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DEVELOPMENT OF THE VEINS. 1049 



eveloped 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 inter- 

 mediate cell tract, i.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 hemi- 

 azygos 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 

 sardinal vein, but the greater part is a persistent transverse anastomosis between the posterior 

 .irdinal 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 

 leep veins of the lower limb which are secondary formations. 



Visceral Veins. The portal vein represents portions of both vitelline veins and of the middle 

 inastomosis 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 

 ;he superior and inferior mesenteric veins are dorsal splanchnic longitudinal venous anastomoses, 

 ihe splenic vein being merely a tributary from a lymphoid organ developed in the dorsal 

 neso-gastrium. 



The anterior facial vein is a combination of somatic and splanchnic veins of several segments, 

 md the internal maxillary vein is probably of similar nature. The thyreoid and bronchial veins 

 'eturn blood from organs developed from diverticula from the walls of the alimentary canal ; they 

 ire, therefore, more or less modified splanchnic veins ; so also apparently are the vesical and the 

 niddle and inferior hsemorrhoidal veins. 



The cardiac veins are simply " vasa vasorum," and they belong therefore to the splanchnic 

 ;roup of vessels, but it is impossible to say whether they are segmental or intersegmental. The 

 oronary sinus into which they open is a portion of the sinus venosus of the heart, and therefore 

 f an originally segmental vessel. 



