The three large branches which arise from the first part of the subclavian artery spring from the parent trunk very close to each other, and are deeply seated at their origin under cover of the internal jugular vein. They proceed, however, from different sides of the parent vessel, pursue different directions, and are distributed to remotely separate parts.

Thus, one (vertebral) springs from the upper and back part of the subclavian, and ascends in the neck to reach the interior of the skull ; another (internal mammary) proceeds from the lower or under side of the vessel, and ascends into the fore part of the chest and abdomen ; whilst the third (thyroid axis) arises from the fore part of the artery, and divides into three branches, one of which (inferior thyroid) is distributed in the fore part of the neck, whilst the other two (suprascapular and transverse Cervical) pass outwards across the neck to the shoulder. Lastly, from the second part of the right subclavian of the subclavian artery, two other branches arise by a common trunk ; of which one (deep cervical) passes upwards behind the neck, and the other (superior intercostal) descends into the back part of the thorax.

Vertebral artery

The vertebral artery [vertebralis,] which is usually the first and largest branch of the subclavian, passes upwards through the foramina in the transverse processes of the cervical vertebrae; and after a winding course enters the skull through the foramen magnum, and terminates in front of the medulla oblongata, by uniting with the corresponding vessel of the opposite side, to form the basilar artery. The singular course of this vessel, thus generally indicated, requires a more precise examination.

Arising from the upper and back part of the subclavian, the vertebral artery passes upwards and a little backwards, and enters the foramen in the base of the transverse process of the sixth cervical vertebra— not infrequently of some higher vertebra. The vessel then ascends in a vertical direction along the canal formed by the series of foramina in the transverse processes of the vertebras, as far as to the upper border of the axis; here, in consequence of the greater width of the atlas, it inclines outwards to the foramen in the root of the transverse process of that bone. Having passed through this foramen, the artery alters its direction and winds backwards behind the articulating process of the atlas, resting in the shallow groove on the posterior arch of the vertebra. After piercing the ligament (posterior occipito-atloidean) connecting the atlas to the occiput, and also the dura mater, it enters the skull through the foramen magnum of the occipital bone, and finally, proceeding upwards and forwards, turns round the side of the medulla oblongata, and converging towards the corresponding vessel of the opposite side, unites with it opposite the lower border of the pons Varolii, to form the basilar artery.

As it is directed upwards from its origin towards the spine, the vertebral artery lies behind the internal jugular vein, and on approaching the vertebrae passes between the longus colli and the scalenus anticus muscle. On the left side, the thoracic duct crosses in front of the vessel from within outwards.

Whilst within the canal formed by the foramina of the cervical vertebras, it is accompanied by a plexus of the sympathetic nerves and by the vertebral vein, which is before it. Here the artery is placed between the intertransverse muscle in the intervals between the bones, and is crossed behind by the cervical nerves as they emerge from the intervertebral foramina. The first and second nerves have peculiar positions in relation to this artery, an account of which will be found in the description of these nerves.

In the interval between the atlas and the occipital bone the vertebral artery is covered by the superior oblique, complexus, and rectus posticus major muscles. Within the skull, it turns round the side of the medulla oblongata between the origin of the ninth cranial nerve and the anterior root of the suboccipital, and then lies between the anterior surface of the medulla and the basilar process of the occipital bone.

Peculiarities of the vertebral artery

There is no recorded instance of the origin of this vessel being transferred from the first part of the subclavian artery to the parts situated beneath and beyond the scalenus muscle.

On the right side, the origin of the vertebral artery in some cases approaches more nearly to the commencement of the subclavian than usual. The right vertebral has also been seen to arise from the common carotid of the same side, and in the examples of this peculiarity which have been observed, the right subclavian artery was given as a separate vessel from the aorta. The change in the subclavian artery is, however, far from being necessarily, or even generally, accompanied by an alteration m the place of origin of the vertebral. Lastly, the rijdit vertebral artery has been observed, but only in two instances, to be a branch of the aorta.

The left vertebral artery is not infrequently derived from the aorta, in which case it generally arises between the left carotid and subclavian arteries, but sometimes it is the last of the branches from the arch.

The left vertebral artery, in a few instances, and the right vertebral in one, have been found to arise by more than a single root. In most of these cases there were two roots, both of which proceeded from the subclavian artery, or one from that vessel and one from the aorta. Whatever their mode of origin, these two parts unite to form a single vessel, either before they enter the canal in the vertebrae, or after one of the vessels has passed through the transverse process of one or two vertebrae. An example of three roots to a vertebral artery has been placed on record.

