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The inferior mesenteric artery may give hepatic, renal, or middle colic branches ; occasionally it is absent, being replaced by branches of the superior mesenteric, and sometimes, as in ruminants and some rodents, its left colic branch does not anastomose with the middle colic artery.

All these variations of the unpaired visceral branches of the abdominal aorta are merely due to modifications of the usual processes by which the vessels are developed.

The hepatic, splenic, and left gastric arteries may arise directly from the aorta, a condition which is due to the retention of a greater number of the splanchnic arteries than usual A doub superior mesenteric artery results from persistence of both the right and left splanchnic vessels from which the superior mesenteric artery is formed, these remaining separate instead of fusing together. All the other variations are the results of the obliteration of the usual channels, combined with the enlargement of anastomoses which exist both between the splanchnic arteries of adjacent segments and between the splanchnic and intermediate visceral arteries.

THE ARTERIES OF THE HEAD AND NECK. Innominate Artery.-From what has already been said, with reference to the branches of the arch of the aorta, it will be noted that the innominate artery may be absent. On the other hand there may be two innominate arteries, a right and a left, each ending in corresponding common carotid and subclavian trunks, and the two vessels may themselves arise by a common stem.

The branches given off by the innominate artery may be increased in number, or the innominate may vary from the normal only as regards length. As a consequence of such inodifications in length, the origins of the right common carotid and right subclavian arteries may be situated at a higher or lower level than usual, whilst, in the absence of the innominate artery, both these branches may arise directly from the aorta.

Common Carotid Arteries. When the right common carotid artery arises separately from the arch of the aorta, it may be the first, or, much more rarely, the second branch. In the former case the fourth right aortic arch has been obliterated, and the right subclavian artery springs from the descending aorta ; in the latter case either the innominate stem has been absorbed into the arch of the aorta, or the ventral root of the fourth right aortic arch has fused with part of an elongated fourth left arch.

Whether arising as the first or second branch, the origin may be to the left of the median plane, and the trunk may pass in front of the trachea, or behind the æsophagus, before it ascends into the neck.

The left common carotid artery varies, as regards its origin, much more frequently than the right vessel ; not uncommonly, and apparently because of the fusion of the ventral roots of the fourth aortic arches, it arises from a stem common to it and to the right common carotid and right subclavian arteries.

Both common carotids may vary as regards their termination. They may divide at a higher or lower level than usual, the former more commonly than the latter; whilst in a few exceptional cases the common carotid does not divide, but is continued directly into the internal carotid, and from this the branches usually given off by the external carotid are derived.

This arrangement is probably due to obliteration of the ventral roots of the first and second aortic arches, the arches persisting and being divided into the branches which generally arise from their ventral extremities.

Usually the common carotids give off no branches, but not infrequently one or more of the branches of the external carotids arise from them.

The external carotid artery may be absent, or it may, in rare cases, arise directly from the arch of the aorta. The number of its branches may be diminished either by fusion of their roots or by transference to the internal or common carotid arteries. On the other hand, the number of its branches may be increased ; thus, the sterno-mastoid artery, the hyoid branch usually given off by the superior thyreoid artery, or the ascending palatine branch of the external maxillary, may arise from it. Sometimes the branches may arise in the usual way, but may deviate from the course generally taken ; more particularly is this the case with the internal maxillary artery, which may pass either between the heads, or entirely lateral or medial to both heads of the external pterygoid muscle.

The internal carotid artery is rarely absent, but its absence has been noted upon one side, more commonly the left; and upon both sides. Occasionally it springs from the arch of the aorta, and in its course through the neck it may vary somewhat in length and in tortuosity. One or more of the branches usually derived from the external carotid artery may arise from it, and it sometimes gives off a large meningeal branch to the posterior fossa of the skull

. Its posterior commnunicating branch may replace the posterior cerebral artery; on the other hand, the upper part of the internal carotid may be absent, and the posterior communicating artery may become the middle cerebral artery. The anterior cerebral branch of the internal carotid may be absent, or rather it may arise from the corresponding artery of the opposite side; or there may be three anterior cerebral arteries, the third arising from the anterior communicating artery which connects the two anterior cerebrals together. The ophthalmic artery, as it traverses the orbit, may puss either above or below the optic nerve. It is occasionally replaced by a branch of the middle meningeal artery.

