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emerges from the foramen in the transverse process of the atlas, between the anterior division of the sub-occipital nerve medially and the rectus capitis lateralis laterally, and runs almost horizontally backwards and medially, round the lateral and posterior aspects of the corresponding superior articular process of the atlas. In this part of its course it enters the sub-occipital triangle, where it lies in the groove on the upper surface of the posterior arch of the atlas (sulcus arteriæ vertebralis). It is separated from the bone by the sub-occipital nerve, and is overlapped superficially by the adjacent borders of the superior and inferior oblique muscles. Finally, this

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part of the artery passes anterior to the oblique ligament of the atlas and enters the vertebral canal.

The fourth part pierces the spinal dura mater and runs upwards into the cranial cavity. It passes between the roots of the hypoglossal nerve, posteriorly, and the first dentation of the ligamentum denticulatum, anteriorly, pierces the arachnoid, and, gradually inclining to the front of the medulla oblongata, reaches the lower border of the pons, where it unites with its fellow of the opposite side to form the basilar artery

Branches.-From the first part. As a rule there are only a few small muscular twigs from this portion of the artery. From the second part.—(1) Muscular branches which vary in number and size.

Ther supply the deep muscles of the neck, and anastomose with the profunda cervicis, the ascending cervical, and the occipital arteries.

(2) Spinal branches pass from the medial side of the second part of the vertebral artery, through the intervertebral foramina, into the vertebral canal, where they give off twigs which pass along the roots of the spinal nerves to reinforce the anterior and posterior spinal arteries; they supply the bodies of the vertebrae and the intervertebral fibrocartilages, and they anastomose with corresponding arteries above and below.

From the third part.—(1) Muscular branches to the sub-occipital muscles.

(2) Anastomotic branches which unite with the descending branch (0.T. princeps cervicis) of the occipital and with the profunda cervicis artery.

From the fourth part.—(1) Meningeal.—One or two small branches given off before the vertebral artery pierces the dura mater. They ascend into the posterior fossa of the skull, where they anastomose with meningeal branches of the occipital and ascending pharyngeal arteries, and occasionally with branches of the middle meningeal artery.

(2) Posterior Spinal.-The posterior spinal branch springs most commonly from the posterior inferior cerebellar branch of the vertebral (Stopford, 1916), but occasionally it arises from the vertebral directly. It runs downwards upon the side of the medulla oblongata and the spinal medulla, either in front of or behind the posterior nerve-roots. It is a slender artery, which is continued to the lower part of the spinal medulla by means of reinforcements from the spinal branches of the vertebral and intercostal arteries. It gives off branches to the pia mater, which form more or less regular anastomoses on the medial and lateral sides of the posterior nerve-roots, and it ends by joining the anterior spinal artery.

(3) The anterior spinal branch arises near the termination of the vertebral. It runs obliquely downwards and medially, in front of the medulla oblongata, and unites with its fellow of the opposite side to form a single anterior spinal artery, which descends along the anterior median fissure of the spinal medulla, and is continued as a fine vessel along the filum terminale. The anterior spinal artery is reinforced as it descends by anastomosing twigs from the spinal branches of the vertebral, intercostal, and lumbar arteries. It gives off branches which pierce the pia mater and supply the spinal medulla, and it unites below with the posterior spinal arteries.

(4) The posterior inferior cerebellar is the largest branch of the vertebral artery. It arises a short distance below the pons and passes obliquely backwards round the medulla oblongata, at first between the fila of the hypoglossal nerve, and then between the fila of the accessory and vagus nerves, into the vallecula of the cerebellum, where it divides into lateral and medial terminal branches.

The trunk of the artery gives branches to the medulla oblongata and to the chorioid plexus of the fourth ventricle. Some of these branches supply the nuclei of the glossopharyngeal, the vagus, and the accessory nerves, the spino-thalamic, spino-cerebellar, rubrospinal, olivo-cerebellar tracts, and possibly also the vestibular root of the acoustic and the spinal root of the fifth nerve (Bury and Stopford). The medial terminal runs backwards between the inferior vermis and the hemisphere of the cerebellum; it supplies the former structure, and anastomoses with its fellow of the opposite side. The lateral branch passes to the lower surface of the hemisphere and anastomoses with the superior cerebellar artery.

