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developed and superadded transverse communicating channels, which are formed between the more primitive longitudinal anastomoses.
The primitive dorsal longitudinal anastomosing channels include on each side (1) the anterior cardinal vein, (2) the posterior cardinal vein, and (3) the duct of Cuvier; the last-named vessel, which opens into the sinus venosus of the primitive heart, is, originally, part of the anterior cardinal vein; it becomes enlarged and receives a special name after the union of the posterior with the anterior cardinal vein.
The cardinal veins return blood not only from the limbs and body wall, but they are also, in the early stages, the only vessels by which blood is returned from the derivatives of the intermediate cell tract, .e. the kidneys, the genital glands, and the suprarenal glands. At a later period other longitudinal anastomoses, called the subcardinal veins, appear, and into these a large part of the blood from the derivatives of the intermediate cell tract is poured. It is from these vessels, and from the transverse communications which are established between the cardinal and subcardinal veins of opposite sides, that the chief veins of the head and neck and the body are formed; there is in addition, however, a later-formed vessel, the upper part of the inferior vena cava, which is developed independently of the veins previously mentioned. Moreover, it must not be forgotten that the veins of the extremities are, like the extremities themselves, secondary structures, and that they are developed at a later period than the veins of the trunk, with which, however, they ultimately communicate.
In the light of these facts the morphology of the chief veins of the head and neck, the trunk and limbs may now be considered.
The cavernous sinuses are remnants of the primary head vein. The other blood sinuses of the cranium are either secondarily formed vessels, or anastomoses between the tributaries of the anterior cardinal veins, or anastomoses between those tributaries and other newly formed veins.
The internal jugular veins are also portions of the anterior cardinal veins.
The right innominate vein is a part of the right anterior cardinal vein. A small part of the left innominate vein is formed from the left anterior cardinal vein, the greater part is derived from a transverse anastomosis between the two anterior cardinal veins.
Other remnants of the anterior cardinal veins are the upper parts of the superior vena cava and left superior intercostal vein.
The basilic vein and its prolongations, the axillary vein and the subclavian vein, are derived from the ulnar or post-axial primitive veins of the superior extremities. The external jugular vein is a secondary formation, and the cephalic vein is the radial or pre-axial vein of the upper extremity which opens first into the external jugular vein and at a later period into the axillary vein.
The superior vena cava represents the lower part of the anterior cardinal vein and the right duct of Cuvier, and the oblique vein of the left atrium represents the left duct of Cuvier.
The azygos vein is the upper or cephalic part of the right posterior cardinal vein, and the vertical parts of the hemiazygos and accessory hemiazygos veins are remnants of the left posterior cardinal vein, whilst the transverse portions of the hemiazygos and accessory hemiazygos veins represent transverse anastomoses between the posterior cardinal veins.
The inferior vena cava is a compound structure representing parts of five different structures. Its upper end is the transformed cephalic end of the right vitelline vein. The portion posterior to the liver is a secondary outgrowth from the right vitelline vein. The part between the liver and the right renal vein represents a part of the right subcardinal vein and an anastomosis between it and the posterior cardinal vein, and the remainder is a portion of the right posterior cardinal vein.
The right common iliac vein is a part of the right posterior cardinal vein, but the left is a compound structure. Its lower part probably represents a portion of the left posterior cardinal vein, but the greater part is a persistent transverse anastomosis between the posterior cardinal veins.
The hypogastric veins are remnants of the posterior cardinal veins.
The popliteal and the inferior gluteal veins are remnants of the primitive fibular vein of the lower limb, and the external iliac vein is the trunk formed by the union of the tibial and the deep veins of the lower limb which are secondary formations.
Visceral Veins.-The portal vein represents portions of both vitelline veins and of the middle anastomosis between them.
The right gastric vein is a splanchnic ventral longitudinal anastomosing vein. The left gastric vein is partly a ventral and partly a dorsal splanchnic longitudinal anastomosis, and the superior and inferior mesenteric veins are dorsal splanchnic longitudinal venous anastomoses, the splenic vein being merely a tributary from a lymphoid organ developed in the dorsal meso-gastrium.
The anterior facial vein is a combination of somatic and splanchnic veins of several segments, and the internal maxillary vein is probably of similar nature. The thyreoid and bronchial veins return blood from organs developed from diverticula from the walls of the alimentary canal; they are, therefore, more or less modified splanchnic veins; so also apparently are the vesical and the middle and inferior hæmorrhoidal veins.
The cardiac veins are simply "vasa vasorum," and they belong therefore to the splanchnic group of vessels, but it is impossible to say whether they are segmental or intersegmental. The coronary sinus into which they open is a portion of the sinus venosus of the heart, and therefore of an originally segmental vessel."