The vertebral artery may have a double origin-one from the subclavian, and one from the inferior thyreoid artery or from the aorta.

The right vertebral may arise from the common carotid or from the arch of the aorta. Occasionally it springs from the descending aorta, an arrangement associated with the persistence of the dorsal roots of the fourth and fifth right arches.

The left vertebral artery not infrequently springs from the arch of the aorta, arising between the left common carotid and left subclavian arteries ; this is evidently due to the absorption of the stem of the seventh segmental artery into the aortic arch. Very exceptionally the left vertebral is a branch of an intercostal artery.

In its course upwards either vertebral artery may enter the vertebrarterial foramen of any of the lower six cervical vertebra.

The cases in which it does not enter one of the lowest of these are apparently associated with its formation, in part, from the precostal instead of from the postcostal anastomosing channels.

The artery may enter the vertebral canal with the second instead of with the first cervical nerve, or, after leaving the foramen in the transverse process of the third vertebra, it may divide into two branches, one of which accompanies the second and the other the first cervical nerve; the two branches unite together again in the vertebral canal to form a single trunk.

Sometimes, though rarely, it gives off superior intercostal and inferior thyreoid branches. The upper end of one of the vertebrals is sometimes very small, or it may be entirely wanting ; in the latter case the basilar artery is formed by the direct continuation of the opposite vertebral.

The basilar artery may be double in part or the whole of its extent, or its cavity may be divided by a more or less complete septum. It may terminate in one instead of two posterior cerebral arteries, the missing vessel being supplied by the enlargement of the posterior communicating branch of the internal carotid.

THE ARTERIES OF THE UPPER LIMB. Subclavian Arteries. The variations, so far as regards the origins of the subclavian arteries, have already been mentioned (p. 1051). Other interesting modifications are met with in respect of its position and branches,

The subclavian artery may reach as high as 25 or 37 mm. (1-1} in.) above the clavicle, though as a rule it does not reach higher than 19 mm. above that bone. On the other hand, it may not rise even to the level of the upper border of the clavicle. These differences appear to be associated with the descent of the clavicle and sternum, which occurs as age increases.

The artery may pass in front of or through the scalenus anterior instead of behind it, or the vein may accompany it behind the muscle.

The branches of the subclavian artery may be modified with reference to their points of origin; thus, those of the first part may be further medial or lateral than usual, the transverse scapular or some other branch of the thyreo-cervical trunk may arise separately from the third part of the subclavian, and not uncommonly the descending branch of the transverse cervical artery is a branch of that part. The abnormalities of the vertebral branch have already been described"; those of the thyreo-cervical trunk and its branches are numerous but not important.

The internal mammary artery, usually a branch of the first part of the subclavian, is very variable as regards its origin. It may arise from the second or third parts, or from the thyreocervical, or it may spring from the aorta, or from the innominate or axillary arteries. All these variations are due to obliteration of the normal origin and the opening up of anastomoses. The internal mammary artery sometimes descends in front of the cartilages of one or more of the lower true ribs; and occasionally it gives off a large lateral branch (a. mammaria lateralis) which descends on the inner side of the chest wall, close to the mid-axillary line, –a point of importance in paracentesis.

A few cases have also been noticed in which a bronchial artery has arisen from the internal mammary.

The superior intercostal branch of the costo-cervical trunk may be absent, and the profunda cervicis branch may arise directly from the subclavian trunk. The superior intercostal is sometimes formed from a postcostal instead of a precostal primitive channel. In such cases it passes between the necks of the ribs and the transverse processes of the vertebræ instead of, as usual, in front of the necks of the ribs.