Arteria Basilaris.— The basilar artery is formed by the junction of the two vertebral arteries; it commences at the lower border and terminates at the

upper border of the pons (Varolii), bifurcating at its termination into the two posterior cerebral arteries.

Course and Relations.—It runs upwards, in the median part of the cisterna pontis, in a shallow groove on the front of the pons, behind the sphenoidal ection of the basi-cranial axis and between the two abducent nerves.

Branches.-(1) Pontine, a series of small arteries which pass across the front and round the sides of the pons, supplying the pons, the brachia pontis (0.T. middle peduncles of the cerebellum), and the roots of the trigeminal nerve.

(2) The internal auditory, a pair of long slender branches. Each internal auditory branch may spring either from the basilar or from the anterior inferior cerebellar artery of the same side (Stopford, 1916). It enters the corresponding internal acoustic meatus with the facial and acoustic nerves, and, after it has passed through the lamina cribrosa, it is distributed to the internal ear.

(3) The anterior inferior cerebellar, two branches which arise, one on each side, from the middle of the basilar artery. They pass backwards, on the anterior parts of the lower surfaces of the lateral lobes of the cerebellum, and anastomose with the posterior inferior cerebellar branches of the vertebral arteries.

(4) The superior cerebellar branches, two in number, arise near the termination of the basilar. Each passes laterally, at the upper border of the pons, directly below the oculo-motor nerve of the same side, and, after turning round the lateral side of the pedunculus cerebri, below the trochlear nerve, it reaches the upper surface of the cerebellum, where it divides into a medial and a lateral branch. The medial branch supplies the upper part of the vermis, and the anterior medullary velum. The lateral branch is distributed over the upper surface of the lateral lobe ; it anastomoses with the inferior cerebellar arteries.

(5) Arteriæ Cerebri Posteriores.—The posterior cerebral arteries (Figs. 762 and 764) are the two terminal branches of the basilar. They run backwards and upwards, between the peduncles of the cerebrum and the uncinate gyri and parallel to the superior cerebellar arteries, from which they are separated by the oculomotor and trochlear nerves. Each posterior cerebral artery is connected with the internal carotid by the posterior communicating artery; it gives branches to the inferior surface of the cerebrum, and is continued backwards, beneath the splenium of the corpus callosum, to the calcarine fissure, where it divides into calcarine and parieto-occipital branches, which pass to the lateral surface of the occipital lobe. It supplies the medial and tentorial surfaces of the occipital lobe and the posterior part of its lateral surface.

Branches.—(A) Central or basal.This group includes (al) A postero-medial set of small vessels which pass, on the medial side of the corresponding cerebral peduncle, to the posterior perforated substance. They supply the peduncle, the posterior part of the thalamus, the corpora mamillaria, and the walls of the third ventricle.

(a?) A postero-lateral set of small vessels, which pass round the lateral side of the peduncle. They supply the corpora quadrigemina, the brachia, the pineal body, the peduncle, the posterior part of the thalamus, and the corpora geniculata.

(ao) A posterior chorioidal set of small branches which pass through the upper part of the chorioidal fissure; they enter the posterior part of the tela chorioidea of the third ventricle, and end in the chorioid plexus, in the body of the lateral ventricle, and the upper part of its inferior cornu. They also supply the adjacent parts of the fornix.

(B) Cortical.—(11) The anterior temporal, frequently a single branch of variable size, but not uncommonly replaced by several small branches. It supplies the anterior parts of the uncus, the hippocampal gyrus, and the fusiform gyrus.

(52) The posterior temporal is a larger branch than the anterior. It supplies the posterior part of the hippocampal gyrus, part of the fusiform gyrus, and the lingual gyrus.

(13) The calcarine branch is the continuation of the posterior cerebral artery along the calcarine fissure, it is especially associated with the supply of the visual area of the cortex of the brain. It supplies the cuneus, the lingual gyrus, and the posterior part of the lateral surface of the occipital lobe.

(54) The parieto-occipital branch, smaller than the calcarine, passes along the corresponding fissure to the cuneus and præcuneus.

