4. The Arteries of the Upper Extremity. a. The Subclavian Artery

The artery which supplies the upper extremity continues as a single trunk from its commencement down to the elbow; but different portions of it have received different names, according to the regions through which they pass. That part of the vessel which extends from its origin to the outer border of the first rib is termed the subclavian; beyond this point to the lower border of the axilla it is named the axillary; and from the lower margin of the axillary space to the bend of the elbow it is termed brachial; here the trunk ends by dividing into two branches the radial and ulnar. 1 The Subclavian Artery (A. Subclavia) (Fig. 520).—On the right side the subclavian artery arises from the innominate artery behind the right sternoclavicular articulation; on the left side it springs from the arch of the aorta. The two vessels, therefore, in the first part of their course, differ in length, direction, and relation with neighboring structures. 2 In order to facilitate the description, each subclavian artery is divided into three parts. The first portion extends from the origin of the vessel to the medial border of the Scalenus anterior; the second lies behind this muscle; and the third extends from the lateral margin of the muscle to the outer border of the first rib, where it becomes the axillary artery. The first portions of the two vessels require separate descriptions; the second and third parts of the two arteries are practically alike. 3 First Part of the Right Subclavian Artery (Figs. 505, 520).—The first part of the right subclavian artery arises from the innominate artery, behind the upper part of the right sternoclavicular articulation, and passes upward and lateralward to the medial margin of the Scalenus anterior. It ascends a little above the clavicle, the extent to which it does so varying in different cases. 4

FIG. 520– Superficial dissection of the right side of the neck, showing the carotid and subclavian arteries. (See enlarged image) Relations.—It is covered, in front, by the integument, superficial fascia, Platysma, deep fascia, the clavicular origin of the Sternocleidomastoideus, the Sternohyoideus, and Sternothyreoideus, and another layer of the deep fascia. It is crossed by the internal jugular and vertebral veins, by the vagus nerve and the cardiac branches of the vagus and sympathetic, and by the subclavian loop of the sympathetic trunk which forms a ring around the vessel. The anterior jugular vein is directed lateralward in front of the artery, but is separated from it by the Sternohyoideus and Sternothyreoideus. Below and behind the artery is the pleura, which separates it from the apex of the lung; behind is the sympathetic trunk, the Longus collie and the first thoracic vertebra. The right recurrent nerve winds around the lower and back part of the vessel. 5 First Part of the Left Subclavian Artery (Fig. 505).—The first part of the left subclavian artery arises from the arch of the aorta, behind the left common carotid, and at the level of the fourth thoracic vertebra; it ascends in the superior mediastinal cavity to the root of the neck and then arches lateralward to the medial border of the Scalenus anterior. 6 Relations.—It is in relation, in front, with the vagus, cardiac, and phrenic nerves, which lie parallel with it, the left common carotid artery, left internal jugular and vertebral veins, and the commencement of the left innominate vein, and is covered by the Sternothyreoideus, Sternohyoideus, and Sternocleidomastoideus; behind, it is in relation with the esophagus, thoracic duct, left recurrent nerve, inferior cervical ganglion of the sympathetic trunk, and Longus colli; higher up, however, the esophagus and thoracic duct lie to its right side; the latter ultimately arching over the vessel to join the angle of union between the subclavian and internal jugular veins. Medial to it are the esophagus, trachea, thoracic duct, and left recurrent nerve; lateral to it, the left pleura and lung. 7 Second and Third Parts of the Subclavian Artery (Fig. 520).—The second portion of the subclavian artery lies behind the Scalenus anterior; it is very short, and forms the highest part of the arch described by the vessel. 8 Relations.—It is covered, in front, by the skin, superficial fascia, Platysma, deep cervical fascia, Sternocleidomastoideus, and Scalenus anterior. On the right side of the neck the phrenic nerve is separated from the second part of the artery by the Scalenus anterior, while on the left side it crosses the first part of the artery close to the medial edge of the muscle. Behind the vessel are the pleura and the Scalenus medius; above, the brachial plexus of nerves; below, the pleura. The subclavian vein lies below and in front of the artery, separated from it by the Scalenus anterior. 9 The third portion of the subclavian artery runs downward and lateralward from the lateral margin of the Scalenus anterior to the outer border of the first rib, where it becomes the axillary artery. This is the most superficial portion of the vessel, and is contained in the subclavian triangle (see page 565). 10

