1911 Encyclopædia Britannica/Arteries

ARTERIES (Gr. ἀρτηρία, probably from αἴρειν, to raise, but popularly connected by the ancients with ἀήρ, air), 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, and anatomists are still undecided as to the normal, i.e. most frequent, arrangement of many of the smaller arteries. From a common-sense point of view it is probably of greater importance to realize how variable the distribution of small arteries is than to remember the names of twigs which are of neither surgical nor morphological importance. Arteries adapt themselves more quickly than most other structures to any mechanical obstruction, and many of the differences between the arterial systems of Man and other animals are due to the assumption of the erect 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 circulation is carried on by the rapid increase in size of the communications 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 in connexion with the anatomy of the lungs (see Respiratory System). That of the aorta will now be briefly described.

The Aorta lies in the cavities of the thorax and abdomen, 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 Aorta. to the left to reach the left side of the body of the 4th thoracic vertebra, and then descends almost vertically. It thus forms the arch of the aorta, which arches over the root of the left lung, and which has attached to its concave surface a fibrous cord, known as the obliterated ductus arteriosus, which connects it with the left branch of the pulmonary artery. The aorta continues its course downward in close relation to the bodies of the thoracic vertebrae, then passes through an opening in the diaphragm (q.v.), enters the abdomen, and descends 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 bifurcation, however, a long slender artery, called the middle sacral, is prolonged downward in front of the sacrum to the end of the coccyx.

It will be convenient to describe the distribution of the arteries under the following headings:—(1) Branches for the head, neck and upper limbs; (2) branches for the viscera of the thorax and abdomen; (3) branches for the walls of the thorax and abdomen; (4) branches for the pelvis and lower limbs.

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 carotid 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.Carotid system.

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 fissure 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 cerebral 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 cerebral 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 the arterial circulation by the anastomoses between arteries not only on the same side, but on opposite sides of the mesial plane. The vertebral 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 sub-maxillary 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 branches to the dura mater, dental branches to the teeth, and other branches to the nose, palate and tympanum; (h) Ascending pharyngeal, which gives branches to the pharynx, palate, tonsils and dura mater.

The subclavian artery is the commencement 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 axillary artery; by that name it extends Subclavian system. 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 foramen 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 muscular branches to the deep muscles of the neck, spinal branches to the spinal cord, meningeal branches to the dura mater, and an inferior cerebellar branch to the under surface of the cerebellum. The basilar artery, formed by the junction of the two vertebrals, extends from the lower to the upper border of the pons Varolii; it gives off transverse branches to the pons, auditory branches to the internal ear, inferior cerebellar branches to the under surface of the cerebellum, whilst it breaks up into four terminal branches, viz. two superior cerebellar to the upper surface of the cerebellum, and two posterior cerebral which supply the tentorial and mesial aspects of the temporo-sphenoidal lobes, the occipital lobes, and the posterior convolutions of the parietal lobes. (b) Thyroid axis, which immediately divides into the inferior thyroid, the supra-scapular, and the transverse cervical branches; the inferior thyroid supplies the thyroid body, and gives off an ascending cervical branch to the muscles of the neck; the supra-scapular supplies the muscles on the dorsum scapulae; the transverse cervical supplies the trapezius and the muscles attached to the vertebral border of the scapula. (c) Internal mammary supplies the anterior surface of the walls of the chest and abdomen, and the upper surface of the diaphragm. (d) Superior intercostal 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 acromio-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 profunda 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 1st and 2nd 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 superficial 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 interosseous muscles, perforating branches to the back of the hand, and recurrent branches to the wrist.

The branches of the aorta which supply the viscera of the thorax are the coronary, the oesophageal, the bronchial and the pericardiac. The coronary arteries, two in number, are the first branches of the aorta, and arise opposite the anterior and left Visceral branches. posterior segments of the semilunar valve, from the wall of the aorta, where it dilates into the sinuses of Valsalva. They supply the tissue of the heart.

The oesophageal, bronchial and pericardiac branches are sufficiently described by their names.

The branches of the aorta which supply the viscera of the abdomen arise either singly or in pairs. The single arteries are the coeliac axis, the superior mesenteric, and the inferior mesenteric, which arise from the front of the aorta; the pairs are the capsular, the two renal, and the two spermatic or ovarian, which arise from its sides. The single arteries supply viscera which are either completely or almost completely invested by the peritoneum, and the veins corresponding to them are the roots of the vena portae. The pairs of arteries supply viscera developed behind the peritoneum, and the veins corresponding to them are rootlets of the inferior vena cava.

