VEINS AND LYMPHATICS IN TKAGULUS 213 



From these figures it appears that the ratio of the inter-renal 

 segment to the paired portion of the cava (cardinal collaterals) 

 is 1 to 1.6 in the 20 mm. embryo as against 1 to 0.75 in the adult 

 animal. In other words, if the increment of growth has remained 

 constant in the inter-renal segment, it has been reduced one 

 half in the cardinal collateral vessels while passing from the 20 

 mm. stage to adult. These facts make it clear that longitudinal 

 expansion of the inter-renal segment plays an important role in 

 determining the position and relations of the post-renal segment 

 of the cava. This assumption is further borne out by the fact 

 that the distance between the sex veins and the renal veins pro- 

 gressively increases in passing from the embryonic stages to the 

 adult. 



The pre- and post-cardinal veins 



The pre-cardinal veins may still be identified almost in their 

 entirety even in these late stages. In the greater part of the 

 neck they constitute a pair of channels, the internal jugular 

 veins. High up in their cephalic portion, they receive a short 

 trunk which has resulted from the confluence of the temporo- 

 facial and internal maxillary veins. This trunk represents the 

 external jugular vein. As the two internal jugular veins approach 

 the thorax they become confluent and form a single large vessel. 

 It is into this confluent element that the sub-clavian, cephalic, 

 internal mammary and vertebral veins enter. On passing into 

 the superior mediastinum the single vessel again becomes a double 

 channel. The vessel on the right is much the larger. This por- 

 tion of the jugular system represents the pre-cava which, after 

 becoming much reduced in size, enters the heart. The left chan- 

 nel proceeds further caudad and finally joins the left post-cardi- 

 nal vein to form the duct of Cuvier. 



The left post-cardinal vein participates in the formation of 

 the left azygos vein. During this process that redundant car- 

 dinal channel, earlier observed in the perimesonephroic plexus 

 and referred to as the intermediate element, seems to take a 

 prominent part. In the cephalic portion of the azygos major 

 vessel, the original post-cardinal channel determines the ultimate 