Circulus Arteriosus (Willis) (Fig. 764).—The cerebral arteries of opposite sides are intimately connected together at the base of the brain by anastomosing channels. Thus, the two anterior cerebral arteries are connected with one another by the anterior communicating artery, whilst the two posterior cerebrals are in continuity through the basilar artery from which they arise. There is also a free anastomosis on each side between the carotid system of cerebral arteries and the vertebral system by means of the posterior communicating arteries, which connect the internal carotid trunks and posterior cerebral arteries.

The vessels referred to form the so-called circulus arteriosus (O.T. circle of Willis) which is situated at the base of the brain, in the interpeduncular and chiasmatic subarachnoid cisterns. It encloses the following structures : the posterior perforated substance, the corpora mamillaria, the tuber cinereum, the infundibulum, and the optic chiasma. The “circle” is irregularly polygonal in outline, and is formed posteriorly by the termination of the basilar and by the two posterior cerebral arteries, postero-laterally by the posterior communicating arteries and the internal carotids, antero-laterally by the anterior cerebral arteries, and in front by the anterior communicating artery.

It is stated that this free anastomosis equalises the flow of blood to the various parts of the cerebrum, and provides for the continuation of a regular blood-supply if one or more of the main trunks should be obstructed.

ARTERIES OF THE UPPER EXTREMITY.

The main arterial stem of each upper extremity passes through the root of the neck, traverses the axillary space, and is continued through the arm to the forearm. In the forearm its extent is short, for it terminates, opposite the neck of the radius, by bifurcating into the radial and ulnar arteries, which run through the forearm to the hand. That portion of the common trunk which lies in the root of the neck is known as the subclavian artery, the part in the axillary space is termed the axillary artery, whilst the remaining part is called the brachial artery.

ARTERIÆ SUBCLAVIÆ.

On the right side the subclavian artery (Figs. 757, 759, 761, and 766) commences at the termination of the innominate artery, posterior to the sternoclavicular articulation, whilst that on the left side arises from the arch of the aorta, behind the upper half of the manubrium sterni.

The right artery is about 75 mm. (3 inches) long; it lies in the root of the neck. The left artery is about 100 mm. (4 inches) long, and is situated not only in the root of the neck, but also in the superior mediastinal part of the thorax. In the root of the neck each artery arches laterally, across the apex of the lung and behind the anterior scalene muscle, and is divided into three parts, which lie respectively to the medial side, behind, and to the lateral side of the muscle. The extent to which the arch rises above the level of the clavicle varies; not uncommonly it reaches the level of the lower part of the thyreoid gland. The first parts of the subclavian arteries differ materially from each other both in extent and relations. The relations of the second and third parts are similar on the two sides.

The first part of the left subclavian artery springs from the arch of the aorta, posterior to the commencement of the left common carotid and on the left side of the trachea. It ascends almost vertically, in the superior mediastinum, to the root of the neck, where it arches upwards and laterally to the medial border of the scalenus anterior muscle.

Relations. Posterior.-In the superior mediastinum it is in relation with the lung and pleura.

Anterior. In front are the left vagus, the left superior cardiac branch of the sympathetic, the left inferior cardiac branch of the vagus, the left phrenic nerve, and the left common carotid artery. It is also crossed obliquely by the left vagus nerve, and it is overlapped on the left side by the left lung and pleura.

Medial.-Medially it is in relation, from below upwards, with the trachea, the left recurrent nerve, the æsophagus, and the thoracic duct, the latter lying in a plane posterior to the esophagus and the artery.

Laterally it is closely invested by the left pleura, and it ascends in a groove on the medial aspect of the left lung.

At the root of the neck, as it turns laterally, it lies behind the commencement of the left innominate vein, and the termination of the left vertebral vein, the phrenic nerve, the sterno-thyreoid and sterno-hyoid muscles, the anterior jugular vein, and, more superficially, the sterno-mastoid muscle and the deep cervical fascia ; the thoracic duct arches obliquely over it; and it lies in front of the apex of the pleural sac and lung.

The first part of the right subclavian artery (Fig. 761) extends from the back of the right sterno-clavicular articulation to the medial border of the scalenus anterior. It is thus limited to the root of the neck.