Relations.—It is covered, in front, by the skin, the superficial fascia, the Platysma, the supraclavicular nerves, and the deep cervical fascia. The external jugular vein crosses its medial part and receives the transverse scapular, transverse cervical, and anterior jugular veins, which frequently form a plexus in front of the artery. Behind the veins, the nerve to the Subclavius descends in front of the artery. The terminal part of the artery lies behind the clavicle and the Subclavius and is crossed by the transverse scapular vessels. The subclavian vein is in front of and at a slightly lower level than the artery. Behind, it lies on the lowest trunk of the brachial plexus, which intervenes between it and the Scalenus medius. Above and to its lateral side are the upper trunks of the brachial plexus and the Omohyoideus. Below, it rests on the upper surface of the first rib. 11 Peculiarities.—The subclavian arteries vary in their origin, their course, and the height to which they rise in the neck. 12 The origin of the right subclavian from the innominate takes place, in some cases, above the sternoclavicular articulation, and occasionally, but less frequently, below that joint. The artery may arise as a separate trunk from the arch of the aorta, and in such cases it may be either the first, second, third, or even the last branch derived from that vessel; in the majority, however, it is the first or last, rarely the second or third. When it is the first branch, it occupies the ordinary position of the innominate artery; when the second or third, it gains its usual position by passing behind the right carotid; and when the last branch, it arises from the left extremity of the arch, and passes obliquely toward the right side, usually behind the trachea, esophagus, and right carotid, sometimes between the esophagus and trachea, to the upper border of the first rib, whence it follows its ordinary course. In very rare instances, this vessel arises from the thoracic aorta, as low down as the fourth thoracic vertebra. Occasionally, it perforates the Scalenus anterior; more rarely it passes in front of that muscle. Sometimes the subclavian vein passes with the artery behind the Scalenus anterior. The artery may ascend as high as 4 cm. above the clavicle, or any intermediate point between this and the upper border of the bone, the right subclavian usually ascending higher than the left. 13 The left subclavian is occasionally joined at its origin with the left carotid. 14 The left subclavian artery is more deeply placed than the right in the first part of its course, and, as a rule, does not reach quite as high a level in the neck. The posterior border of the Sternocleidomastoideus corresponds pretty closely to the lateral border of the Scalenus anterior, so that the third portion of the artery, the part most accessible for operation, lies immediately lateral to the posterior border of the Sternocleidomastoideus. 15 Collateral Circulation.—After ligature of the third part of the subclavian artery, the collateral circulation is established mainly by three sets of vessels, thus described in a dissection: 16 1. A posterior set, consisting of the transverse scapular and the descending ramus of the transverse cervical branches of the subclavian, anastomosing with the subscapular from the axillary. 17 2. A medial set, produced by the connection of the internal mammary on the one hand, with the highest intercostal and lateral thoracic arteries, and the branches from the subscapular on the other. 18 3. A middle or axillary set, consisting of a number of small vessels derived from branches of the subclavian, above, and, passing through the axilla, terminating either in the main trunk, or some of the branches of the axillary below. This last set presented most conspicuously the peculiar character of newly formed or, rather, dilated arteries, being excessively tortuous, and forming a complete plexus. 19 The chief agent in the restoration of the axillary artery below the tumor was the subscapular artery, which communicated most freely with the internal mammary, transverse scapular and descending ramus of the transverse cervical branches of the subclavian, from all of which it received so great an influx of blood as to dilate it to three times its natural size. 101 20 When a ligature is applied to the first part of the subclavian artery, the collateral circulation is carried on by: (1) the anastomosis between the superior and inferior thyroids; (2) the anastomosis of the two vertebrals; (3) the anastomosis of the internal mammary with the inferior epigastric and the aortic intercostals; (4) the costocervical anastomosing with the aortic intercostals; (5) the profunda cervicis anastomosing with the descending branch of the occipital; (6) the scapular branches of the thyrocervical trunk anastomosing with the branches of the axillary, and (7) the thoracic branches of the axillary anastomosing with the aortic intercostals. 21 Branches.—The branches of the subclavian artery are: 22

Vertebral.		Internal mammary.
Thyrocervical.		Costocervical.