The hepatic veins are parts of the primitive vitelline veins; and the pulmonary veins are splanchnic veins returning blood from a diverticula of the gut.
It is noteworthy that some parts of the splanchnic venous system, i.e. the portal vein and the coronary sinus, are portions of the most primitive vascular system, and that others,-the thyreoid, bronchial, mesenteric, vesical, and hæmorrhoidal veins-appear to belong to a somewhat secondary group of splanchnic veins of combined segmental and intersegmental character; more over, some of the secondary group of veins open into the primary splanchnic veins, eg. the superior and inferior mesenteric into the portal vein; some open into the dorsal longitudinal anastomosing veins, e.g. the vesical and hæmorrhoidal veins open into the cardinal veins, which are intersegmental anastomoses; others again open into the internal jugular, which is part of the anterior cardinal vein.
Veins of the Limbs. The veins of the limbs, like the arteries, were probably at one time intersegmental in character, but we have no indisputable proof that this was the case. Looked at from an embryological standpoint, the most primitive limb veins are a superficial distal arch and a post-axial trunk vein in each extremity; at a later period digital veins are connected with the distal arch, and a pre-axial trunk is formed. In the upper extremity the distal arch and its tributaries remain as the dorsal venous arch and the digital veins, and the post-axial vein becomes the basilic, axillary, and subclavian veins. The pre-axial vein of the upper extremity is represented in the adult by the cephalic vein; the latter vessel originally terminated in the external jugular vein, above the clavicle, the union with the axillary portion of the post-axial vessel being a secondary condition; the primary condition is, however, frequently retained in man, and is constant in many monkeys. The anastomosis between the pre-axial and post-axial veins in the region of the elbow, and the connexion of the anastomosing channels, is brought about by newly-formed vessels of secondary character.
The distal arch in the lower extremity and the tributaries connected with it remain in the adult as the dorsal venous arch of the foot and the digital veins. The post-axial vein becomes the small saphenous vein, which was originally continued proximally as the popliteal and inferior gluteal veins to the hypogastric portion of the posterior cardinal vein.
The pre-axial vein of the lower limb becomes the great saphenous vein, which is continued proximally to the cardinal portion of the left common iliac vein as the proximal part of the femoral and the external iliac veins.
The venæ comites of the arteries in both the upper and lower extremities are secondarily developed vessels which become connected with the upper portions of the pre-axial venous trunks.
ABNORMALITIES AND VARIATIONS OF THE VASCULAR
Abnormalities are of special interest to the anatomist because of their morphological significance, and the vascular system is, perhaps more than any other, rich in such abnormalities, many of which are of great practical importance.
With the exception of those irregularities which are directly due to the effect of morbid conditions and external influences, all abnormalities are the result of modifications of normal developmental processes. The exceptions referred to are, however, very numerous; thus disease and external influences may lead to the obliteration of vessels, a condition which is invariably associated with the enlargement of collateral vessels, and it will be obvious that abnormalities so produced may occur in almost any situation.
Abnormalities which are determined by, or are dependent upon, modifications of the usual developmental processes are of greater interest. In the human subject they are generally due either to the retention of conditions which, normally, are only transitory in ontogenetic develop ment, or to the acquirement of conditions which, though not as a rule present at any time in man, occur normally in some animals.
There are, in addition, other variations from the normal, such as the division of the axillary artery into radial and ulnar branches; the higher or lower division of the brachial artery; formation of "vasa aberrantia," e.g. of long slender vessels connecting the axillary or brachial to the radial, ulnar, or interosseous arteries; the altered position of certain vessels, eg. the transference of the subclavian artery to the front of the scalenus anterior, or of the ulnar artery to the front of the superficial flexor muscles; all of which, though undoubtedly due to alterations of ordinary developmental processes, still do not represent any known conditions met with, either temporarily or permanently, in man or in other animals. Their occurrence cannot at present be adequately explained, and their retention in the adult is entirely dependent upon their utility.
To the first and the last of these different groups of abnormalities-it is not necessary to refer further, whilst with regard to the rest it will be sufficient to indicate those of greatest importance. They can only, however, be fully understood and explained on the basis of a comprehensive knowledge of the development and morphology of the vascular system, to the chapters on which the reader is referred.
ABNORMALITIES OF THE HEART.
The heart may be transposed from the left to the right side of the body, a condition which is usually associated with general transposition of the viscera, and with the presence of a right instead of a left aortic arch.