Instead of entering the canal formed by the foramina in the transverse processes of the cervical vertebrae, through the foramen in the transverse process of the sixth vertebra, the vertebral artery of one side not infrequently enters this canal higher up — through the foramen of the fifth, fourth, or third vertebra, — or even, according to several anatomists, of the second. On the other hand, the vertebral artery has been seen to enter the foramen in the transverse process of the seventh vertebra.

One of the vertebral arteries often exceeds the other in size. The left is said to be the larger vessel more frequently than the right.

Branches of the vertebral artery

The branches of the vertebral artery are numerous. Some small ones are given off in the neck, and others of greater size within the skull.

Cervical branches

In the neck, the vertebral, artery sends off at different points of its course several small branches named spinal arteries. Each of these entering the spinal canal through an intervertebral foramen divides into two branches, of which one passes along the roots of the spinal nerve, assists in supplying the spinal cord and its membranes, and anastomoses with the other spinal arteries ; whilst the other branch ramifies on the back part of the bodies of the vertebrae in the same manner as similar branches derived from the intercostal and lumbar arteries, which will be hereafter described. The vertebral artery also gives muscular branches of variable size to the deep- seated cervical muscles.

Cranial branches

The posterior meningeal [a. meningea post, int.] is a small branch which arises opposite to the foramen magnum, and ramifies between the dura mater and the bone in the occipital fossa and upon the falx cerebelli. There are sometimes two of these small vessels.

The posterior spinal, [a. spinalis post.], arising at an obtuse angle from the vertebral, inclines backwards round the medulla oblongata to reach the back part of the spinal cord, along which, aided by reinforcements from small arteries which ascend upon the cervical and dorsal nerves through the intervertebral foramina, it may be traced, lying behind the roots of the nerves, as a minute tortuous vessel, or rather a series of little inosculating vessels, as far as the second lumbar vertebra, where it terminates in ramifications on the cauda equina. Some of its branches run upon the cord around the roots of the nerves, whilst others maintain frequent transverse communications with similar branches from the vessel of the opposite side.

The anterior spinal artery, n, [a. spinalis ant.] somewhat larger than the preceding, arises near the end of the vertebral artery, and descends obliquely in front of the medulla oblongata. Immediately below the foramen magnum, it unites with the corresponding vessel of the opposite side, so as to form a single trunk, which descends a short distance only along the middle line in front of the spinal cord, forming the upper part or commencement of the anterior median artery of the cord. This anterior spinal branch of the vertebral artery supplies therefore only the upper part of the cord, the greater part being provided with a series of small arteries, which are derived in the neck from the vertebral and inferior thyroid arteries, in the back from the intercostal, and below this from the lumbar, ilio- lumbar, and lateral sacral arteries. These small vessels enter the spinal canal at irregular intervals through the intervertebral foramina, and reaching the anterior surface of the cord by passing along the roots of the nerves, communicate with each other along the middle line by means of ascending and descending branches; so that, by a succession of anastomoses, a very slender single vessel, but of varying thickness, named the anterior median artery, appears to extend from one end to the other of the cord. This vessel, or chain of inosculating vessels, is placed beneath the pia mater along the anterior median fissure of the spinal cord. Its numerous branches supply the pia mater and the substance of the cord — some entering its anterior median fissure. At the lower end of the cord it sends branches downwards on the cauda equina. On a part of the spinal cord near the lower end, and in front of the posterior roots of the nerves, may be found another small artery, about equal in size to the anterior spinal.

The inferior cerebellar artery [a. cerebelli inf.] (profunda cerebelli, — Haller), the largest of the branches of the vertebral, arises near the pons, and sometimes from the basilar artery : it turns backwards and outwards, between the hypoglossal and pneumogastric nerves, over the restiform body, and near the side of the opening of the fourth ventricle to reach the under surface of the cerebellum. Here, running backwards between the inferior vermiform process and the hemisphere, it divides into two branches, one of which continues backwards to the notch between the two hemispheres of the cerebellum ; whilst the other, turning outwards, ramifies on the under surface of the cerebellum as far as its outer border, over which the ultimate divisions of each branch anastomose with those of the superior cerebellar arteries. This artery partly supplies the hemisphere and the vermiform process, and gives branches to the choroid plexus of the fourth ventricle.