On the left side all four branches generally arise from the first portion of the vessel; but on the right side (Fig. 520) the costocervical trunk usually springs from the second portion of the vessel. On both sides of the neck, the first three branches arise close together at the medial border of the Scalenus anterior; in the majority of cases, a free interval of from 1.25 to 2.5 cm. exists between the commencement of the artery and the origin of the nearest branch. 23 1. The vertebral artery (a. vertebralis) (Fig. 514), is the first branch of the subclavian, and arises from the upper and back part of the first portion of the vessel. It is surrounded by a plexus of nerve fibers derived from the inferior cervical ganglion of the sympathetic trunk, and ascends through the foramina in the transverse processes of the upper six cervical vertebræ 102 it then winds behind the superior articular process of the atlas and, entering the skull through the foramen magnum, unites, at the lower border of the pons, with the vessel of the opposite side to form the basilar artery. 24 Relations.—The vertebral artery may be divided into four parts: The first part runs upward and backward between the Longus colli and the Scalenus anterior. In front of it are the internal jugular and vertebral veins, and it is crossed by the inferior thyroid artery; the left vertebral is crossed by the thoracic duct also. Behind it are the transverse process of the seventh cervical vertebra, the sympathetic trunk and its inferior cervical ganglion. The second part runs upward through the foramina in the transverse processes of the upper six cervical vertebræ, and is surrounded by branches from the inferior cervical sympathetic ganglion and by a plexus of veins which unite to form the vertebral vein at the lower part of the neck. It is situated in front of the trunks of the cervical nerves, and pursues an almost vertical course as far as the transverse process of the atlas, above which it runs upward and lateralward to the foramen in the transverse process of the atlas. The third part issues from the latter foramen on the medial side of the Rectus capitis lateralis, and curves backward behind the superior articular process of the atlas, the anterior ramus of the first cervical nerve being on its medial side; it then lies in the groove on the upper surface of the posterior arch of the atlas, and enters the vertebral canal by passing beneath the posterior atlantoöccipital membrane. This part of the artery is covered by the Semispinalis capitis and is contained in the suboccipital triangle—a triangular space bounded by the Rectus capitis posterior major, the Obliquus superior, and the Obliquus inferior. The first cervical or suboccipital nerve lies between the artery and the posterior arch of the atlas. The fourth part pierces the dura mater and inclines medialward to the front of the medulla oblongata; it is placed between the hypoglossal nerve and the anterior root of the first cervical nerve and beneath the first digitation of the ligamentum denticulatum. At the lower border of the pons it unites with the vessel of the opposite side to form the basilar artery. 25 Branches.—The branches of the vertebral artery may be divided into two sets: those given off in the neck, and those within the cranium. 26

Cervical Branches.	Cranial Branches.
Spinal.	Meningeal.
Muscular.	Posterior Spinal.
	Anterior Spinal.
	Posterior Inferior Cerebellar.
	Medullary.