The external form of the heart does not as a rule vary much, but occasionally the apex is slightly bifid, a character it normally possesses at an early stage of its development, and which is retained in the adult in many cetaceans and sirenians. The internal conformation of the heart deviates from the normal much more frequently; more particularly is this the case with regard to the septa which separate the right from the left chambers. The interatrial septum may be entirely absent, as in fishes; it may be fenestrated and incomplete, as in some amphibians; or the foramen ovale may remain patent, as in amphibians and reptiles.
The interventricular septum may be absent, as in fishes and amphibians, or incomplete, as in reptiles; when incomplete, it is usually the "pars membranacea septi" which is deficient, but perforations are occasionally found in the muscular portion.
The communication between the infundibular part of the right ventricle and the body of the ventricle may be constricted or the infundibular part may be entirely cut off from the remainder of the cavity.
ABNORMALITIES OF ARTERIES.
The pulmonary artery and the aorta may arise by a common stem, as in fishes and some amphibians, and the common stem may spring either from the right or the left ventricle, or from both. In these cases the truncus arteriosus has remained undivided, and the normal position of the interventricular septum in relation to the lower orifice of the aortic bulb has been altered. Again, owing to malposition of the aortic septum, the pulmonary artery may spring from the left ventricle and the aorta from the right ventricle. In some cases the root of the pulmonary artery is obliterated, and the blood passes to the lungs along the patent ductus arteriosus.
Occasionally the arch of the aorta is on the right side instead of the left, a condition which is normal in birds. More rarely there are two permanent aortic arches, right and left, as in reptiles; the oesophagus and trachea in these cases are enclosed in a vascular collar, the two arches unite dorsally, and the beginning of the descending aorta is double. Quite independent of this condition, however, the two primitive dorsal aortæ sometimes fail, either altogether or partially, to unite together, and the descending aorta is accordingly represented, to a corresponding extent, by two tubes. A more common, though still rare, form of double aorta is that due to the persistence, in whole or in part, of the septum formed by the fused walls of the primitive dorsal aortæ from which the descending aorta is developed.
The length of the descending aorta is determined largely by the extent to which fusion of the two primitive aorta takes place. Accordingly, when this deviates from the normal, the termination of the descending aorta is at a correspondingly higher or lower level than usual, and resulting from this the lengths of the common iliac arteries are almost invariably proportionately modified. The bifurcation of the aorta may be as low as the fifth lumbar vertebra; less frequently it is higher than usual; it is rare, however, to find it as high as the third, and still more rare to find it at the level of the second, lumbar vertebra.
The aorta, instead of bifurcating into two common iliac arteries, may terminate in a common iliac artery on one side and a hypogastric artery on the opposite side, the external iliac artery on the irregular side arising, at a higher level, as a branch of the aortic stem. This arrangement approaches the condition met with in carnivores and many other mammals, in which the aorta bifurcates into two hypogastric arteries, the external iliacs arising from the aorta at a higher level as lateral branches; it is probably due either to a fusion of the secondary roots of the umbilical arteries of opposite sides.
THE BRANCHES OF THE AORTA.
The coronary or cardiac arteries may arise by a single stem. When arising separately both may spring from the same aortic sinus; or again, their interventricular and circumflex branches may arise as distinct vessels from a single aortic sinus. This variability is not very remarkable, seeing that the arteries in question are merely enlarged "vasa vasorum raised to a position of special importance by the development of the heart.
The branches of the arch of the aorta are sometimes increased and sometimes decreased in number.
The highest number recorded is six, viz., right subclavian, right vertebral, right common carotid, left common carotid, left vertebral, and left subclavian. Apparently this condition is the result of the absorption into the arch of the innominate artery and of the roots of the subclavian arteries, to points beyond the origins of the vertebrals. By variations of this process of absorption other combinations may be produced; thus, instead of the roots of the subclavian arteries being absorbed, the right common carotid and innominate arteries may alone be absorbed, in which case the five following branches spring separately from the arch of the aorta: right subclavian, right external carotid, right internal carotid, left common carotid, and left subclavian. The trunk most commonly absorbed is the initial part of the left subclavian; the
number of branches then arising from the arch of the aorta is four, the additional vessel being the left vertebral, which arises between the left common carotid and the left subclavian. Occasionally the usual three branches from the arch are increased to four by the formation of a new vessel, the "thyreoidea ima." This may be placed between the innominate and left carotid trunks, in which case it represents a persistent ventral visceral branch from the ventral root of the fourth left aortic arch; in other cases the thyreoidea ima springs from the innominate artery and represents a ventral visceral branch of the ventral root of the fourth right arch. Very rarely the right vertebral artery arises separately, and forms a fourth branch of the arch of the aorta, the rest of the branches being normal. This condition cannot be accounted for by any modifica tion of the ordinary developmental processes. It may possibly be due to the persistence of an irregular or unimportant anastomosis between the ventral root of an aortic arch and the seventh somatic segmental artery.
