Charles F. W. McClure 175 
terminates in birds at the-caudal end of the pars subcardinalis which, in 
addition to the portion which is developed between the heart and the 
hepatico-subeardinal junction, constitutes the postcava in the adult. 
The subcardinal system is represented in the adult bird by the follow- 
ing veins: The left anterior revehent vein forms the left suprarenal ; the 
right anterior revehent probably atrophies; the section of the right sub- 
cardinal included between the hepatico-subcardinal junction and the 
original anastomosis between the two subcardinals forms the pars sub- 
cardinalis of the posteava, and the two posterior revehent veiris enter 
into the formation of the genital veins. 
In birds the azygos veins, as met with in the rabbit, are not developed ; 
their place being taken, for the most part, by the newly formed posterior 
vertebral veins (Text Fig. 5) which open, on each side (chick) into the 
precava in common with the internal jugular and the subclavian veins 
(see McClure, 03, page 381). 
In the rabbit the fundamental plan of the venous system, as repre- 
sented by Text Fig. 6, undergoes a number of changes before the adult 
stage is reached. 
The section of the right posteardinal vein which hes caudad of its anas- 
tomosis with the pars subeardinalis of the postcava, after forming a col- 
lateral channel on the medial side of the ureter (Text Fig. 6), consti- 
tutes in the adult that portion of the postcava which lies caudad of the 
renal veins (Text Fig. 7). The corresponding section of the left post- 
cardinal atrophies with the exception of a small portion at its proximal 
end which usually persists as the left spermatic vein, and of a portion at 
its caudal end which fuses with the posteardinal of the opposite side to 
form the common internal iliac vein. The postcava of the adult rabbit 
is thus seen to be a compound vessel which is formed from four distinct 
sets of veins: The vena hepatica communis, the hepatic sinusoids and 
portions of the right subeardinal and right postcardinal veins. 
Correlated with the completion of the adult postcava in the rabbit a 
number of changes, also take place in connection with the remaining por- 
tions of the postcardinal and subecardinal veins. The posteardinals which 
lie craniad of the level of the renal veins entirely disappear with the ex- 
ception of the proximal end of the vein of the right side which persists as 
the common trunk of the newly formed azygos veins. Also, with the ex- 
ception of a section of the right subeardinal which enters into the forma- 
tion of the adult postcava, a portion of the left subcardinal which forms 
the left suprarenal, and possibly a portion of the right which forms the 
right suprarenal vein (Hochstetter, 03), the subcardinal veins are com- 
pletely lost at the time the adult stage is reached. 