The coeliac axis is a thick, short artery, which almost immediately divides into the gastric, hepatic and splenic branches. The gastric gives off oesophageal branches and then runs along the lesser curvature of the stomach. The hepatic artery ends in the substance of the liver; but gives off a cystic branch to the gall bladder, a pyloric branch to the stomach, a gastro-duodenal branch, which divides into a superior pancreatico-duodenal for the pancreas and duodenum, and a right gastro-epiploic for the stomach and omentum. The splenic artery ends in the substance of the spleen; but gives off pancreatic branches to the pancreas, vasa brevia to the left end of the stomach, and a left gastro-epiploic to the stomach and omentum.

The superior mesenteric artery gives off an inferior pancreatico-duodenal branch to the pancreas and duodenum; about twelve intestinal branches to the small intestines, which form in the substance of the mesentery a series of arches before they end in the wall of the intestines; an ileocolic branch to the end of the ileum, the caecum, and beginning of the colon; a right colic branch to the ascending colon; and a middle colic branch to the transverse colon.

The inferior mesenteric artery gives off a left colic branch to the descending colon, a sigmoid branch to the iliac and pelvic colon, and ends in the superior haemorrhoidal artery, which supplies the rectum. The arteries which supply the coats of the alimentary tube from the oesophagus to the rectum anastomose freely with each other in the wall of the tube, or in its mesenteric attachment, and the anastomoses are usually by the formation of arches or loops between adjacent branches.

The capsular arteries, small in size, run outward from the aorta to end in the supra-renal capsules.

The renal arteries pass one to each kidney, in which they for the most part end, but in the substance of the organ they give off small perforating branches, which pierce the capsule of the kidney, and are distributed in the surrounding fat. Additional renal arteries are fairly common.

The spermatic arteries are two long slender arteries, which descend, one in each spermatic cord, into the scrotum to supply the testicle. The corresponding ovarian arteries in the female do not leave the abdomen.

The branches of the aorta which supply the walls of the thorax, abdomen and pelvis, are the intercostal, the Parietal branches. lumbar, the phrenic, and the middle sacral.

 

Fig. 1.—Diagram of a pair
of intercostal arteries.

Ao, The aorta transversely divided, giving off
at each side an intercostal artery.
PB, The posterior or dorsal branch.
AB, The anterior or proper intercostal branch.
IM, A transverse section through the internal
mammary artery.

The intercostal arteries arise from the back of the thoracic aorta, and are usually nine pairs. They run round the sides of the vertebral bodies as far as the commencement of the intercostal spaces, where each divides into a dorsal and a proper intercostal branch; the dorsal branch passes to the back of the thorax to supply the deep muscles of the spine; the proper intercostal branch (AB.) 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 subcostal 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 supplies 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 phrenic arteries, two in number, pass to 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.

Injections made by Sir W. Turner have shown that, both in the thoracic and abdominal cavities, slender anastomosing communications exist between the visceral and parietal branches.

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

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 Iliac
system. 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 downward 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.

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 Structure
of
arteries. muscular tissue, the contractile fibre-cells of which are for the most part 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 muscular coat is relatively strong; the vaso-motor nerves terminate in the muscular coat. In the first part of the aorta, pulmonary artery 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, distributed in their external, elastic and muscular coats.

 Fig. 2.—Diagram of the structure of an artery. A, tunica adventitia; E, elastic coat; M, muscular coat; F, fenestrated coat; En, endothelium continuous with the endothelial wall of C, the capillaries.

Embryology

 Fig. 3.—Diagram of the Embryonic Arterial Arches.
1, 2, 3, 4, 5, 6, point to the six arches. (The black parts are obliterated in the adult human subject.)
V.Ao. Ventral Aorta.
A.Ao. Arch of Aorta.
D.Ar. Ductus Arteriosus.
In. Innominate Artery.
R.I.C.-L.I.C. Right and Left Internal Carotid Arteries.
D.B. Duct of Botalli.
R.S.-L.S. Right and Left Subclavian Arteries.
R.V.-L.V. Right and Left Vertebral Arteries.
P.A. Posterior Auricular Artery.
Oph. Ophthalmic Artery.
D.Ao. Dorsal Aorta.
P.T. Pulmonary trunk.
R.P.A.-L.P.A. Right and Left Pulmonary Arteries.
R.C.C.-L.C.C. Right and Left Common Carotid Arteries.
E.C. External Carotid Artery.
Oc. Occipital Artery.
I.M. Internal Maxillary Artery.

 Fig. 4.—Diagram of the Human Aorta and its branches. S.T., Superficial Temporal Artery.

