Part 27

=A. basilaris= (_c_).--This arises by the union of the two vertebral arteries in the manner just described. It passes craniad along the ventral middle line of the medulla and pons, giving numerous small branches to these structures. A large branch, =A. cerebelli inferior posterior= (_d_), passes on each side to the caudal surface of the cerebellum, on which it ramifies. At the cranial margin of the pons the basilar artery divides. From each division a very large branch passes on each side across the pedunculi cerebri to the cranial part of the cerebellum; this is =A. cerebelli anterior= (_e_). Just craniad of this, arising from nearly the same point, the smaller =A. cerebri posterior= (_f_) passes laterad to the caudal part of the cerebrum; it is joined by the internal carotid artery (_g_). The small continuations of the two halves of =A. basilaris= then pass craniad at the side of the hypophysis to join the posterior communicating branches from the carotid plexus. As other branches of the carotid plexus unite across the middle line craniad of the optic chiasma, an arterial circle is formed on the base of the brain, surrounding the hypophysis and the optic chiasma. This is known as the =circulus arteriosus=, or circle of Willis (see page 289).

_b._ =A. mammaria interna= (Fig. 118, _h_).--The internal mammary artery rises from the ventral surface of the subclavian opposite the first rib, and passes in the mediastinum ventrocaudad to reach the sternum opposite the third intercostal space. It sends two or three small branches craniad to the midventral part of the thoracic wall and then extends caudad at the side of the sternum. It sends off lateral branches to the ventral thoracic wall, branches to the mediastinum and pericardium, a branch to the diaphragm which anastomoses with the phrenic; and finally it passes out of the thoracic cavity caudad of the last costal cartilage, extends caudad in a zigzag course at the lateral border of the rectus muscle and anastomoses with the inferior epigastric artery.

_c._ =Truncus costocervicalis= (Fig. 118, _g_).--The costocervical axis arises from the subclavian opposite the first rib, passes craniodorsad, and divides almost at once into two branches. The smaller one of these, the =superior intercostal= (=A. intercostalis suprema=), passes caudad and, dividing, supplies the first and second intercostal spaces, and then passes to the deep muscles of the back. The other branch divides almost immediately into two. One of these, =A. transversa colli=, passes laterad in front of the first rib and enters the serratus anterior muscle. In this it passes dorsad, giving off branches to this muscle and to the levator scapulæ, till it reaches M. rhomboideus, which it likewise supplies. The other branch (=A. cervicalis profunda=), which seems to form a continuation of the main artery, passes directly dorsad and leaves the thoracic cavity between the heads of the first and second ribs. Here it passes into the deep muscles of the neck; it can be traced in the substance of the complexus muscle as far forward as the atlas.

_d._ =Truncus thyrocervicalis= (Fig. 118, _j_).--The thyrocervical axis (or thyroid axis) arises from the subclavian beneath the first rib, a short distance distad of the origin of the costocervical axis. It passes laterocraniad and dorsad, lying on the mediocranial side of the brachial plexus. A short distance from its origin it gives off a branch, varying much in size, the =cervicalis ascendens= (Fig. 119, _d_), which passes craniad on the ventral side of the neck, supplying the sternomastoid, sternohyoid, the cervical portion of the scalenus, and sometimes other muscles of the neck region. Other branches pass from the thyroid axis to the inner surface of the clavotrapezius (Fig. 122, _a_) and to the ventral end of the pectoral muscles. At the level of the cranial border of the scapula the artery takes the name =A. transversa scapulæ=, or =suprascapularis=. This divides into three main branches. The first branch, sometimes large, sometimes small, passes to the lymphatic gland in the hollow of the shoulder, and to the adjacent muscles; it supplies the clavotrapezius, acromiotrapezius, levator scapulæ ventralis, splenius, occipitoscapularis, and rhomboideus. A second branch passes between the subscapularis and supraspinatus muscles, dividing into various branches which supply the muscles named. The third branch pierces the supraspinatus muscle, just craniad of the acromion process, and ramifies in that muscle.

_e._ =A. axillaris= (Fig. 122, _g_).--The axillary artery is the continuation of the subclavian laterad of the first rib. It lies caudad of the brachial plexus and parallel to it. It sends off the following branches:

1. =A. thoracica anterior= (Fig. 122, _h_).--The anterior thoracic is a slender artery which leaves the ventral side of the axillary opposite the first rib and passes caudomediad, to supply the medial ends of the pectoral muscles (_g_).

