Arteries

ARTERIES, in anatomy, the elastic tubes which carry the blood away from the heart to the tissues. As, after death, they are always found empty, the older anatomists believed that they contained air, and to this belief they owe the name, which was originally given to the windpipe (trachea). Two great trunks, the aorta and pulmonary artery, leave the heart and divide again and again until they become minute vessels to which the name of arterioles is given. The larger trunks are fairly constant in position and receive definite names, but as the smaller branches are reached there is an increasing inconstancy in their position. Many arteries are tortuous, especially when they supply movable parts such as the face or scalp, but when one or two sharp bends are found they are generally due to the artery going out of its way to give off a constant and important branch. Small arteries unite or anastomose with others near them very freely, so that when even' a large artery is obliterated a collateral cir culation is carried on by the rapid increase in size of the com munications between the branches coming off above and below the point of obstruction. Some branches, however, such as those going to the basal ganglia of the brain and to the spleen, are known as "end arteries" and these do not anastomose with their neighbours at all; thus, if one is blocked, arterial blood is cut off from its area of supply. As a rule, there is little arterial anastomosis across the middle line of the body near the surface, though the scalp, lips and thyroid body are exceptions.

The distribution of the pulmonary artery is considered with the anatomy of the lungs (see RESPIRATORY' SYSTEM). Aorta.—The Aorta lies in the cavities of the thorax and abdo men, and arises from the base of the left ventricle of the heart. It ascends forward, upward, and to the right as far as the level of the second right costal cartilage, then runs backward, and to the left to reach the left side of the body of the 4th thoracic verte bra, and then descends almost vertically. It thus forms the aortic arch, which has attached to its concave surface a fibrous cord, the ductus arteriosus, which connects it with the left branch of the pulmonary artery. The aorta continues downward close to the bodies of the thoracic vertebrae, then passes through an opening in the diaphragm (q.v.), enters the abdomen, and de scends in front of the bodies of the lumbar vertebrae as low as the 4th, where it usually divides into two terminal branches, the common iliac arteries. Above and behind the angle of bifur cation, however, a long slender artery, called the middle sacral, is prolonged downward in front of the sacrum to the end of the coccyx.

Carotid System.

The branches for the head, neck and upper limbs arise as three large arteries from the transverse part of the aorta; they are named innominate, left common carotid and left subclavian. The innominate artery is the largest and passes upward and to the right, to the root of the neck, where it divides into the right common carotid and the right subclavian. The carotid arteries supply the two sides of the head and neck; the subclavian arteries the two upper extremities. The common caro tid artery runs up the neck by the side of the windpipe, and on a level with the upper border of the thyroid cartilage divides into the internal and external carotid arteries.

The internal carotid artery ascends through the carotid canal in the temporal bone into the cranial cavity. It gives off an ophthalmic branch to the eyeball and other contents of the orbit, and then divides into the anterior and middle cerebral arteries. The middle cerebral artery extends outward into the Sylvian fis sure of the brain, and supplies the island of Reil, the orbital part, and the outer face of the frontal lobe, the parietal lobe, and the temporo-sphenoidal lobe; it also gives a choroid branch to the choroid plexus of the velum interpositum. The anterior cere bral artery supplies the inner face of the hemisphere from the anterior end of the frontal lobe as far back as the internal parieto occipital fissure. At the base of the brain not only do the two internal carotids anastomose with each other through the anterior communicating artery, which passes between their anterior cere bral branches, but the internal carotid on each side anastomoses with the posterior cerebral branch of the basilar, by a posterior communicating artery. In this manner a vascular circle, the circle of Willis, is formed, which permits of freedom of circulation be tween the arteries on opposite sides of the mesial plane. The verte bral and internal carotid arteries, which are the arteries of supply for the brain, are distinguished by lying at some depth from the surface in their course to the organ, by having curves or twists in their course, and by the absence of large collateral branches.

The external carotid artery ascends through the upper part of the side of the neck, and behind the lower jaw into the parotid gland, where it divides into the internal maxillary and superficial temporal branches. This artery gives off the following branches: (a) Superior thyroid to the larynx and thyroid body; (b) Lingual to the tongue and sublingual gland; (c) Facial to the face, palate, tonsil and submaxillary gland; (d) Occipital to the sterno-mastoid muscle and back of the scalp; (e) Posterior auricular to the back of the ear and the adjacent part of the scalp ; (f) Superficial temporal to the scalp in front of the ear, and by its transverse facial branch to the back part of the face; (g) Internal maxillary, giving muscular branches to the muscles of mastication, meningeal to the dura mater, dental to the teeth, and other branches to the nose, palate and tympanum; (h)' Ascending pharyngeal, which supplies the pharynx, palate, tonsils and dura mater.

Subclavian System.