Relations. Posterior.—Behind this part of the artery, and intervening between it and the upper two thoracic vertebræ, are the recurrent nerve, the posterior part of the ansa subclavia, and the apex of the right pleural sac. Anterior.-In front it is in relation with the right vagus, the cardiac branches of the vagus and the sympathetic, the anterior portion of the ansa subclavia, the internal jugular and vertebral veins, and more superficially the sterno-hyoid and sterno-thyreoid muscles, the anterior jugular vein, the sternal end of the clavicle, the sterno-clavicular ligaments, and the sterno-mastoid muscle. The recurrent nerve passes below it and intervenes between it and the apex of the pleural sac.

The second part of the subclavian artery, on each side, extends from the medial to the lateral border of the scalenus anterior, behind which it lies.

Relations. Posteriorly and below it is in relation with the pleural sac. Anteriorly it is covered by the anterior scalene and the sterno-mastoid muscles. The anterior scalene separates it from the subclavian vein, which lies at a slightly lower level, from the transverse cervical and transverse scapular arteries, from the anterior jugular vein, and, on the right side, from the phrenic nerve.

The third part of the subclavian artery is the most superficial portion. It extends from the lateral border of the anterior scalene to the outer border of the first rib, lying partly in the clavicular portion of the posterior triangle of the neck and partly behind the clavicle and the subclavius muscle.

Relations.—It rests upon the upper surface of the first rib. Immediately posterior to it is the lowest trunk of the brachial plexus, which separates it from the middle scalene muscle. Anterior to it, and at a slightly lower level, lies the subclavian vein. The external jugular vein crosses the medial part of this portion of the artery, and just before its termination it receives the transverse cervical and transverse scapular veins; those vessels also pass superficial to the artery, which is thus covered superficially by venous trunks; it is also crossed vertically, behind the veins, by the nerve to the subclavius muscle. The lateral section of this part of the artery lies posterior to the clavicle and the subclavius muscle. It is crossed anteriorly by the transverse scapular artery, but the layer of deep cervical fascia which binds the posterior belly of the omo-hyoid to the posterior border of the subclavian groove intervenes between the two vessels. More superficially the third part of the artery is covered by the superficial layer of the deep fascia, the supra-clavicular branches of the cervical nerves, the platysma, and the skin.

BRANCHES OF THE SUBCLAVIAN ARTERY.

(1) The vertebral artery is distributed almost entirely to the head and neck and its chief function is to supply the posterior part of the brain. Its description has therefore been given with that of the other cerebral arteries (see p. 905).

(2) Truncus Thyreocervicalis.—The thyreo-cervical trunk (Figs. 757 and 759) arises close to the medial border of the scalenus anterior, from the upper and front part of the subclavian artery, directly above the origin of the internal mammary artery. After a short upward course of about 4 mm. (two lines), it ends, under cover of the internal jugular vein, by dividing into three branches-viz., the inferior thyreoid, the transverse cervical, and the transverse scapular.

(A) Arteria Thyreoidea Inferior.-The inferior thyreoid artery (Figs. 757, 759 ascends along the anterior border of the scalenus anterior, and turns medially, opposite the cricoid cartilage, to the middle of the posterior border of the corresponding lobe of the thyreoid gland; it then curves medially and downwards, and descends to the lower end of the lobe of the thyreoid gland, where it divides into ascending and inferior terminal branches.

Relations.Posterior are the vertebral artery, and the longus colli muscle ; the recurrent nerve passes either anterior or posterior to the vessel, opposite the lower border of the thyreoid gland. It is covered anteriorly by the carotid sheath, which contains the common carotid artery, the internal jugular vein, and the vagus nerve ; the middle cervical ganglion of the sympathetic lies in front of the artery as it bends medially ; and on the left side the thoracic duct also passes in front of it.

Branches. It gives off the following branches :

(a) Muscular.—Numerous small branches pass to the scalenus anterior, the longus colli, the infra-hyoid muscles, and the inferior constrictor of the pharynx.

(6) The ascending cervical usually springs from the inferior thyreoid near its origin, but it may arise separately from the thyreo-cervical trunk. It ascends, parallel with and medial to the phrenic nerve, in the angle between the longus capitis and the scalenus anterior, to both of which it gives branches. It also gives off spinal branches which pass through the intervertebral foramina to the vertebral canal. It anastomoses with branches of the vertebral, occipital, ascending pharyngeal, and deep cervical arteries.

(c) Esophageal. – Small branches given to the walls of the esophagus, which anastomose with the esophageal branches of the thoracic aorta.

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