2. =A. thoracica longa= (_p_).--The long thoracic artery is larger than the preceding, leaves the axillary a short distance laterad of it, and passes caudad to the middle portions of the pectoral muscles (8) and continues to the inner surface of the latissimus dorsi.

A short distance beyond the long thoracic the axillary divides into two. The more cranial one of these is the =subscapular= (_l_); the other is the =brachial= (_g_′).

3. =A. subscapularis= (_l_).--The subscapular artery passes laterad and gives off a short distance from its origin the =A. thoracicodorsalis= (_t_) and =A. circumflexa humeri posterior= (see below); it may also give rise to the =circumflexa anterior humeri= (_l_′), and to the =profunda brachii= (_u_); these two branches, however, rise more frequently from the brachial artery (_g_′) and are described in connection with it. The subscapular artery then passes through the triangular interval between the scapular end of the long head of the triceps, the latissimus dorsi, and the glenoid border of the scapula. Within this interval it sends muscular branches to the long head of the triceps, the subscapularis, and the latissimus dorsi. That to the latter muscle is very large. At the border of the scapula, opposite the tuberosity of the spine, it turns craniad, passes over the lateral surface of the infraspinatus muscle, supplying it; crosses the spine, and sends branches into the supraspinatus fossa in both directions parallel to the scapular spine. These supply the supraspinatus, acromiotrapezius, and spinotrapezius, and anastomose with the branches of the transversa scapulæ. As the subscapularis turns craniad onto the surface of the infraspinatus it sends dorsad a small branch, the =circumflexa scapulæ=, which passes in the infraspinatus fossa close to its glenoid border as far as the glenovertebral angle, supplying the infraspinatus and latissimus dorsi by lateral branches.

=A. thoracicodorsalis= (_t_).--This arises from the subscapular a short distance from its origin and passes across the teres major to the latissimus dorsi (7), giving branches to both these muscles and to the epitrochlearis.

=A. circumflexa humeri posterior.=--The posterior circumflex artery arises from the A. subscapularis close to the origin of the latter. It passes between the subscapularis muscle and the teres major, close to the border of the biceps, then between the lateral and long heads of the triceps. It gives off a branch to the inner surface of the spinodeltoid and acromiodeltoid, then passes distad to supply the lateral and long heads of the triceps.

4. =A. brachialis= (_g_′).--The brachial artery is the continuation of the axillary (_g_) into the arm beyond the origin of the subscapular (_l_). It passes along that side of the biceps which lies next to the humerus. It passes thus, accompanied by the brachial vein (_f_′) and median and ulnar nerves, through the bicipital arch and afterwards between the biceps (4) and the intermediate division of the medial head of the triceps to the supracondyloid foramen of the humerus, through which it passes with the median nerve to reach the concavity of the elbow. It gives off the following branches:

_a._ =A. circumflexa humeri anterior= (_l_′).--The anterior circumflex artery usually leaves the brachial near its origin (but may arise from the subscapular (_l_) or one of its branches); it passes to the biceps (4) near the origin of the latter and sends a branch proximad to the head of the humerus.

_b._ =A. profunda brachii= or superior profunda (_u_).--This arises from the first part of the brachial or it may come off from one of the branches of the axillary (e.g., the subscapularis). It passes along with the radial nerve onto the dorsal side of the humerus and supplies the triceps muscle. It also sends branches to the epitrochlearis (6) and latissimus dorsi (7).

_c._ =Rami musculares.=--Muscular branches are given off near the supracondyloid foramen to the biceps (4), epitrochlearis (6), and brachialis muscles. A nutrient artery leaves the brachial proximad of the supracondyloid foramen, either separately or in common with the muscular branches, and passes into the nutrient foramen at the junction of the middle and distal thirds of the shaft of the humerus, supplying the bone.

_d._ =A. collateralis ulnaris= (superior) (or A. anastomotica magna) (_z_).--This leaves the brachial artery just proximad of the supracondyloid foramen and passes to the convexity of the elbow, supplying the structures about the olecranon.

_e._ =A. collateralis radialis superior= (Fig. 130).--This rises from the axillary artery just proximad of the supracondyloid foramen (Fig. 122, _x_), in company with the vena mediana cubiti (_y_), passes across the surface of the biceps (4), beneath the pectoantibrachialis, into the concavity of the elbow. Here it gives branches to the pectoantibrachialis, clavobrachialis, and extensor muscles of the forearm. It then passes onto the ventroradial border of the forearm (Fig. 130) and runs along this border, in company with the vena cephalica (Fig. 130, _c_) and the superficial radial nerve (_g_), to the wrist, sending off branches to the integument. At the wrist it turns onto the dorsum of the hand, passing in a gentle curve to the ulnar side and distad, and giving off a branch for the space between each pair of metacarpal bones. These branches (Fig. 130, _e_) (=Aa. digitales dorsales=) pass distad and anastomose with branches coming from the palm.