The axillary artery does not vary much as regards its origin or course. Its relations may be modified by the existence of a muscular or tendinous "axillary arch,” which, passing from the latissimus dorsi to the pectoralis major, crosses the distal part of the artery superficially; and a further interesting modification is associated with an anomalous arrangement of its branches. Occasionally the sub-scapular, circumflex, and profunda and superior ulnar collateral arteries arise from the axillary by a common stem. In those cases the chief branches of the brachial plexus are grouped round the common stem instead of round the main trunk. The arrangement is due to the persistence of a different part of the original vascular plexus.

Sometimes the axillary artery divides into the radial and ulnar arteries, and more rarely the interosseous artery may spring from it.

Obviously there is no brachial artery when the radial and ulnar arteries are formed by the division of the axillary ; its place is taken by the two abnormal vessels which, as a rule, are separated by the median nerve as they run through the arm; the radial is usually more superficial than the ulnar, and crosses laterally in front of it at the bend of the elbow.

The brachial artery is rarely prolonged beyond its usual point of bifurcation; not uncommonly, however, it bifurcates at a more proximal level. Of the two terminal branches of the brachial, one may divide into radial and interosseous, the other forming the ulnar; or one may divide into radial and ulnar, whilst the other is the interosseous artery. Occasionally the brachial artery terminates by dividing into three branches-viz., the radial, the ulnar, and the interosseous In any case, the branch which gives origin to or becomes the interosseous was, in all probability, the original trunk.

Division of the brachial artery at a more proximal level than usual occurs most commonly in the proximal third of the arm, and least commonly in the distal third ; the resulting trunks are often united near the bend of the elbow by a more or less oblique anastomosis.

In cases of proximal division of the brachial artery the radial branch may pierce the deep fascia of the arm near the bend of the elbow, and passes distally in the forearm immediately deep to the skin ; in other cases the radial runs deeper, and passes behind the tendon of the biceps. The ulnar branch sometimes runs, on the medial intermuscular septum, towards the medial epicondyle, and then laterally towards the middle of the bend of the elbow, under a band of fascia from which the proximal fibres of the pronator teres arise, or round the supracondylar process of the humerus if it is present. More commonly the ulnar branch runs distally towards the inedia: epicondyle, and crosses superficial to the flexor muscles or deep to the paimaris longus; and il a few cases it is subcutaneous. In rare cases the ulnar artery accompanies the ulnar nerve behind the medial epicondyle; in those cases it has obviously been formed by enlargement of the ordinary superior ulnar collateral and dorsal ulnar recurrent arteries.

Instead of following its usual course along the brachialis muscle, the brachial artery may accompany the median nerve behind a supracondylar or epicondylic process, or ligament, as in many carnivora ; it may pass in front of the median nerve instead of behind it. "It may give off a “vas aberrans” or a median artery, and any of its ordinary branches may be absent.

The vas aberrans given off from the brachial artery usually ends in the radial artery, sometimes in the radial recurrent, and rarely in the ulnar artery.

The ulnar artery may be absent, being replaced by the median artery or the interosseous artery, and it may terminate in the deep instead of in the superficial volar arch. It rarely arises more distally than usual, and when it arises at a more proximal point it most commonly passes superficial to the muscles which spring from the medial epicondyle. Moreover, in those cases it frequently has no interosseous branch, the latter vessel springing from the radial artery, and in all probability variations of this description are produced by the ulnar artery taking origin from the main trunk, which is represented by the radio-interosseous vessel, at a more proximal level than usual. Even when it commences in the usual way the ulnar artery may pass superficial to the muscles arising from the medial epicondyle, and in those cases its interosseous and recurrent branches spring from the radial artery.

The volar and dorsal interosseous arteries may arise separately from the ulnar instead of by a common interosseous trunk. The recurrent branches of the ulnar may spring from the interosseous, and the interosseous itself may be a branch of the radial.