Spinal Branches (rami spinales) enter the vertebral canal through the intervertebral foramina, and each divides into two branches. Of these, one passes along the roots of the nerves to supply the medulla spinalis and its membranes, anastomosing with the other arteries of the medulla spinalis; the other divides into an ascending and a descending branch, which unite with similar branches from the arteries above and below, so that two lateral anastomotic chains are formed on the posterior surfaces of the bodies of the vertebræ, near the attachment of the pedicles. From these anastomotic chains branches are supplied to the periosteum and the bodies of the vertebræ, and others form communications with similar branches from the opposite side; from these communications small twigs arise which join similar branches above and below, to form a central anastomotic chain on the posterior surface of the bodies of the vertebræ. 27 Muscular Branches are given off to the deep muscles of the neck, where the vertebral artery curves around the articular process of the atlas. They anastomose with the occipital, and with the ascending and deep cervical arteries. 28 The Meningeal Branch (ramus meningeus; posterior meningeal branch) springs from the vertebral opposite the foramen magnum, ramifies between the bone and dura mater in the cerebellar fossa, and supplies the falx cerebelli. It is frequently represented by one or two small branches. 29 The Posterior Spinal Artery (a. spinalis posterior; dorsal spinal artery) arises from the vertebral, at the side of the medulla oblongata; passing backward, it descends on this structure, lying in front of the posterior roots of the spinal nerves, and is reinforced by a succession of small branches, which enter the vertebral canal through the intervertebral foramina; by means of these it is continued to the lower part of the medulla spinalis, and to the cauda equina. Branches from the posterior spinal arteries form a free anastomosis around the posterior roots of the spinal nerves, and communicate, by means of very tortuous transverse branches, with the vessels of the opposite side. Close to its origin each gives off an ascending branch, which ends at the side of the fourth ventricle. 30 The Anterior Spinal Artery (a. spinalis anterior; ventral spinal artery) is a small branch, which arises near the termination of the vertebral, and, descending in front of the medulla oblongata, unites with its fellow of the opposite side at the level of the foramen magnum. One of these vessels is usually larger than the other, but occasionally they are about equal in size. The single trunk, thus formed, descends on the front of the medulla spinalis, and is reinforced by a succession of small branches which enter the vertebral canal through the intervertebral foramina; these branches are derived from the vertebral and the ascending cervical of the inferior thyroid in the neck; from the intercostals in the thorax; and from the lumbar, iliolumbar, and lateral sacral arteries in the abdomen and pelvis. They unite, by means of ascending and descending branches, to form a single anterior median artery, which extend as far as the lower part of the medulla spinalis, and is continued as a slender twig on the filum terminale. This vessel is placed in the pia mater along the anterior median fissure; it supplies that membrane, and the substance of the medulla spinalis, and sends off branches at its lower part to be distributed to the cauda equina. 31 The Posterior Inferior Cerebellar Artery (a. cerebelli inferior posterior) (Fig. 516), the largest branch of the vertebral, winds backward around the upper part of the medulla oblongata, passing between the origins of the vagus and accessory nerves, over the inferior peduncle to the under surface of the cerebellum, where it divides into two branches. The medial branch is continued backward to the notch between the two hemispheres of the cerebellum; while the lateral supplies the under surface of the cerebellum, as far as its lateral border, where it anastomoses with the anterior inferior cerebellar and the superior cerebellar branches of the basilar artery. Branches from this artery supply the choroid plexus of the fourth ventricle. 32 The Medullary Arteries (bulbar arteries) are several minute vessels which spring from the vertebral and its branches and are distributed to the medulla oblongata. 33 The Basilar Artery (a. basilaris) (Fig. 516), so named from its position at the base of the skull, is a single trunk formed by the junction of the two vertebral arteries: it extends from the lower to the upper border of the pons, lying in its median groove, under cover of the arachnoid. It ends by dividing into the two posterior cerebral arteries. 34 Its branches, on either side, are the following: 35

Pontine.		Anterior Inferior Cerebellar.
Internal Auditory.		Superior Cerebellar.
Posterior Cerebral.

The pontine branches (rami ad pontem; transverse branches) are a number of small vessels which come off at right angles from either side of the basilar artery and supply the pons and adjacent parts of the brain. 36 The internal auditory artery (a. auditiva interna; auditory artery), a long slender branch, arises from near the middle of the artery; it accompanies the acoustic nerve through the internal acoustic meatus, and is distributed to the internal ear. 37 The anterior inferior cerebellar artery (a. cerebelli inferior anterior) passes backward to be distributed to the anterior part of the under surface of the cerebellum, anastomosing with the posterior inferior cerebellar branch of the vertebral. 38 The superior cerebellar artery (a. cerebelli superior) arises near the termination of the basilar. It passes lateralward, immediately below the oculomotor nerve, which separates it from the posterior cerebral artery, winds around the cerebral peduncle, close to the trochlear nerve, and, arriving at the upper surface of the cerebellum, divides into branches which ramify in the pia mater and anastomose with those of the inferior cerebellar arteries. Several branches are given to the pineal body, the anterior medullary velum, and the tela chorioidea of the third ventricle. 39 The posterior cerebral artery (a. cerebri posterior) (Figs. 516, 517, 518) is larger than the preceding, from which it is separated near its origin by the oculomotor nerve. Passing lateralward, parallel to the superior cerebellar artery, and receiving the posterior communicating from the internal carotid, it winds around the cerebral peduncle, and reaches the tentorial surface of the occipital lobe of the cerebrum, where it breaks up into branches for the supply of the temporal and occipital lobes. 40 The branches of the posterior cerebral artery are divided into two sets, ganglionic and cortical: 41

Ganglionic	Posterior-medial.	Cortical	Anterior Temporal.
	Posterior Choroidal.		Posterior Temporal.
	Postero-lateral.		Calcarine.
			Parietoöccipital.

