with areolar tissue, whilst the left vein is more directly in front of the corresponding common carotid artery.

A dilatation, the inferior bulb, is present at the inferior extremity of the vein it is bounded, either above or below, by a valve of two or three semilunar cusps. Sometimes both the superior and inferior ends of the bulb are bounded. by valves.

Relations. The vein lies anterior to the transverse processes of the cervical vertebræ, the rectus capitis lateralis, longus capitis, and scalenus anterior muscles, the ascending cervical artery, which runs upwards in the interval between the attachments of the two latter muscles, and the phrenic nerve; the transverse scapular and the transverse cervical arteries intervene between it and the scalenus anterior. At the root of the neck the vein lies in front of the first part of the subclavian artery and the origins of the vertebral artery and the thyreo-cervical trunk, and on the left side it is anterior to the terminal part of the thoracic duct.

On the antero-medial side of the internal jugular vein, immediately below the skull, are the internal carotid artery and the last four cerebral nerves; in the rest of its extent it is in relation, medially, first with the internal and then with the common carotid artery, whilst to its medial side and somewhat posteriorly, between it and the large arteries, lies the vagus nerve.

Each internal jugular vein is covered, superficially, in the whole of its length, by the sterno-mastoid muscle; near its upper end it is crossed by the styloid process, the stylopharyngeus and stylo-hyoid muscles, and the posterior belly of the digastric, whilst in its lower half, the omo-hyoid, the sterno-hyoid, and the sterno-thyreoid muscles are superficial to it, under cover of the sterno-mastoid. Just below the transverse process of the atlas, and under cover of the sterno-mastoid, the vein. is crossed, on its lateral side, by the accessory nerve and by the occipital artery; about the middle of its course it is crossed by the communicans cervicis nerve, and near its lower end by the anterior jugular vein; the latter vessel, however, is separated from it by the sterno-hyoid and sternothyreoid muscles. Superficial to the vein are numerous deep cervical lymph glands.

Tributaries. (a) A vein from the cochlea and (b) the inferior petrosal sinus join it near its commencement. (c) Pharyngeal branches from the venous plexus on the wall of the pharynx. (d) Emissary veins from the cavernous sinus. (e) The common facial vein, which receives the anterior and posterior facial veins. (f) The lingual veins, which return part of the blood from the tongue. (g) The vena comitans hypoglossi, which accompanies the hypoglossal nerve. (h) The superior thyreoid vein, which accompanies - the corresponding artery. (i) The middle thyreoid vein, which passes backwards from the corresponding lobe of the thyreoid gland and crosses the middle of the lateral aspect of the common carotid artery. (j) The occipital vein occasionally terminates in the internal = jugular vein. In many cases, however, it ends in the sub-occipital plexus, which is : drained by the vertebral and deep cervical veins (see p. 963).

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The common facial vein is formed by the union of the anterior and posterior facial veins. It accompanies the first part of the external maxillary artery in the carotid triangle, and terminates in the anterior border of the internal jugular vein. Just before it disappears beneath the sterno-mastoid, the common facial vein frequently gives off a large branch which descends along the anterior border of the sterno-mastoid to the supra-sternal fossa, where it joins the anterior jugular vein.

The anterior facial vein (Fig. 785) commences at the medial commissure of the eyelids as the angular vein, which is formed by the union of the supra-orbital and frontal veins. It passes downwards and backwards, in the face, to the lower and anterior part of the masseter muscle, which it crosses, lying in the same plane as the external maxillary artery, but following a much straighter course. After crossing the lower border of the mandible it passes across the submaxillary triangle, superficial to the submaxillary gland, and separate from the external maxillary artery, which there lies in a deeper plane. It terminates, a short distance below the angle of the mandible, by uniting with the posterior facial vein to form the common facial vein.

The anterior facial vein receives tributaries corresponding with all the branches of the external maxillary artery, except the ascending palatine and the tonsillar, which have no accompanying veins, the blood from the region which they supply being returned for the most part through the pharyngeal plexus. The anterior facial vein also communicates with the pterygoid plexus around the external pterygoid muscle, by means of an anastomosing channel, called the deep facial vein, which passes posteriorly, between the masseter and buccinator muscles, into the infra-temporal fossa.

The posterior facial vein, see p. 968.

The inferior thyreoid veins have already been described (see p. 964).

Venæ Subclavia.-The subclavian vein, of each side, is the direct continuation of the main vein of the upper extremity, i.e. the axillary vein; but through its tributary, the external jugular vein, it receives blood both from the head and from the superficial parts of the neck.

From its commencement, at the external border of the first rib, it runs medially, below and anterior to the corresponding artery from which it is separated by the lower part of the scalenus anterior muscle, and it terminates, behind the medial end of the clavicle, in the innominate vein of the corresponding side. As it passes medially it forms a slight curve, the convexity of which is directed

upwards.

Each subclavian vein possesses a single bicuspid valve which is situated immediately to the distal side of the opening of the external jugular vein.

Relations. The subclavian vein is in relation anteriorly with the posterior layer of the costo-coracoid membrane, which separates it from the subclavius muscle, and the nerve to the subclavius, and with the back of the medial end of the clavicle, from which it is partly separated, however, by the fibres of the sterno-hyoid and sterno-thyreoid

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muscles.

It is closely attached, anteriorly, to the posterior surface of the costo-coracoid membrane; consequently it is expanded when the clavicle is moved forwards, an arrangement which constitutes a distinct danger when operations are being performed in the neighbourhood of the vein; for, in the event of the vessel being wounded, forward movement of the clavicle may cause air to be sucked into the vein, with fatal results.

Posterior to the vein, and on a higher plane, is the first part of the subclavian artery, but it is separated from the second part by the scalenus anterior. As soon as it reaches the medial border of the anterior scalene the subclavian vein unites with the internal jugular vein, immediately anterior to the internal mammary artery.

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The upper surface of the first rib is below the vein.

Tributaries. Whilst the subclavian vein is the direct continuation of the axillary vein, and receives, therefore, the blood from the upper extremity, it has, as a general rule, only one named tributary, viz., the external jugular vein.

Vena Jugularis Externa.-The external jugular vein (Fig. 785) is formed on the superficial surface of the sterno-mastoid muscle, a little below and posterior to the angle of the mandible, by the union of the posterior auricular vein with a branch from the posterior facial vein (O.T. temporo-maxillary). In many cases the branch

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FIG. 785.-SUPERFICIAL VEINS OF THE HEAD AND NECK.

from the posterior facial vein is so preponderantly large that it is more correct to describe the external jugular vein as commencing as a branch of the posterior facial vein. After its formation the external jugular vein descends, with a slight obliquity backwards, to the anterior part of the subclavian portion of the posterior triangle of the neck, where it pierces the deep fascia, crosses in front of the third part of the subclavian artery, and terminates in the subclavian vein.

Whilst on the surface of the sterno-mastoid muscle it is covered by the superficial fascia, and platysma muscle, and it lies parallel with the great auricular nerve; after crossing the nervus cutaneus colli (O.T. trans. cervical) it reaches the posterior border of the sterno-mastoid, where it receives a tributary called the posterior external jugular vein, which commences in the superficial tissues of the upper and back part of the neck, and runs downwards and forwards, across the

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