PARKER AND TOZIER : POSTCARDINAL VEINS IN SWINE. 139 



It will be recalled from the earlier part of this description that the 

 two components (postcardinal and accessory vein) which make up the 

 azygos and the hemiazygos were united at about the level of the tenth 

 rib. The hemiazygos from the region of the heart to the tenth rib is 

 therefore to be regarded as the persistent anterior portion of the left 

 postcardinal. As the corresponding part of the azygos has aborted, the 

 right postcardinal of this region is entirely absent. Consequently the 

 variable portion of this system — most of which lies posterior to the tenth, 

 or at least to the ninth rib — represents the parts derived from the 

 accessory veins. 



Although the main stem of the hemiazygos from the heart to the 

 region of the tenth rib has been stated to be derived exclusively from 

 the left postcardinal, it is possible that occasionally a portion of its 

 posterior extent may come from a fusion of both right and left post- 

 cardinals ; for in one instance we found between the levels of the ninth 

 and tenth intercostals (compare Fig. 4, C) an " island " formation which 

 was so narrow that the right and left components may be said to have 

 almost completely united. While the rareness of such cases makes it 

 improbable that a process of fusion is at all usual, the possibility of its 

 occurrence cannot be ignored, and, where fusion does occur, the incor- 

 poration of a part of the right postcardinal into what becomes the main 

 stem of the hemiazygos is at least a possibility. Aside from this, how- 

 ever, the right postcardinal certainly plays no part in the ultimate for- 

 mation of the system of veins under consideration. 



Historico-critical Remarks. 



The postcardinals of swine were first described by Rathke ('30, p. 64), 

 whose account, though mainly taken from the sheep, applies, according 

 to this author, almost equally well to the pig. The same account was 

 subsequently somewhat amplified and published by Eathke ('32, p. 82) 

 in a second paper. In both these papers the postcardinals are called 

 posterior venae cavse (hintere Hohlvenen), for Eathke believed at this 

 time that the right postcardinal persisted throughout its whole extent 

 as the adult postcava. He further believed that the thoracic portion of 

 the left postcardinal became the hemiazygos. This interpretation agreed 

 well with the fact that, as Rathke ('30, p. 67) pointed out, adult sheep 

 and pigs have no azygos veins, structures that might be supposed to 

 represent the right postcardinals. Stark, as we gather from the histor- 

 ical account given by Hochstetter ('93, p. 611), subsequently showed 