Ganglionic.—The postero-medial ganglionic branches (Fig. 519) are a group of small arteries which arise at the commencement of the posterior cerebral artery: these, with similar branches from the posterior communicating, pierce the posterior perforated substance, and supply the medial surfaces of the thalami and the walls of the third ventricle. The posterior choroidal branches run forward beneath the splenium of the corpus callosum, and supply the tela chorioidea of the third ventricle and the choroid plexus. The postero-lateral ganglionic branches are small arteries which arise from the posterior cerebral artery after it has turned around the cerebral peduncle; they supply a considerable portion of the thalamus. 42 Cortical.—The cortical branches are: the anterior temporal, distributed to the uncus and the anterior part of the fusiform gyrus; the posterior temporal, to the fusiform and the inferior temporal gyri; the calcarine, to the cuneus and gyrus lingualis and the back part of the convex surface of the occipital lobe; and the parietoöccipital, to the cuneus and the precuneus. 43 2. The thyrocervical trunk (truncus thyreocervicalis; thyroid axis) (Fig. 520) is a short thick trunk, which arises from the front of the first portion of the subclavian artery, close to the medial border of the Scalenus anterior, and divides almost immediately into three branches, the inferior thyroid, transverse scapular, and transverse cervical. 44 The Inferior Thyroid Artery (a. thyreoidea inferior) passes upward, in front of the vertebral artery and Longus colli; then turns medialward behind the carotid sheath and its contents, and also behind the sympathetic trunk, the middle cervical ganglion resting upon the vessel. Reaching the lower border of the thyroid gland it divides into two branches, which supply the postero-inferior parts of the gland, and anastomose with the superior thyroid, and with the corresponding artery of the opposite side. The recurrent nerve passes upward generally behind, but occasionally in front, of the artery. 45 The branches of the inferior thyroid are: 46

Inferior Laryngeal.		Esophageal.
Tracheal.		Ascending Cervical.
Muscular.

The inferior laryngeal artery (a. laryngea inferior) ascends upon the trachea to the back part of the larynx under cover of the Constrictor pharyngis inferior, in company with the recurrent nerve, and supplies the muscles and mucous membrane of this part, anastomosing with the branch from the opposite side, and with the superior laryngeal branch of the superior thyroid artery. 47 The tracheal branches (rami tracheales) are distributed upon the trachea, and anastomose below with the bronchial arteries. 48 The esophageal branches (rami æsophagei) supply the esophagus, and anastomose with the esophageal branches of the aorta. 49 The ascending cervical artery (a. cervicalis ascendens) is a small branch which arises from the inferior thyroid as that vessel is passing behind the carotid sheath; it runs up on the anterior tubercles of the transverse processes of the cervical vertebræ in the interval between the Scalenus anterior and Longus capitis. To the muscles of the neck it gives twigs which anastomose with branches of the vertebral, and it sends one or two spinal branches into the vertebral canal through the intervertebral foramina to be distributed to the medulla spinalis and its membranes, and to the bodies of the vertebræ, in the same manner as the spinal branches from the vertebral. It anastomoses with the ascending pharyngeal and occipital arteries. 50 The muscular branches supply the depressors of the hyoid bone, and the Longus colli, Scalenus anterior, and Constrictor pharyngis inferior. 51 The Transverse Scapular Artery (a. transversa scapulæ suprascapular artery) passes at first downward and lateralward across the Scalenus anterior and phrenic nerve, being covered by the Sternocleidomastoideus; it then crosses the subclavian artery and the brachial plexus, and runs behind and parallel with the clavicle and Subclavius, and beneath the inferior belly of the Omohyoideus, to the superior border of the scapula; it passes over the superior transverse ligament of the scapula which separates it from the suprascapular nerve, and enters the supraspinatous fossa (Fig. 521). In this situation it lies close to the bone, and ramifies between it and the Supraspinatus, to which it supplies branches. It then descends behind the neck of the scapula, through the great scapular notch and under cover of the inferior transverse ligament, to reach the infraspinatous fossa, where it anastomoses with the scapular circumflex and the descending branch of the transverse cervical. Besides distributing branches to the Sternocleidomastoideus, Subclavius, and neighboring muscles, it gives off a suprasternal branch, which crosses over the sternal end of the clavicle to the skin of the upper part of the chest; and an acromial branch, which pierces the Trapezius and supplies the skin over the acromion, anastomosing with the thoracoacromial artery. As the artery passes over the superior transverse ligament of the scapula, it sends a branch into the subscapular fossa, where it ramifies beneath the Subscapularis, and anastomoses with the subscapular artery and with the descending branch of the transverse cervical. It also sends articular branches to the acromioclavicular and shoulder-joints, and a nutrient artery to the clavicle. 52

