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 frorn 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 authoy, almost equally well to the pig. The same account was 
subsequently somewhat amplified and published by Rathke ('32, p. 82) 
in a second paper. In both these papers the postcardinals are called 
posterior ven® cave (hintere Hohlvenen), for Rathke 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 