The basilar artery, [a. basilaris] the single trunk, formed by the junction of the right and left vertebralis in the middle line, is so named from its lying on the basilar process of the occipital bone. It extends from the posterior to the anterior border of the pons Varolii, beneath the median groove of which it lies, under cover uf the arachnoid. The length of this artery is therefore about equal to the breadth of the pons, at the anterior border of which it divides into two terminal branches, the posterior arteries of the cerebrum.

The basilar artery supplies many small branches to the substance of the pons. On each side it also gives several transverse branches, one of which accompanies the acoustic nerve into the internal auditory meatus and labyrinth of the ear, and another of more considerable size, the anterior cerebellar artery (cerebelli inferior anterior), which passes backwards along the fore part of the eras cerebelli to the anterior part of the under surface of the cerebellum.

The superior cerebellar artery, [a. cerebelli sup.] — The superior arteries of the cerebellum arise so close to the bifurcation of the basilar, that this artery is described by several anatomists as dividing into four branches. Each one turns backwards and outwards immediately behind the third nerve, and entering the groove between the pons Varolii and the eras cerebri, turns round the latter lose to the fourth nerve, to reach the upper surface of the cerebellum, where it divides into branches. Of these some extend outwards, and one or more backwards along the superior vermiform process, to reach the circumference" of the cerebellum, where they anastomose with the branches of the inferior cerebellar arteries : other branches run inwards to supply the vermiform process and valve of Vieussens, and in part the velum interpositum — an extension of the pia mater into the interior of the brain. The posterior cerebral artery, p, (posterior aut profunda cerebri,— Haller), is larger than the preceding vessel, and is separated from it at its origin by the third nerve, which comes forward between the two vessels. It turns backwards round the eras cerebri, at first parallel with the last-named vessel, and then runs out- wards and upwards on the under surface of the posterior lobe of the cerebrum passing near the posterior extremity of the corpus callosum. It divides beneath the posterior lobe into many branches, which ramify upon its under, inner or median, and outer surfaces, and anastomose with the other cerebral arteries. Immediately after its origin, the posterior cerebral artery gives off numerous small parallel branches, which perforate the substance of the brain between its crura, at the point which is called from this circumstance the posterior perforated spot (locus perforatus). As it turns backwards, a short distance from its origin, this artery is joined by the posterior communicating artery, or communicating artery of Willis, a branch of the internal carotid, and in this way contributes, as already described, to form the circle of Willis. Lastly, the posterior cerebral gives origin to a small branch, the posterior choroid (choroidea posterior), which arises from it soon after its junction with the communicating artery, turns backwards over the crus cerebri and the tubercula quadrigemina, supplying these with branches, and ending in the velum interpositum and choroid plexus, which enter the interior of the brain beneath the corpus callosum. Peculiarities of the branches of the vertebral artery. — In the neck, the vertebral artery has been found, though very rarely, to give branches which are usually derived from the subclavian, as the superior intercostal and the inferior thyroid. In two instances, the septum formed by the juxtaposition of the vertebral arteries behind the basilar, has been observed by Dr. Davy to be perforated by an opening as large as a probe. Not unfrequendy the interior of the basilar artery itself is traversed by a fibrous band, which is attached to the sides of the vessel. This band varies in situation and dimensions, and is considered by Dr. Davy to be congenital, and not the result of disease. In one instance the basilar artery was joined by a large branch of the internal carotid. The posterior cerebral artery is occasionally given off on one side from the internal carotid artery. Sometimes a small " aberrant" branch is connected with the side of the basilar. Lastly, the basilar artery has been found to be perforated by a small foramen, owing to a partial Assuring of the vessel, along the median line.

Thyroid axis

Thyroid axis (inferior thyroid artery: thvreoidea inferior arteria — Haller). — The name " axis" is applied to this artery because immediately after its origin it divides into branches, which diverge in different directions, viz. the inferior or ascending thyroid, the transverse cervical, and the suprascapular. The thyroid axis springs from the fore part of the subclavian artery close to the inner side of the anterior scalenus muscle. It is a short thick trunk, and usually divides a line or two from its origin into the branches above named.