FIG. 521– The scapular and circumflex arteries. (See enlarged image) The Transverse Cervical Artery (a. transversa colli; transversalis colli artery) lies at a higher level than the transverse scapular; it passes transversely above the inferior belly of the Omohyoideus to the anterior margin of the Trapezius, beneath which it divides into an ascending and a descending branch. It crosses in front of the phrenic nerve and the Scaleni, and in front of or between the divisions of the brachial plexus, and is covered by the Platysma and Sternocleidomastoideus, and crossed by the Omohyoideus and Trapezius. 53 The ascending branch (ramus ascendens; superficial cervical artery) ascends beneath the anterior margin of the Trapezius, distributing branches to it, and to the neighboring muscles and lymph glands in the neck, and anastomosing with the superficial branch of the descending ramus of the occipital artery. 54 The descending branch (ramus descendens; posterior scapular artery) (Fig. 521) passes beneath the Levator scapulæ to the medial angle of the scapula, and then descends under the Rhomboidei along the vertebral border of that bone as far as the inferior angle. It supplies the Rhomboidei, Latissimus dorsi and Trapezius, and anastomoses with the transverse scapular and subscapular arteries, and with the posterior branches of some of the intercostal arteries. 55 Peculiarities.—The ascending branch of the transverse cervical frequently arises directly from the thyrocervical trunk; and the descending branch from the third, more rarely from the second, part of the subclavian. 56

FIG. 522– The internal mammary artery and its branches. (See enlarged image) 3. The internal mammary artery (a. mammaria interna) (Fig. 522) arises from the under surface of the first portion of the subclavian, opposite the thyrocervical trunk. It descends behind the cartilages of the upper six ribs at a distance of about 1.25 cm. from the margin of the sternum, and at the level of the sixth intercostal space divides into the musculophrenic and superior epigastric arteries. 57 Relations.—It is directed at first downward, forward, and medialward behind the sternal end of the clavicle, the subclavian and internal jugular veins, and the first costal cartilage, and passes forward close to the lateral side of the innominate vein. As it enters the thorax the phrenic nerve crosses from its lateral to its medial side. Below the first costal cartilage it descends almost vertically to its point of bifurcation. It is covered in front by the cartilages of the upper six ribs and the intervening Intercostales interni and anterior intercostal membranes, and is crossed by the terminal portions of the upper six intercostal nerves. It rests on the pleura, as far as the third costal cartilage; below this level, upon the Transversus thoracis. It is accompanied by a pair of veins; these unite above to form a single vessel, which runs medial to the artery and ends in the corresponding innominate vein. 58 Branches.—The branches of the internal mammary are: 59

Pericardiacophrenic.		Intercostal.
Anterior Mediastinal.		Perforating.
Pericardial.		Musculophrenic.
Sternal.		Superior Epigastric.

