IOI2 A MANUAL OF ANATOMY 



anterior cardinal vein, and the trunk so formed is called the duct of Cuvier 

 (right and left), which opens into the sinus venosus. 



Amongst other tributaries, the posterior cardinal veins receive segmental 

 veins, such as the lumbar and intercostal veins, and veins from the inter- 

 mediate cell-mass, and they are connected with each other by transverse 

 retro-aortic vessels, which constitute the transverse retro-aortic posterior 

 cardinal anastomoses. One of these, situated in the lower lumbar region, 

 enlarges and forms the transverse iliac vein. This vessel extends some.what 

 obliquely from the left posterior cardinal vein, at a point a little above the 

 place where it receives the left external iliac vein, to the right posterior car- 

 dinal vein, its incHnation in crossing being upwards. The transverse iliac 

 vein gives rise to the greater part of the left common iliac vein, the com- 

 mencement of the latter vessel being formed by that portion of the left posterior 

 cardinal vein which intervenes between the place where it receives the left 

 external iliac vein and the left extremity of the transverse iliac vein. After 

 the formation of the transverse iliac vein, the blood from the left half of the 

 pelvis and left hind-limb is now diverted through the transverse iliac vein 

 into the right posterior cardinal vein. 



In the thoracic region, two (sometimes one) of the retro-aortic posterior 

 cardinal anastomoses persist as the lower and upper transverse azygos veins. 



Between the anterior cardinal veins, and also between the posterior cardinal 

 veins, there are, as stated, several transverse anastomotic vessels. Four 

 (sometimes three) of these are of special importance, and may be here summed 

 up as follows, in order from above downwards : 



1. Transverse jugular between anterior cardinals, forming the left in- 



nominate vein. 



2. Upper retro-aortic posterior cardinal anastomosis in thorax, forming 



the upper transverse azygos vein. 



3. Lower retro-aortic posterior cardinal anastomosis in thorax, forming 



the lower transverse azygos vein. 



4. Transverse iliac, between posterior cardinals in lower lumbar region, 



forming greater part of left common iliac vein. 



The posterior cardinal veins constitute a somewhat circuitous route to the 

 heart. Moreover, in the region of the Wolffian bodies they become, to a 

 certain extent, subdivided into sinusoids. New and more direct channels 

 are therefore formed. 



New Secondary Venous Channels. — As the development of the hind-Hmbs 

 proceeds, two new secondary venous channels are formed, which are auxiliaries 

 of the posterior cardinal veins, and relieve the obstruction caused by the 

 developing Wolffian bodies. They are called the subcardinal veins (F. T. 

 Lewis). Each subcardinal vein is formed by a longitudinal anastomotic 

 vessel, which connects the small veins passing from the mesentery to the 

 posterior cardinal veins, and it lies ventral to the mesial part of the Wolffian 

 body. The subcardinal veins run parallel with the posterior cardinal veins, 

 on their ventral aspect, and each extends from the caudal end of a posterior 

 cardinal vein to near its cephalic end, being connected with the posterior 

 cardinal at either extremity. Between these points the two veins (posterior 

 cardinal and subcardinal) are connected, at frequent intervals, by dorso- 

 ventral anastomoses. Each vein is also connected with its fellow of the 

 opposite side by transverse anastomoses, those between the posterior cardinal 

 veins being retro-aortic, whilst those between the subcardinal veins are pre- 

 aortic. The subcardinal anastomotic vessels, for the most part, disappear, 

 but one of them becomes large, and lies below the root of the superior mesen- 

 teric artery, in front of the aorta and opposite the future renal veins. It is 

 known as the great pre-aortic transverse subcardinal anastomotic vessel. It 

 also forms an indirect anastomosis between the adjacent parts of the pos- 

 terior cardinal veins. At the level of this anastomotic vessel each renal vein 

 opens into the corresponding posterior cardinal vein. 



One of the dorso-ventral anastomotic vessels on the right side connects 