_f._ =A. radialis= (Fig. 123).--Distad of the convexity of the elbow the brachial artery takes the name =radial artery=. It passes from the supracondyloid foramen on the medial side of the biceps tendon (1) and beneath the pronator teres muscle (5), giving off small branches, as far as the middle of the forearm, where it gives off the ulnar artery (_k_). It then passes from beneath the pronator teres (5) and lies on the surface of the fifth part of the flexor profundus (8), covered only by the fascia and integument. Near the wrist it sends off a branch (_m_) toward the ulnar side of the arm, which passes into the palm on the surface of the tendon of the flexor profundus, giving twigs to the digits and to the pad in the palm. A branch from this may join the ulnar, and the common trunk thus formed passes into the hand and gives origin to the branches described under the ulnar artery. The presence and size of this branch of the radial varies with the size of the ulnar artery, it being smaller or absent as the ulnar is larger. It is shown at _m_, Fig. 123. The radial artery (_g_) now turns gradually dorsad and passes beneath the tendon of the extensor brevis pollicis onto the dorsum of the hand. It passes over the oblique groove on the dorsal surface of the base of the second metacarpal beneath the tendon of the extensor carpi radialis longus and passes between the bases of the second and third metacarpals into the palm of the hand. Here it passes to the ulnar side and anastomoses with the ulnar artery to form the palmar arch.

=Branches of the Radial Artery.=

1. =A. radialis recurrens= (_i_).--The radial recurrent arises in the concavity of the elbow and sends a branch proximad to the structures in the concavity; it then continues to the radial side of the forearm at the elbow, supplying adjacent parts of the brachialis (2), the extensor carpi radialis, and the extensor communis digitorum.

2. =A. ulnaris recurrens= (_j_).--The ulnar recurrent arises on the medial side of the tendon of the biceps and supplies structures in the concavity of the elbow on the ulnar side, also the pronator teres (5) and proximal ends of the flexor carpi radialis and flexor profundis digitorum.

3. =Rami musculares.=--Muscular branches are given off along the course of the artery, to adjacent muscles. A nutrient artery to the radius leaves the radial artery about two centimeters distad of the supracondyloid foramen.

4. =Aa interosseæ= (=anterior= (_l_) and =posterior=) are given off usually separately between the origin of the ulnar artery and the biceps tendon. Sometimes they arise as a common trunk which soon divides. The =posterior interosseous artery= passes distad, supplying the flexor muscles of the forearm. It also sends a branch to the pronator teres. The =anterior interosseous= (_l_) may arise from the ulnar artery. It passes dorsad to the interosseous membrane, on which it runs distad; sends a nutrient branch to the ulna, and then passes to the wrist in the substance of the pronator quadratus, which it supplies. It sends a branch to the dorsal surface of the wrist; this ramifies on the carpus, forming a network of small arteries.

5. =A. ulnaris= (_k_).--The ulnar artery passes beneath the second, third, and fourth parts of the flexor profundus digitorum (but outside of the origin of the fifth part), to the inner surface of the flexor carpi ulnaris. It supplies the flexor carpi ulnaris, the flexor profundus and palmaris longus, and passes on the inner surface of the flexor carpi ulnaris to the wrist. Near the wrist it sends a branch onto the side of the forearm, and another to its midventral part, and ends in a small branch to the wrist on the radial side of the pisiform bone. This branch anastomoses with the radial to form the palmar arch, described below.

6. The =palmar arch= (Fig. 124) is formed by the termination of the radial artery (_a_) in the palm and its junction with the end of the ulnar (_f_). The radial artery reaches the palm between the bases of the second and third metacarpals and passes thence toward the ulnar side and distad, piercing the interosseus muscle of the third digit and lying on the outer surface of the interossei of the third and fourth digits beneath the adductors of the second and fifth digits. A small communicating branch from the radial passes to it between the first and second metacarpals.

Branches of the palmar arch:

=A. princeps pollicis et indicis= (_c_) leaves the palmar arch near its radial end and sends a branch onto the ulnar side of the thumb and one onto the radial side of the index.