The Pericardiacophrenic Artery (a. pericardiacophrenica; a. comes nervi phrenici) is a long slender branch, which accompanies the phrenic nerve, between the pleura and pericardium, to the diaphragm, to which it is distributed; it anastomoses with the musculophrenic and inferior phrenic arteries. 60 The Anterior Mediastinal Arteries (aa. mediastinales anteriores; mediastinal arteries) are small vessels, distributed to the areolar tissue and lymph glands in the anterior mediastinal cavity, and to the remains of the thymus. 61 The Pericardial Branches supply the upper part of the anterior surface of the pericardium; the lower part receives branches from the musculophrenic artery. 62 The Sternal Branches (rami sternales) are distributed to the Transversus thoracis, and to the posterior surface of the sternum. 63 The anterior mediastinal, pericardial, and sternal branches, together with some twigs from the pericardiacophrenic, anastomose with branches from the intercostal and bronchial arteries, and form a subpleural mediastinal plexus. 64 The Intercoastal Branches (rami intercostales; anterior intercostal arteries) supply the upper five or six intercostal spaces. Two in number in each space, these small vessels pass lateralward, one lying near the lower margin of the rib above, and the other near the upper margin of the rib below, and anastomose with the intercostal arteries from the aorta. They are at first situated between the pleura and the Intercostales interni, and then between the Intercostales interni and externi. They supply the Intercostales and, by branches which perforate the Intercostales externi, the Pectorales and the mamma. 65 The Perforating Branches (rami perforantes) correspond to the five or six intercostal spaces. They pass forward through the intercostal spaces, and, curving lateralward, supply the Pectoralis major and the integument. Those which correspond to the second, third, and fourth spaces give branches to the mamma, and during lactation are of large size. 66 The Musculophrenic Artery (a. musculophrenica) is directed obliquely downward and lateralward, behind the cartilages of the false ribs; it perforates the diaphragm at the eighth or ninth costal cartilage, and ends, considerably reduced in size, opposite the last intercostal space. It gives off intercostal branches to the seventh, eighth, and ninth intercostal spaces; these diminish in size as the spaces decrease in length, and are distributed in a manner precisely similar to the intercostals from the internal mammary. The musculophrenic also gives branches to the lower part of the pericardium, and others which run backward to the diaphragm, and downward to the abdominal muscles. 67 The Superior Epigastric Artery (a. epigastrica superior) continues in the original direction of the internal mammary; it descends through the interval between the costal and sternal attachments of the diaphragm, and enters the sheath of the Rectus abdominis, at first lying behind the muscle, and then perforating and supplying it, and anastomosing with the inferior epigastric artery from the external iliac. Branches perforate the anterior wall of the sheath of the Rectus, and supply the muscles of the abdomen and the integument, and a small branch passes in front of the xiphoid process and anastomoses with the artery of the opposite side. It also gives some twigs to the diaphragm, while from the artery of the right side small branches extend into the falciform ligament of the liver and anastomose with the hepatic artery. 68 4. The costocervical trunk (truncus costocervicalis; superior intercostal artery) (Fig. 513) arises from the upper and back part of the subclavian artery, behind the Scalenus anterior on the right side, and medial to that muscle on the left side. Passing backward, it gives off the profunda cervicalis, and, continuing as the highest intercostal artery, descends behind the pleura in front of the necks of the first and second ribs, and anastomoses with the first aortic intercostal. As it crosses the neck of the first rib it lies medial to the anterior division of the first thoracic nerve, and lateral to the first thoracic ganglion of the sympathetic trunk. 69 In the first intercostal space, it gives off a branch which is distributed in a manner similar to the distribution of the aortic intercostals. The branch for the second intercostal space usually joins with one from the highest aortic intercostal artery. This branch is not constant, but is more commonly found on the right side; when absent, its place is supplied by an intercostal branch from the aorta. Each intercostal gives off a posterior branch which goes to the posterior vertebral muscles, and sends a small spinal branch through the corresponding intervertebral foramen to the medulla spinalis and its membranes. 70 The Profunda Cervicalis (a. cervicalis profunda; deep cervical branch) arises, in most cases, from the costocervical trunk, and is analogous to the posterior branch of an aortic intercostal artery: occasionally it is a separate branch from the subclavian artery. Passing backward, above the eighth cervical nerve and between the transverse process of the seventh cervical vertebra and the neck of the first rib, it runs up the back of the neck, between the Semispinales capitis and colli, as high as the axis vertebra, supplying these and adjacent muscles, and anastomosing with the deep division of the descending branch of the occipital, and with branches of the vertebral. It gives off a spinal twig which enters the canal through the intervertebral foramen between the seventh cervical and first thoracic vertebræ. 71

Contents -SUBJECT INDEX -BIBLIOGRAPHIC RECORD -ILLUSTRATION INDEX