The subclavian artery is the com mencement of the great arterial trunk for the upper limb. It passes across the root of the neck and behind the clavicle, where it enters the armpit, and becomes the axil:ary artery; by, that name it extends as far as the posterior fold of the axilla, where it enters the upper arm, takes the name of brachial, and courses as far as the bend of the elbow ; here it bifurcates into the radial and ulnar arteries. From the subclavian part of the trunk the following branches arise :—(a) Vertebral, which enters the fora men at the root of the transverse process of the 6th cervical vertebra, ascends through the corresponding foramina in the vertebrae above, lies in a groove on the arch of the atlas, and enters the skull through the foramen magnum, where it joins its fellow to form the basilar artery; it gives off numerous branches in its course. The basilar artery extends from the lower to the upper border of the pons Varolii ; it gives off transverse branches to the pons, auditory and inferior cerebellar branches and breaks up into four terminal branches; viz., two superior cerebellar and two posterior cerebral; (b) Thyroid axis, which immediately divides into the inferior thyroid, the suprascapular, and the transverse cervical branches; (c) Internal mammary, which supplies the anterior surface of the walls of the chest and abdomen, and the upper surface of the diaphragm; (d) Superior intercostal, which supplies the first intercostal space, and by its deep cervical branch the deep muscles of the back of the neck.

The axillary artery supplies thoracic branches to the wall of the chest, the pectoral muscles, and the fat and glands of the axilla; an acronzio-thoracic to the parts about the acromion; anterior and posterior circumflex branches to the shoulder joint and deltoid muscle; a subscapular branch to the muscles of the posterior fold of the axilla.

The brachial artery supplies muscular branches to the muscles of the upper arm; a nutrient branch to the humerus; superior and inferior pro f unda branches and an anastomotic to the muscles of the upper arm and the region of the elbow joint.

The ulnar artery extends down the ulnar side of the front of the fore-arm to the palm of the hand, where it curves outward toward the thumb, and anastomoses with the superficial volar or other branch of the radial artery to form the superficial palmar arch. In the fore-arm the ulnar gives off the interosseous arteries, which supply the muscles of the fore-arm and give nutrient branches to the bones ; two recurrent branches to the region of the elbow; carpal branches to the wrist joint : in the hand it gives a deep branch to the deep muscles of the hand, and from the superficial arch arise digital branches to the sides of the little, ring, and middle fingers, and the ulnar border of the index finger.

The radial artery extends down the radial side of the front of the fore-arm, turns round the outer side of the wrist to the back of the hand, passes between the i st and and metacarpal bones to the palm, where it joins the deep branch of the ulnar, and forms the deep palmar arch. In the fore-arm it gives off a recurrent branch to the elbow joint ; carpal branches to the wrist joint ; and muscular branches, one of which, named super ficial volar, supplies the muscle of the thumb and joins the ulnar artery : in the hand it gives off a branch to the thumb, and one to the radial side of the index, interosseous branches to the inter osseous muscles, perforating branches to the back of the hand, and recurrent branches to the wrist.

costal branch ; the dorsal branch passes to the back of the thorax to supply the deep muscles of the spine; the proper intercostal branch runs outward in the intercostal space to supply its muscles, and the lower pairs of intercostals also give branches to the diaphragm and wall of the abdomen. Below the last rib a sub costal artery runs.

The lumbar arteries arise from the back of the abdominal aorta, and are usually four pairs. They run round the sides of the lumbar vertebrae, and divide into a dorsal branch which sup plies the deep muscles of the back of the loins, and an abdominal branch which runs outward to supply the wall of the abdomen. The distribution of the lumbar and intercostal arteries exhibits a transversely segmented arrangement of the vascular system, like the transversely segmented arrangement of the bones, muscles and nerves met with in these localities, but more especially in the thoracic region.

The two phrenic arteries supply the under surface of the diaphragm.

The middle sacral artery, as it runs down the front of the sacrum, gives branches to the back of the pelvic wall.

The arteries to the pelvis and hind limbs begin at the bifurca tion of the aorta into the two common iliacs.

Iliac System.

The common iliac artery, after a short course, divides into the internal and external iliac arteries. The internal iliac enters the pelvis and divides into branches for the supply of the pelvic walls and viscera, including the organs of generation, and for the great muscles of the buttock. The external iliac descends behind Poupart's ligament into the thigh, where it takes the name of femoral artery. The femoral descends along the front and inner surface of the thigh, gives off a profunda or deep branch, which, by its circumflex and perforating branches, supplies the numerous muscles of the thigh ; most of these extend to the back of the limb to carry blood to the muscles situated there. The femoral artery then runs to the back of the limb in the ham, where it is called popliteal artery. The popliteal divides into two branches, of which one, called anterior tibial, passes between the bones to the front of the leg, and then down ward to the upper surface of the foot ; the other, posterior tibial, continues down the back of the leg to the sole of the foot, and divides into the internal and external plantar arteries; branches proceed from the external plantar artery to the sides of the toes, and constitute the digital arteries. From the large arterial trunks in the leg many branches proceed, to carry blood to the different structures in the limb.

Structure of Arteries.

The wall of an artery consists of several coats (see fig. 2.) . The outermost is the tunica adventitia, composed of connective tissue immediately internal to this is the yellow elastic coat ; within this again the muscular coat, formed of involuntary muscular tissue arranged transversely to the long axis of the artery ; in the larger arteries the elastic coat is much thicker than the muscular, but in the smaller the muscu lar coat is relatively strong; the vasomotor nerves terminate in the muscular coat. In the greater part of the aorta and arteries of the retina there is no muscular coat. Internal to the muscular coat is the elastic fenestrated coat, formed of a smooth elastic membrane perforated by small apertures. Most internal of all is a layer of endothelial cells, which form the free surface over which the blood flows. The arteries are not nourished by the blood which flows through them, but by minute vessels, vasa vasorum, dis tributed in their external, elastic and muscular coats.