Decrease in the number of branches from the arch of the aorta is most frequently due to fusion of the ventral roots of the fourth aortic arches, the result being that a stem is formed common to the right subclavian and the right and left common carotid arteries; whilst the left subclavian, arising separately, is the only other branch which springs from the arch of the aorta. If the fusion of the ventral roots proceeds further and includes those of the third arches, the result, as regards the branches given off from the arch of the aorta, is the same, viz., there is a common stem for the right subclavian and both carotids, and a separate left subclavian trunk; but the common stem now gives off the right subclavian artery, and then continues as a single vessel for some distance before it divides into the two common carotids, of which the left crosses in front of the trachea. This arrangement is common in many quadrumana and in some other mammals.
It is only in rare cases when the number of branches from the arch of the aorta is reduced to two, that these consist of a right subclavian artery and of a single stem common to the two carotids and the left subclavian artery. In such cases, however, the right common carotid crosses in front of the trachea, and the variation is one of practical importance, but it does not appear to exist as a normal condition in any mammal. Probably it is due to fusion of the ventral roots of the fourth aortic arches, with absorption of the left fourth arch and the left subclavian into the stem so formed, whilst the right subclavian is relatively displaced. The two common carotids may arise by a common stem, and the left subclavian arise separately from the arch of the aorta, whilst the right subclavian springs from the descending aorta. This arrangement probably results from the disappearance of the fourth right arch, the fusion of the ventral roots of the fourth arches of opposite sides and the persistence of the dorsal roots of the right fourth and sixth arches.
Sometimes two innominate arteries, right and left, replace the usual three branches of the arch of the aorta. This is the normal arrangement in bats, moles and hedgehogs. It is obviously the result of the disappearance of that portion of the arch which intervenes between the left carotid and left subclavian arteries, and the consequent fusion of these two vessels.
In a similar way may be explained the rarer condition in which the three ordinary branches of the arch arise by one single stem, which divides into right and left innominate arteries. In most ruminants, in the horse and in the tapir, this arrangement is constant.
It will be evident that other combinations and modifications may be met with in the branches of the arch of the aorta as the result of fusions and absorption.
The right subclavian or the right vertebral may spring from the commencement of the descending aorta.
The bronchial arteries obviously correspond to splanchnic arteries and their continuations to diverticula from the walls of the gut; therefore the usual origin of the right bronchial artery from the first right aortic intercostal artery must result from the persistence of an anastomosis between a splanchnic artery and the first part of a somatic intersegmental artery; the origin of the right from the upper left bronchial artery, which sometimes occurs, is due to the fusion of the roots of two splanchnic arteries. The occasional origin of a bronchial vessel from an internal mammary artery can result only from the persistence and enlargement of an anastomosis between a splanchnic artery and the ventral branch of a somatic segmental artery. The origin of a bronchial branch from a subclavian artery may have the same or a different significance on opposite sides of the body. A bronchial artery arising from the left subclavian artery corresponds with the origin of the right bronchial artery from the first aortic intercostal artery; it is due to the persistence of an anastomosis between a splanchnic artery and the root of a somatic intersegmental artery; and the origin of a bronchial artery from a right subclavian artery may be due to a similar cause. It may, on the other hand, be due to the enlargement of an anastomosis between a splanchnic branch of the descending aorta and a splanchnic branch of the fourth right aortic arch. When, as occasionally happens, the bronchial artery arises from the inferior thyreoid, it is due to the persistence and enlargement of an anastomosis between splanchnic arteries.
Intercostal Arteries.-Variations of the intercostal arteries are not very common, but they are significant and interesting. Corresponding vessels of opposite sides may arise from a common stem which has been formed by the fusion of the roots of two somatic intersegmental arteries after or simultaneously with the fusion of the primitive dorsal aorta. The number of intercostal arteries may be reduced, one artery supplying two or more intercostal spaces; in these cases the roots of origin of some of the somatic intersegmental arteries in the thoracic region have disappeared, and the precostal anastomoses between their ventral branches have persisted. Occasionally the number of the aortic intercostal arteries is increased, an additional artery
being given to the second intercostal space, which is usually supplied by the superior intercostal artery; this is brought about by the persistence of the root of the tenth somatic intersegmental artery and the disappearance of the precostal anastomosis between the ventral branches of the ninth and tenth somatic intersegmental arteries. Very rarely the first aortic intercostal artery sends a branch upwards between the necks of the ribs and the transverse processes of the upper thoracic region; this branch supplies the upper intercostal spaces, the superior intercostal artery being small or absent, and it terminates by becoming the profunda cervicis artery. It is due to the persistence of the postcostal anastomoses in the upper thoracic region, and is a repetition of a condition regularly present in some carnivores.