Peculiarities of the thyroid axis

Exceptions to the regular place of origin of the thyroid axis at the inner side of the scalenus muscle are very rare. The vessel has, in two cases, been found to arise beyond that muscle. Sometimes the thyroid axis is associated at its origin with another branch. Thus, it gave origin to the internal mammary in about one case in twenty ; and once only, in nearly three hundred cases, to the vertebral; once to the superior intercostal, and once to the profunda cervicis. The deviations from the ordinary arrangement of the branches of the thyroid artery, will be examined under each branch. The inferior thyroid artery 7 (ramus thyreoideae thyreoideus, — Haller.) — This name is by most authorities applied to the common trunk here recognised as the axis. The artery passes directly upwards resting on the longus colli muscle, and, after a short course, bends inwards and downwards, behind the sheath of the large cervical vessels, and also behind the sympathetic nerve (the middle cervical ganglion of which, when present, often rests upon the vessel). The artery now makes another curve in the opposite direction to the former one, and is distributed to the under surface of the thyroid body. Its branches com- municate freely with those of the superior thyroid artery (an offset from the external carotid), and with the corresponding artery of the other side. The inferior thyroid artery usually supplies a laryngeal branch of irregular size, which ascends on the trachea to the back of the larynx, and is distributed to the muscles and mucous membrane in that situation. It also gives off tracheal branches which ramify over the trachea, upon which they anastomose below with the bronchial arteries. Other small branches are supplied to the oesophagus, and one or more descend upon the trachea into the chest. Peculiarities. — The inferior thyroid artery occasionally arises as an independent branch — generally from the subclavian artery, and rarely from the common carotid or the vertebral. This artery is often smaller than usual, or it may be entirely wanting on one or both sides, — the deficiencies being generally compensated for by an enlargement of one or both superior thyroid arteries. On the contrary, one or both inferior thyroid arteries are sometimes larger than usual when the superior arteries are small. Instances have occurred — very rarely, however, — of the presence of two inferior thyroid arteries, one passing over the common carotid artery. In this place may be noticed, in connection with the peculiarities of the inferior thyroid arteries, that there is sometimes found an additional artery for the thyroid body, which has been named the lowest thyroid artery (thyroidea ima, of Neubauer and Erdmann). This artery usually arises from the innominate trunk, but in rare instances has been observed to come from the right common carotid artery, or from the aorta itself. It is of very different size in different cases, and compensates in various degrees for deficiencies or absence of the other thyroid arteries. This unusual thyroid artery ascends to its destination in front of the trachea, and its existence would therefore complicate the operation of tracheotomy. The ascending cervical artery (ramus thyreoids adscendens, — Haller ;) [a. cervicalis ascendens]. At the point where the inferior thyroid, changing its direction, turns inwards behind the carotid artery, a small branch thus named arises from it and proceeds upwards, close to the phrenic nerve, on the line of separation between the scalenus anticus and rectus anticus major, giving branches (muscular) to these muscles, and a few whieh pass transversely outwards across the neck. These muscular branches communicate with others sent outwards from the vertebral artery. To the spinal canal the ascending cervical artery sends one or two branches (spinal branches), which enter the intervertebral foramina along the cer- vical nerves, and assist in supplying the bodies of the vertebrae, the spinal cord, and its membranes. Peculiarities. — The ascending cervical artery is occasionally derived from the subclavian artery or from one of the branches of that vessel, as from the transverse cervical, or the suprascapular, or from the trunk common to those two arteries. It is sometimes much larger than usual, and takes the place of the occipital artery, a branch of the external carotid. A branch of the ascending cervical not infrequently compensates for a small profunda cervicis artery.