The small median artery, the companion artery of the median nerve, usually a branch of the volar interosseous, may spring from the axillary, brachial, or ulnar arteries ; it may be much larger than usual, and it may terminate either by breaking up into digital branches, or by joining one or more digital branches of the superficial volar arch or the arch itself.

The radial artery may be absent, its place being taken by branches of the ulnar or interosseous arteries ; it may arise, more proximal than usual

, from the axillary, or from the brachial. It may terminate in muscular branches in the volar part of the forearm, or as the superficial volar, or in carpal branches; the distal portion of the artery, in those cases, is usually replaced by branches of the ulnar or interosseous arteries. Occasionally the radial divides some distance proximal to the wrist into two terminal branches, one of which gives off the carpal branches, and becomes the superficial volar, whilst the other crosses superficial to the extensor tendons and passes to the dorsum of the wrist.

The radial artery may run a superficial course, or, and especially when it commences at a more distal level than usual, it may pass deep to the pronator teres and the radial origin of the flexor digitorum sublimis. In some cases it passes to the dorsum of the wrist across the brachroradialis, and in others it lies superficial, instead of deep to, the extensor tendons of the thumb.

Its branches may be diminished or increased in number. The radial recurrent may spring from the brachial or ulnar arteries, or may be represented by several branches from the proximal part of the radial. The dorsal artery of the index digit may be large, and may replace the princeps pollicis and the volaris indicis radialis.

On the contrary, the dorsal carpal artery and dorsal digital branches of the radial may be small, or the former may be replaced by branches of the metacarpal arteries, and the latter by the proximal perforating branches of the deep volar arch.

The princeps pollicis and volaris indicis radialis arteries may be absent, their places being taken either by branches of the superficial volar arch or by the dorsalis indicis radialis artery,

The superficial volar arch is sometimes absent; its branches are then given off from the deep arch. On the other hand, it may be larger than normal, and it may be completed on the radia: side by the volaris indicis radialis

, the princeps pollicis, or the comes nervi mediani arteries The deep volar arch is much more rarely absent than the superficial arch. When abeent its branches are supplied by the superficial arch, the proximal perforating arteries, or the volar carpal arch.

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THE ILIAC ARTERIES AND THEIR BRANCHES. The common iliac artery may be longer or shorter than usual, a modification whah ia determined largely, though not altogether, by the point at which the bifurcation of the aorta

takes place. If exceptionally long, it is usually tortuous. In rare cases in man the artery is absent. It occasionally gives off the middle or a lateral sacral artery, and ilio-lumbar, spermatic, or accessory renal branches may arise from it.

The hypogastric artery varies as regards length. It is usually longer, and arises at a higher level when the common iliac is short. In rare cases it has been found to arise from the aorta without the intervention of a common iliac. Frequently it does not, even in appearance, end in anterior and posterior divisions, but obviously forms a single trunk, as in the fætus, from which the several branches are given off.

The visceral branches vary much in number and size, and the middle hæmorrhoidal may not be present, its place being taken by branches from the vesical arteries. A renal branch sometimes arises from the hypogastric artery.

The ilio-lumbar branch may arise from the common iliac instead of from the hypogastric artery; the superior gluteal and inferior glutæal arteries may arise by a common stem, or the superior glutæal may be absent, and its place taken by a branch from the femoral artery ; the inferior glutæal artery may, as in the fætus, constitute the main artery of the lower limb, and run distally to become continuous with the popliteal artery. Probably the arteria comitans nervi ischiadici represents the original continuity of the two vessels. Occasionally the lateral sacral arteries do not arise from the hypogastric trunks.