There are no very important variations of the œsophageal, pericardial, and mediastinal arteries.
Lumbar Arteries. Variations of the lumbar arteries are very similar to those of the intercostal arteries, and they are due to similar causes. The lumbar arteries of opposite sides may arise by common stems from the back of the aorta; and the last pair of lumbar arteries may arise in common with the middle sacral artery. Further, a lumbar artery may have its area of distribution extended into the adjacent segment.
The inferior phrenic arteries are very variable; they may arise by a common trunk either from the cœliac artery or from the aorta; they may arise separately either from the aorta or from the cœliac artery and more commonly from the latter vessel; or again, one may spring from the aorta or coeliac artery, and the other from the coronary, renal, or even from the superior mesenteric artery.
The middle sacral artery usually springs from the back of the aorta above its bifurcation; it may be considerably above, or more rarely it may spring directly from the bifurcation. Not infrequently it arises from the last lumbar artery or from a stem common to the two last lumbar arteries, and occasionally it arises from a common or internal iliac artery. Sometimes it gives off the last pair of lumbar arteries, and, in a few cases, an accessory, renal, or a hæmorrhoidal branch arises from it. The vessel is not always present, it may be double, entirely or in part, and it may bifurcate at its termination.
The renal arteries frequently deviate from the normal arrangement.
The arteries of opposite sides may spring from a common stem, or there may be two or more renal arteries on one or both sides. The accessory arteries are more common on the left than on the right side, and an accessory artery arising below the ordinary vessel is more common than one arising above it.
Accessory renal arteries may be derived not only from the aorta, but also from the common iliac or hypogastric arteries; they have been described as arising also from the inferior phrenic, spermatic, lumbar, or middle sacral arteries, and even from the external iliac artery. As the kidney is developed in the region of the first sacral vertebra, and afterwards ascends to its permanent position, it is not surprising that it occasionally receives arteries from the main stem of more than one of the segments of the body through which it has passed, and it is usually found that the lower the position of the kidney in the abdomen the more likely it is to receive its arteries from the lower part of the aorta or from the common iliac arteries. The accessory renal arteries which spring from the inferior phrenic, the spermatic, and lumbar arteries can only be the result of the persistence and enlargement of anastomosing channels between the renal and either another intermediate visceral, or a somatic artery.
The testicular or ovarian arteries may be double on one or both sides; the arteries of the two sides may spring from a common trunk, or each may arise from the renal, accessory renal, or suprarenal arteries. The right artery may pass behind instead of in front of the inferior vena cava. The spermatic and ovarian arteries arise from the upper lumbar portion of the aorta, because the testes and ovaries are developed in and obtain their arterial supply in that region, and the vessels are elongated as the testes and ovaries descend to their permanent positions. The occurrence of two spermatic arteries on one side is probably an indication that the testis was developed in two segments of the body. The origin of a spermatic artery from a renal or suprarenal artery is due to the obliteration of the root of the original vessel and the enlargement of an anastomosis between the intermediate visceral arteries of adjacent segments. The cœliac artery may be absent, its branches arising separately from the aorta or from some other source. Sometimes it gives off only two branches, usually the left gastric and splenic, and occasionally it gives four branches, the additional branch being either a second left gastric artery or a separate gastro-duodenal artery.
The hepatic artery may spring directly from the aorta or from the superior mesenteric artery, and the left hepatic artery arises occasionally from the left gastric artery. Accessory hepatic arteries are not uncommon, and they originate either from the left gastric, superior mesenteric, renal, or inferior mesenteric artery.
The left gastric artery is occasionally double; it may spring directly from the aorta, and it may give off the left hepatic or an accessory hepatic artery.
The splenic artery may arise from the middle colic, from the left hepatic, or from the superior or inferior mesenteric artery.
The superior mesenteric artery may be double, and it may supply the whole of the alimentary canal from the second part of the duodenum to the end of the rectum, the inferior mesenteric artery being absent. In addition to its ordinary branches it may give off a hepatic, a splenic, a pancreatic, a gastric, a gastro-epiploic or a gastro-duodenal branch. Very rarely it gives off an omphalo-mesenteric branch, which passes to the region of the umbilicus and becomes connected with capillary vessels in the falciform ligament of the liver.