The earliest appearance of the blood vessels is dealt with under Vascular System. Here will be briefly described the fate of the main vessel which carries the blood away from the truncus arteriosus of the developing heart (q.v.). This ventral aorta, if traced forward, soon divides into two lateral parts, the explanation being that there were originally two vessels, side by side, which fused to form the heart, but continued separate anteriorly. The two parts run for a little distance toward the head of the embryo, ventral to the alimentary canal, and then turn toward the dorsum, passing one on either side of that tube to form the first aortic arch. Having reached the dorsum they turn backward toward the tail end and form the dorsal aortae; here, according to A. H. Young (Studies in Anatomy, Owens College, 1891 and 1900) they again turn toward the ventral side and become, after a transitional stage, the hypogastric, placental, allantoic or umbilical arteries. This authority does not believe that the middle sacral artery of the adult is the real continuation of the single median dorsal aorta into which the two parallel dorsal vessels just mentioned soon coalesce, though until recently it has always been so regarded. The anterior loop between the ventral and dorsal aortae already described as the first aortic arch is included in the maxillary or first visceral arch of the soft parts (see fig. 3, 1). Later, four other well-marked aortic arches grow behind this in the more caudal visceral arches, so that there are altogether five arterial arches on each side of the pharynx, through which the blood can pass from the ventral to the dorsal aorta. Of these arches the first soon disappears, but is probably partly represented in the adult by the internal maxillary artery, one branch of which, the infraorbital, is enclosed in the upper jaw, while another, the inferior dental, is surrounded by the lower jaw. Possibly the ophthalmic artery also belongs to this arch. The second arch also disappears, but the posterior auricular and occipital arteries probably spring from it, and at an early period it passed through the stapes as the transitory stapedial artery. The third arch forms the beginning of the internal carotid. The fourth arch becomes the arch of the adult aorta, between the origins of the left carotid and left subclavian, on the left side, and the first part of the right subclavian artery on the right. The apparent fifth arch on the left side (fig. 3, 6) remains all through foetal life as the ductus arteriosus, and, as the lungs develop, the pulmonary arteries are derived from it. J. E. V. Boas and W. Zimmermann have shown that this arch is in reality the sixth, and that there is a very transitory true fifth arch in front of it (fig. 3, 5). The part of the ventral aorta from which this last arch rises is a single median vessel due to the same fusion of the two primitive ventral aortae which precedes the formation of the heart, but a spiral septum has appeared in it which divides it in such a way that while the anterior or cephalic arches communicate with the left ventricle of the heart, the last one communicates with the right (see Heart). The fate of the ventral and dorsal longitudinal vessels must now be followed. The fused part of the two ventral aortae, just in front of the heart, forms the ascending part of the adult aortic arch, and where this trunk divides between the fifth and fourth arches (strictly speaking, the sixth and fifth), the right one forms the innominate (fig. 3, In.) and the left one a very short part of the transverse arch of the aorta until the fourth arch comes off (see fig. 4). From this point to the origin of the third arch is common carotid, and after that, to the head, external carotid on each side. The dorsal longitudinal arteries on the head side of the junction with the third arch form the internal carotids. Between the third and fourth arches they are obliterated, while on the caudal side of this, until the point of fusion is reached on the dorsal side of the heart, the left artery forms the upper part of the dorsal aorta while the right entirely disappears. Below this point the thoracic and abdominal aortae are formed by the two primitive dorsal aortae which have fused to form a single median vessel. As the limbs are developed, vessels bud out in them. The subclavian for the arm comes from the fourth aortic arch on each side, while in the leg the main artery is a branch of the caudal arch which is curving ventralward to form the umbilical artery. From the convexity of this arch the internal iliac and sciatic at first carry the blood to the limb, as they do permanently in reptiles, but later the external iliac and femoral become developed, and, as they are on the concave side of the bend of the hip, while the sciatic is on the convex, they have a mechanical advantage and become the permanent main channel.

For further details see O. Hertwig, Handbuch der vergleichenden und experimentellen Entwickelungslehre der Wirbeltiere (Jena, 1905).

Comparative Anatomy

In the Acrania the lancelet (Amphioxus) shows certain arrangements of its arteries which are suggestive of the embryonic stages of the higher vertebrates and Man. There is a median ventral aorta below the pharynx, from which branchial arteries run up on each side between the branchial clefts, where the blood is aerated, to join two dorsal aortae which run back side by side until the hind end of the pharynx is reached; here they fuse to form a median vessel from which branches are distributed to the straight intestine. There is no heart, but the ventral aorta is contractile, and the blood is driven forward in it and backward in the dorsal aortae. The branchial arteries are very numerous, and cannot be homologized closely with the five (originally six) pairs of aortic arches in Man.