Suprascapular artery

The suprascapular artery, 12 (ramus transversus scapularis, — Haller), [a. transversa scapulae] a smaller vessel than the succeeding branch, the transverse cervical, arises almost constantly from the thyroid axis, and runs from within outwards deeply at the root of the neck. At first it descends obliquely towards the clavicle, resting upon the scalenus anticus, and covered by the sterno-mastoid muscle ; then crosses the subclavian artery, and continues transversely outwards behind and parallel with the clavicle and subclavius muscle, and below the pos- terior belly of the omo-hyoid muscle. At the outer side of the neck, this artery approaches the upper margin of the scapula; and here, under cover of the trapezius muscle, it runs downwards with the suprascapular nerve towards the notch at the root of the coracoid process in the upper border of the scapula. At this point the nerve usually passes beneath the ligament stretched across the notch, whilst the artery turns over it to enter the supraspinous fossa ; where, lying close to the bone, it gives off branches which ramify in the supraspi- nous fossa, beneath the supraspinatus muscle, and sends a small communicating branch into the subscapular fossa. Peculiarities. — The suprascapular artery has in some cases been observed to spring directly from the subclavian, or to arise from that vessel by a common trunk with the transverse cervical, or more rarely with the internal mammary. It has been even found to proceed from the axillary artery, and from the sub- scapular branch of that vessel. Branches. — At the root of the neck, whilst under cover of the sterno-mastoid, the suprascapular artery gives off a small branch which runs inwards through the attachment of that muscle, and sup- plies it. In its course across the neck, the artery sends small unnamed branches to the adjacent muscles. It also gives a supra-acromial branch, which passes obliquely downwards through the attachment of the trapezius to reach the cutaneous surface of the acromion, on which it ramifies, anastomosing with offsets from the acromial thoracic artery. As the artery passes over the notch of the scapula, a small branch arises from it, and turns forwards over the neck of the bone to enter the subscapular fossa, where it ramifies beneath the subscapular muscle, and anastomoses with the posterior scapular and subscapular arteries. After having given off this branch, the artery enters the supraspinous fossa and ramifies between the bone and the supraspinatus muscle, to which it is chiefly distributed. The suprascapular artery also supplies branches to the bone and to the shoulder-joint; and sends downwards a communicating branch, which descends close upon the neck of the scapula, .between the glenoid cavity and the spine of that bone and beneath the acromion process, in order to reach the infraspinous fossa, and join with the dorsal branch of the subscapular artery.

Transverse cervical artery

The transverse cervical artery, [a. transversa colli,] the third branch of the thyroid axis, passes outwards a short distance above the clavicle, and therefore higher than the suprascapular artery. It crosses over the scaleni muscles and the brachial nerves, — sometimes passing through the divisions of the latter. Beneath the anterior margin of the trapezius, and near the outer edge of the levator anguli scapulas, it divides into two branches, the superficial cervical, and the posterior scapular. The transverse cervical artery lies deeply in its entire course ; being covered by (besides the platysma and the fascia) the sterno-mastoid, the omo-hyoid, and the trapezius muscles. Its terminal branches are the two following : — The superficial cervical (superficialis cervicis) ascends beneath the anterior border of the trapezius, and distributes branches to the trapezius, levator anguli scapulae, and sterno-mastoid muscles, as well as to the cervical glands and the integuments in the interval between those muscles. The descending or posterior scapular branch, [a. posterior scapulae,] fig. 243, b, which may be considered the continuation of the transverse cervical, passes backwards to the posterior angle of the scapula under cover of the levator anguli scapulae, and then changing its direction, runs downwards beneath the rhomboidei muscles and the base of the scapula, as far as the inferior angle of that bone. It anastomoses freely on both sides of the scapula with the divisions of the supra-scapular and the subscapular arteries ; and supplies branches to the rhomboidei, serratus magnus, and latissimus dorsi ; communicating at the same time with the posterior muscular branches of some of the intercostal arteries.

Peculiarities in the transverse cervical artery, and its branches

The frequent varieties which occur in the arrangement of the transverse cervical artery and its two branches, have occasioned them to be very differently described and named by different anatomists. The condition above noticed is that most commonly met with, — viz., that the transverse cervical artery is the third branch given off by the thyroid axis, and divides near the levator anguli scapular into the superficial cervical and the posterior scapular arteries. In a number of cases, however, so great as to have been regarded by some as representing the ordinary condition, the superficial cervical portion of the artery only is derived from the thyroid axis, whilst the posterior scapular arises as a separate vessel from the subclavian artery, most commonly beyond the scalenus muscle. In a third class of cases, not nearly so common, the vessel derived from the thyroid axis is very small, and represents only in part the superficial cervical artery ; whilst a large vessel arising from the third part of the subclavian divides near the levator anguli scapulae into two branches, of which one ascends and represents the remaining and larger portion of the superficial cervical artery, whilst the other forms the posterior scapular. The transverse cervical artery is sometimes derived directly from the subclavian, or it arises from that vessel in common with the suprascapular, or occasionally with that and the internal mammary artery also. When the transverse cervical artery arises separately from the subclavian artery, its place of origin may be beneath or even beyond that muscle. The transverse cervical artery sometimes gives off the ascending cervical. When the superficial cervical is separated from the posterior scapular, it some- times arises from other sources than the thyroid axis, as from the suprascapular, or the subclavian. The posterior scapular was observed, when derived from the subclavian as a separate branch, (a very common arrangement, it will be remembered,) to take its origin beyond the scalenus in more than two-thirds of a considerable number of cases, and, in less than one-third, beneath that muscle ; in one case only did it arise to the inner side of the muscle.