The =palmar interosseæ= (_d_) are three in number. They leave the palmar arch (_b_) and pass distad and dorsad in the intervals between the four ulnar digits. The radial one passes along the ulnar side of the first digit, the ulnar one along the radial side of the fifth digit. The middle one divides and supplies the contiguous sides of the third and fourth digits. Each of these interosseous arteries sends off muscular branches (_e_) to the short muscles in the palm, and branches into the fibrous pad which occupies the palm of the hand. The palmar arch also sends branches onto the dorsum of the hand at the sides of the fourth metacarpal. Branches pass from these proximad to the wrist.

B. AORTA ABDOMINALIS. THE ABDOMINAL AORTA.--The abdominal aorta emerges into the abdomen from between the crura of the diaphragm, at about the level of the second lumbar vertebra. It passes caudad along the dorsal middle line, lying to the left of the inferior vena cava. It gives off =parietal= branches to the body wall, and =visceral= branches to the viscera, and ventrad of the first sacral vertebra it gives off two large branches on each side, the =external iliac= (Fig. 126, _k_) and the =hypogastric= (Fig. 126, _l_)--a very small median vessel, the =sacralis media= (Fig. 126, _o_), continuing the course of the aorta and passing into the tail.

The aorta gives off the following branches: =A. cœliaca=; =A. mesenterica superior=; =Aa. adrenolumbales=; =Aa. renales=; A. =mesenterica inferior=; =Aa. iliolumbales=; =Aa. lumbales= (seven pairs); =Aa. iliacæ externæ=; =Aa. hypogastricæ=.

A. =A. cœliaca= (Fig. 125).--The cœliac artery is a large branch which is given off from the aorta one centimeter or less caudad of the opening in the diaphragm. It passes directly ventrad about three or four centimeters, then divides, usually at once, into three branches. The most cranial of these is the =hepatic= (_d_), the next is the =gastrica sinistra= (_e_), while the third and largest, seeming to form a continuation of the cœliac, is the =splenic= (_f_) (=A. lienalis=). The cœliac artery may give rise also, before its division, to the two =phrenic= arteries (which, however, usually arise from the adrenolumbales), and either before or at the point of division to one or two small =Aa. ventriculi dorsales=, which, however, frequently arise from the gastrica sinistra.

Branches of the cœliac artery:

1. =A. hepatica= (_d_).--The hepatic artery passes cranioventrad, pierces the descending limb of the great omentum and passes craniad to the liver, lying, together with the portal vein and common bile-duct, in the ventral boundary of the foramen epiploicum (foramen of Winslow) and contained, together with the last-named vessels, in a fibrous sheath called the capsule of Glisson. Just before entering the sheath it gives off the =gastroduodenalis= (_g_). At its termination the hepatic artery divides, sending branches to the lobes of the liver and a =cystic= artery to the gall-bladder.

_a._ =A. gastroduodenalis= (_g_).--This arises from the hepatic near the pylorus and passes caudad, dividing one or two centimeters from its origin into three branches, =A. pylorica= (_h_), =A. pancreaticoduodenalis superior= (_j_), and =A. gastroepiploica dextra= (_i_). The =pylorica= (_h_) (which may arise directly from the hepatic) passes to the pylorus, thence along the lesser curvature of the stomach, dividing into numerous branches and anastomosing with the gastrica sinistra. The =pancreaticoduodenalis superior= (_j_) passes to the duodenum, supplying it and the duodenal part of the pancreas and anastomosing with the pancreaticoduodenalis inferior (_k_). The =gastroepiploica dextra= (_i_) passes along the greater curvature of the stomach from the pyloric end and supplies the walls of the stomach, sending branches also to the ascending limb of the great omentum. These branches anastomose with the terminal branches of the splenic artery.

2. =A. gastrica sinistra= (_e_).--This arises from the cœliac artery and passes to the lesser curvature of the stomach, extending along this to the right. It gives off many branches to the walls of the stomach, and anastomoses with A. pylorica.

=A. ventriculi dorsalis= (_l_).--One or two small arteries which arise either from the gastrica sinistra or the cœliac artery, and pass toward the dorsal part of the greater curvature of the stomach near its cardiac end.

3. =A. lienalis= (_f_).--The splenic artery is the largest of the branches of the cœliac axis, of which it appears to be the direct continuation. It divides into two large branches, one to the cranial end, the other to the caudal end, of the spleen (1). From the latter a large branch passes to the pancreas and descending limb of the great omentum.