In the fish the ventral aorta gives rise to five afferent branchial arteries carrying the blood to the gills, though these may not all come off as independent trunks from the aorta. From the gills the afferent branchials carry the blood to the median dorsal aorta. As pectoral and pelvic fins are now developed, subclavian and iliac arteries are found rising from the dorsal aorta, though the aorta itself is continued directly backward as the caudal artery into the tail. In the Dipnoi or mud fish, in which the swim bladder is converted into a functional lung, the hindmost afferent branchial artery, corresponding to the fifth (strictly speaking the sixth) aortic arch of the human embryo, gives off on each side a pulmonary artery to that structure.

The arrangement of the branchial aortic arches in the tailed Amphibia (Urodela), and in the tadpole stage of the tailless forms (Anura), makes it probable that the generalized vertebrate has six (if not more) pairs of these instead of the five which are evident in the human embryo. Four pairs of arches are present, the first of which is the carotid and corresponds to the third of Man; the second is the true aortic arch on each side; the third undergoes great reduction or disappears when the gills atrophy, and is very transitory in the Mammalia (fig. 3, 5), while the fourth is the one from which the pulmonary artery is developed when the lungs appear, and corresponds to the nominal fifth, though really the sixth arch, of the higher forms (fig. 3, 6). The dorsal part of this sixth arch remains as a pervious vessel in the Urodela, joining the pulmonary arch to the dorsal aorta. In the central part of the carotid arch the vessel breaks up into a plexus, for a short distance forming the so-called carotid gland, which has an important effect upon the adult circulation of the Amphibia. In the Reptilia the great arteries are arranged on the same plan as in the adult Amphibia, but the carotid arch retains its dorsal communication with the systematic aortic arch on each side, and this communication is known as the duct of Botalli (fig. 3, D.B.). In this class, as in the Amphibia, one great artery, the coeliaco-mesenteric, usually supplies the liver, spleen, stomach and anterior part of the intestines; this is a point of some interest when it is noticed how very close together the coeliac axis and superior mesenteric arteries rise from the abdominal aorta in Man.

In the Birds the right fourth arch alone remains as the aorta, the dorsal part of the left corresponding arch being obliterated. From the arch of the aorta rise two symmetrical innominates, each of which divides later into a carotid and subclavian. The blood path from the aorta to the hind limb in the Amphibia, Reptilia and Aves, is a dorsal one, and passes through the internal iliac and sciatic to the back of the thigh, and so to the popliteal space; the external iliac is, if it is developed at all, only a small branch to the pelvis.

In the Mammalia the fourth left arch becomes the aorta, the corresponding right one being obliterated, but several cases have been recorded in Man in which both arches have persisted, as they do in the reptiles (H. Leboucq, Ann. Sci. Med. Gand, 1894, p. 7). Examples have also been found of a right aortic arch, as in birds, while a very common human abnormality is that in which the dorsal part of the fourth right arch persists, and from it the right subclavian artery arises (see fig. 3).

The commonest arrangement of the great branches of the aortic arch in Mammals is that in which the innominate and left carotid arise by a single short trunk, while the left subclavian comes off later; this is also Man’s commonest abnormality. Sometimes, especially among the Ungulata, all the branches may rise from one common trunk; at other times two innominate arteries may be present; this is commonest in the Cheiroptera, Insectivora and Cetacea. It is extremely rare to find all four large arteries rising independently from the aorta, though it has been seen in the Koala (F. G. Parsons, “Mammalian Aortic Arch,” Journ. of Anat. vol. xxxvi. p. 389). The human arrangement of the common iliacs is not constant among mammals, for in some the external and internal iliacs rise independently from the aorta, and this is probably the more primitive arrangement. The middle sacral artery has already been referred to. A. H. Young and A. Robinson believe, on embryological grounds, that this artery in mammals is not homologous with the caudal artery of the fish, and is not the direct continuation of the aorta; it is an artery which usually gives off two or more collateral branches, and sometimes, as in the Ornithorynchus and some edentates, breaks up into a network of branches which reunite and so form what is known as a rete mirabile. These retia mirabilia are often found in other parts of the mammalian body, though their function is still not satisfactorily explained. The way in which the blood is carried to the foot in the pronograde mammals differs from that of Man; a large branch called the internal saphenous comes off the common femoral in the lower third of the thigh, and this runs down the inner side of the leg to the foot. This arrangement is quite convenient as long as the knee is flexed, but when it comes to be extended, as in the erect posture, the artery is greatly stretched, and it is much easier for the blood to pass to the foot through the anterior and posterior tibials. A vestige of this saphenous artery, however, remains in Man as the anastomotica magna.

The literature of the Comparative Anatomy of the Arteries up to 1902 will be found in R. Wiedersheim’s Vergleichende Anatomie der Wirbeltiere (Jena, 1902). The morphology of the Iliac Arteries is described by G. Levi, Archivio Italiano di Anat. ed Embriol., vol. i. (1902).  (F. G. P.)