Internal mammary artery

The internal mammary artery, [a. mammaria interna], remarkable for its length and the number of its branches, arises from the under side of the subclavian, opposite to the thyroid axis. It runs forwards and downwards behind the clavicle, to reach the inner surface of the cartilage of the first rib, lying between it and the sac of the pleura ; from this point it inclines a little inwards for a .short space, and then descends vertically behind the costal cartilages, a short distance from the border of the sternum, as far as to the interval between the sixth and seventh costal cartilages, where the internal mammary artery is considered to end, by dividing into two branches. One of the branches into which the artery divides (musculo-phrenic) inclines outwards, along the margin of the thorax ; whilst the other, under the name superior epigastric, continues in the original direction of the trunk, onwards to the abdomen. Covered at its origin, like the other large branches of the subclavian artery, by the internal jugular vein, the internal mammary soon passes behind the subclavian vein, and is crossed in front by the phrenic nerve, which is between the vein and the artery.* In the chest it has at first the costal cartilages and the internal intercostal muscles before, and the pleura behind ; but lower down it lies between the cartilages and the triangularis sterni muscle — the muscle separating the vessel from the pleura. — This artery has two companion veins, which, however, are united into a single venous trunk at the upper part of the chest. Peculiarities. — The internal mammary is occasionally found connected at its origin with the thyroid axis, or with the scapular arteries — these being detached from the thyroid. It occasionally springs from the second or third part of the subclavian artery (the latter being the more frequent position of the two). The internal mammary is very rarely transferred away from the subclavian to another artery. The axillary, the innominate, and the aorta, have been found to give origin to it. Of each of the last two changes but a single example has been recorded.

Branches of the internal mammary artery

The branches of this long artery are numerous, and are chiefly distributed to the walls of the chest and abdomen. The superior phrenic or comes nervi phrenici, a very slender branch, arises high in the chest, and descends with the phrenic nerve (as its name implies), between the pleura and the pericardium, to the diaphragm, in which it is distributed, anastomosing with the other phrenic branches derived from the musculo-phrenic division of the internal mammary artery, and with the inferior phrenic arteries which come from the abdominal aorta. The mediastinal or thymic branches, [a. mediastinicse s. fhymicse ; ] of very small size, ramify in the cellular tissue of the anterior mediastinum, and supply the remains of the thymus body, which, when in full development, receives its principal branches from the internal mammary artery. Pericardiac branches are given off directly to the upper part of the pericardium, the lower part of which receives some from the musculo-phrenic division. Branches, named sternal, are also supplied to the triangularis sterni muscle and to both surfaces of the sternum. The anterior intercostal arteries, [a. intercostales ant.] two in each space, arise from the internal mammary, either separately or by a trunk common to the two, which soon divides. The arteries pass outwards, at first between the pleura and the internal intercostal muscles, and afterwards between the two layers of intercostal muscles ; they lie one near the upper, and one near the lower rib in each of the upper five or six intercostal spaces, and inosculate with the corresponding intercostal branches derived from the aortic intercostals. These branches supply the intercostal and pectoral muscles, and give some offsets to the mamma and integument. The anterior or perforating branches, [rami perforantes] pass forwards from the internal mammary artery through from four to six intercostal spaces, and turning outwards ramify partly in the pectoralis major, and partly in the integument on the front of the chest. Some of these perforating branches (those placed nearest to the organ) supply the mammary gland, and in the female they are of compara- tively large size, especially during lactation ; and some offsets ramify on the sternum and on the articulations of that bone with the cartilages of the ribs. The musculo-phrenic artery, [a. musculo-phrenica] the outer of the two branches into which the internal mammary artery divides, inclines downwards and outwards behind the cartilages of the false ribs, perforating the attachment of the diaphragm at the eigth or ninth rib, and becoming gradually reduced in size as it reaches the last intercostal space. It gives branches backwards into the diaphragm ; others, which pass outwards to form the anterior intercostals of each space, and are disposed precisely like those which are derived higher up from the internal mammary itself; and some which descend into the abdominal muscles.