In some instances the obturator artery arises from the inferior epigastric artery instead of from the hypogastric. The condition is apparently due to obliteration of the usual origin of the obturator artery and to the subsequent enlargement of the anastomosing pubic branches of the obturator and inferior epigastric arteries. The course of the abnormal obturator artery is of importance. From its origin it descends, into the pelvis minor, on the medial side of the external iliac vein, and in the majority of cases on the lateral side of the crural ring, but in three-tenths of the cases, and more frequently in males than in females, it descends on the medial side of the ring:

The obturator artery sometimes gives off an accessory pudendal branch which passes along the side of the prostate, pierces the urogenital diaphragm, and terminates by dividing into the profunda artery of the penis and the dorsal artery of the penis. When this occurs the internal pudendal artery is small, and it terminates in the artery to the bulb. Occasionally the accessory pudendal arises from the internal pudendal artery in the pelvis, or from one of the vesical arteries.

The external iliac artery may be much smaller than usual, especially if the inferior glutæal artery persists as the main vessel of the lower limb. It may give off two deep circumflex iliac branches, a dorsal artery of the penis, a medial circumflex artery of the thigh, or vas aberrans, and its deep circumflex iliac and inferior epigastric branches may arise at higher or lower levels than usual.

THE ARTERIES OF THE LOWER LIMB. The femoral artery is small, and ends in the profunda and circumflex branches, when the inferior glutæal artery forms the principal vessel of the lower limb. The profunda branch, which arises usually from the lateral side of the feinoral trunk, about 37 mm. (1} in.) distal to the inguinal ligament, may commence at a more proximal or a more distal level, and from the back or the medial side of the femoral trunk. In rare cases when the profunda arises at a more proximal level than usual it may cross anterior to the femoral vein, above the entrance of the great saphenous vein, after which it passes distally and laterally posterior to the femoral vessels Johnston, Anat. Anz., Bd., 42, 1912). Absence of the profunda has been noted, and in those cases the branches usually given off by it spring directly from the femoral artery.

The femoral artery may be double for a portion of its extent, or it may be joined by a vas aberrans given off from the external iliac artery. In addition to its ordinary branches, it may furnish one or both of the circumflex arteries of the thigh, and sometimes it gives off, near the origin of the profunda, a great saphenous artery, such as exists normally in many mammals. This vessel runs distally through femoral trigone and the adductor canal, and accompanies the saphenous nerve to the medial side of the foot.

The deep circumflex iliac, the obturator, and the inferior epigastrio arteries are occasionally given off from the femoral.

The popliteal artery may exceptionally form the direct continuation of the inferior glutæal artery. It sometimes divides at a more proximal or more distal level than usual, and the division may be into either two or three branches ; if three terminal branches are present, they are the anterior and posterior tibial and the peroneal arteries, and if only two, either the anterior and posterior tibial, or the anterior tibial and the peroneal arteries.

Occasionally the artery is double for a short portion of its course, and it has been found to cross first posterior to the medial head of the gastrocnemius to the medial side of the knee, and then anterior to the medial head of the gastrocnemius to regain the popliteal fossa. The number of its branches may be reduced, or they may be increased by the addition of a vas aberrans which connects it with the posterior tibial artery. Its superficial sural branch may enlarge to form a well-ınarked small saphenous artery:

The posterior tibial artery may be small or altogether absent, its place being taken by branches of the peroneal artery; again, it may be longer or shorter than usual, in conformity with the more proximal or more distal division of the popliteal trunk. The peroneal artery is large, if either the anterior or the posterior tibial artery is small. The perforating branch of the peroneal is almost invariably large when the anterior tibial artery is small; in some cases, indeed, it replaces the whole of the dorsalis pedis continuation of the latter vessel ; in other however, only the lateral tarsal and arcuate branches are so replaced. The peroneal sometime arises from a stem common to it and the anterior tibial artery.

The anterior tibial artery may be absent, its place being taken by branches of the posterior tibial and peroneal arteries. It is longer than normal when the popliteal artery divides at a more proximal level than usual, and in those cases it may pass either posterior or anterior to the popliteus muscle. Occasionally the anterior tibial artery and its dorsalis pedis continuation are larger than normal, and the terminal part of the dorsalis pedis takes the place, more or less completely, of the lateral plantar artery.

