THE VASCULAR SYSTEM 229 



normal post-caval. It would therefore appear that the post-caval vein 

 is normally formed by a dorsal extension of the right hepatic vein 

 anastomosing with the fused inter-renal portion of the post-cardinals. 

 The dorsal extension of the right hepatic vein has been named by 

 Hochstetter, who was the first to record the abnormality referred to, 

 the 'Mittelstiick' of the post-caval. The post-caval vein is to be 

 regarded therefore rather as a modification of existing vessels than 

 as a new structure. 



B. The post-cardinal veins (v.card.p.). 



Venae vertebrales ....... Funk. 



Veines de I'oviducte ....... Rusconi. 



Venae azygoi ....... Hochstetter. 



In view of the above considerations the post-cardinal system should 

 logically be regarded as arising from the posterior end of the kidneys 

 as a median vessel, but since this section has already been included 

 under the post-caval vein it will be described from the point where 

 the post-cardinal veins separate from the latter vessel. 



As already mentioned, the post-cardinal system separates from the 

 post-caval as a single vessel which almost immediately divides into 

 two, and these pass anteriorly on either side of the dorsal aorta. One 

 case has been found in which the right post-cardinal passed dorsal 

 to the aorta to reach the right side, but this is abnormal, the veins 

 usually remaining ventral to the artery. This division takes place at 

 the anterior end of the kidneys, and occasionally a remnant of the 

 primitive connexion between the post-cardinals and Jacobson's vein 

 may persist. 



During their course alongside the aorta the post-cardinal veins re- 

 ceive tributaries from the muscles of the back. These join the main 

 veins at about the centre of each vertebra and appear at first sight 

 to enter alternately from the right and left sides, but on closer inspec- 

 tion each tributary is found to collect from both sides. The tributary 

 from the side opposite to that of the main factor passes ventrally 

 between the muscle and the vertebra, and then crosses over the middle 

 of the vertebra and dips dorsalwards again between the vertebra and 

 the muscle on the other side to join the main factor within the sub- 

 stance of the muscle. A little dissection is therefore necessary to dis- 

 play the relations. Besides these dorsal factors the post-cardinal veins 

 also receive some three or four anterior oviducal veins from the an- 

 terior portion of the oviduct connecting them with the longitudinal 

 oviducal vtin (v.od.long.). This vessel runs along the ventral aspect 

 of the anterior three-quarters of the oviduct (i.e. along the portion 