Each of these sets of branches anastomose with those derived from other sources and distributed to the same parts. The superior epigastric artery, [a. epigastrica sup.] the abdominal division of the internal mammary, continues in the direction of that artery, and descends be- hind the seventh costal cartilage to gain the wall of the abdomen, in which it lies behind the rectus, between the muscle and its sheath. From this artery, branches are furnished to the upper part of the rectus, and anastomose with the ascending ramifications of the epigastric artery, a branch of the external iliac. Small vessels are likewise supplied to the broad muscles of the belly, and to the skin ;. some are distributed to the diaphragm, and one runs forwards upon the side and front of the xiphoid cartilage, where it anastomoses with that of the opposite side. Unusual branches. — The internal mammary artery occasionally gives origin to the suprascapular; or furnishes a bronchial artery. An unusual branch has also been observed descending vertically from the artery at the side of the thorax, and crossing the middle of a few of the ribs, on their inner surface, in contact with the pleura.

Superior intercostal artery

The superior intercostal artery [a. intercostalis sup. s. truncus costo-cervicalis] generally arises from the upper and back part of the subclavian, under the anterior scalenus muscle on the right side, and immediately at the inner side of the muscle on the left side. Taking its course backwards, it speedily gives oft' the deep cervical branch (profunda cervicis), and bends backwards and downwards in front of the neck of the first, or first two ribs, and ends in the first or second intercostal space. On the neck of the first rib, the artery is situated on the outer side of the first dorsal ganglion of the sympathetic nerve. In the first intercostal space the superior intercostal artery gives an intercostal branch similar in course and distribution to the aortic intercostals; in the second intercostal space, the branch usually joins with one from the first aortic intercostal. The intercostal vessel sends backwards a small offset to the posterior spinal muscles, and also a small one through the corresponding intervertebral foramen to the spinal cord and its membranes. Peculiarities. — The place of origin of the superior intercostal artery is occa- sionally moved to the inner side of the scalenus anticus on the right side. At the left side it has that position in a majority of cases; but is never, as far as our observation extends, moved in the opposite direction — to the outer side of the muscle. It has been found, very rarely, however, to proceed from the vertebral artery, or from the thyroid axis. In a few instances the intercostal artery was observed to pass between the necks of one or two ribs and the corresponding transverse processes of the dorsal vertebra : and in one case, after arising from the vertebral artery, it descended through the foramen in the transverse process of the last cervical vertebra, and then continued, as in the cases just mentioned, between the necks of the ribs and the contiguous transverse processes of the vertebrae of the back. This intercostal artery is sometimes, though very rarely, wanting.

Deep cervical artery

The deep cervical branch 10 (profunda cervicis) often described as a separate branch from the subclavian, arises in most cases (13 out of 14) from the superior intercostal. Resembling the posterior branch of an aortic intercostal artery, it generally passes backwards in the interval between the transverse process of the last cervical vertebra and the first rib, to reach the posterior aspect of the neck. Here it ascends in the interval between the transverse and spinous processes, as high as the second vertebra, under cover of the complexus muscle, between it and the semi-spinalis colli. To these and other contiguous muscles it furnishes ramifications. Some of the branches communicate with those given outwards by the vertebral artery, whilst others ascend to anastomose with the cervical branch (princeps cervicis) of the occipital artery. Peculiarities. — The deep cervical artery occasionally takes its course backwards between the last two cervical vertebrae. In several instances among a large number in which the condition of this artery was noted, it was observed to arise from the intercostal within the thorax, and to proceed backwards below the first rib and the transverse process supporting it ; and even, but with much less frequency, below the second rib and transverse process. It has likewise been seen between the rib and transverse process in passing backwards. The place of origin is occasionally changed. In one case in twenty the deep cervical arose from the subclavian, either beneath the anterior scalenus muscle or at its inner side; and, in a small number of cases, the posterior scapular artery was the source from which it was derived.

The deep, artery of the neck is not infrequently small, the deficiency being compensated for by an additional branch. In most cases, this supplementary artery was observed to take origin from the ascending cervical (a branch of the inferior thyroid) which turned backwards beneath the transverse process of the third cervical vertebra, and supplied the defect at the upper part of the neck. This additional artery has likewise been seen to arise from the superior intercostal (as well as the ordinary profunda); and more rarely from the posterior scapular, or the inferior thyroid.

From Quain's Anatomy.