The medial plantar artery is sometimes very small, and it may be absent; its place is taken by branches of the dorsalis pedis or lateral plantar arteries. The lateral plantar artery also may be small or absent, the plantar arch being formed entirely by the dorsalis pedis.

ABNORMALITIES OF VEINS.

Abnormalities or variations of veins are as frequently met with as those of arteries, and thes are due to similar causes.

THE SUPERIOR VENA CAVA. The superior vena cava may develop on the left side instead of on the right. This peculiarity is due to the persistence of the left duct of Cuvier instead of that on the right side, and it is associated with absence of the coronary sinus, which is replaced by the lower part of the left superior vena cava. An exceptional case is recorded in which the opening of the coronary sinus into the heart was obliterated, and the cardiac veins terminated in a trunk which passed upwards to the left innominate vein. This trunk was obviously formed by enlargement of the left duct of Cuvier and the lower part of the left anterior cardinal vein. Not very uncommonly, as the result of the persistence of both ducts of Cuvier, there are two superior venæ cavæ, the transvert anastomosis which usually forms the left innominate vein being small or entirely absent. In such cases the left innominate vein descends in the left part of the superior mediastinum, crosse the aortic arch, is joined by the left superior intercostal vein, and becomes the left superior vena cava; which descends anterior to the root of the left lung, and terminates in the lower and back part of the right atrium. It receives the great cardiac vein, and, turning to the back of the heart, replaces the coronary sinus. This arrangement is normal in many mammals. Occasionally in man the left superior vena cava terminates in the left atrium, and the coronary sinus, which represents a part of the sinus venosus, has been seen to have a similar ending; both these abnormal endings must be the result of malposition of the interatrial septum.

The vena azygos may be formed on the left side ; it then arches over the root of the left lung, and terminates in the left end of the coronary sinus. This is the normal arrangement in some' mammals, and it is due to the persistence of the left posterior cardinal vein and the let: duct of Cuvier.

Occasionally the azygos vein is the only vessel by which blood is returned to the heart from the lower limbs and the lower parts of the body walls. In such cases that portion of the inferior vena cava which usually extends from the right renal vein to the heart is absent and the azygie vein is the direct continuation of the inferior vena cava. This condition probably results from the absence of those parts of the inferior vena cava which are usually formed from the right vitelline and the right subcardinal veins, and to the enlargement of the whole of the supra. pelvic portion of the right posterior cardinal vein.

The hemiazygos and the accessory hemiazygos veins may be absent. In such cases each left intercostal vein opens separately into the vena azygos. On the other hand the hemiazygur and the accessory hemiazygos veins may form a continuous trunk which may open by a transver anastomosis into the azygos vein, or it may join the left innominate vein. When the hemiazygos ! and the accessory hemiazygos veins form a single trunk, which receives the left intercostal veins and opens into the left innominate vein, the condition is due to the persistence of the whole of the thoracic part of the left posterior cardinal vein and of the lower part of the left anterior cardinal vein.

Cases also occur in which the thoracic part of the posterior cardinal vein is representel by three instead of two stems, either the hemiazygos or the accessory lemiazygos vein being represented by two vessels.

The internal jugular vein may be either smaller or larger than normal. In either case wikpensatory changes in size occur in the transverse sinus and internal jugular vein of the opposite side, or in the external and anterior jugular veins of the same side.

The external jugular vein is sometimes absent, or it may be smaller than usual; in bil cases either the anterior or the internal jugular vein is enlarged. In some of the cases in which the external jugular vein is small it receives no communication from the posterior facial vell, but is merely the continuation of the posterior auricular vein. On the other hand, it may be enlarged, and receive the whole of the posterior facial vein.

The anterior jugular vein may be absent, or it may be unusually large, especially in t. lower part of its extent, and after it has received an occasional tributary from the common facral vein.

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