No. 399. ] FREQUENCY OF ABNORMALITIES. IgI 
iliac vein (* Connecting branch"), which is normal in every 
respect and has, as is usually the case in the cat, the middle 
sacral vein (5) opening intoit. The ilio-lumbar veins (6), which, 
usually in the cat, open into the common postcava (1), in this 
instance (Fig. 4), open into separate vessels on the right and 
left side, respectively. That the vessel (17) on the left side, 
into which the ilio-lumbar vein (6) opens, is the left posterior 
cardinal vein and not the so-called * Connecting branch" is 
evidenced by the fact that the vein in question persists in com- 
mon with the left common iliac (* Connecting branch"). In 
addition to this, the left posterior cardinal (17), in Fig. 4, lies 
ventrad of the aorta at the point where it unites with the cor- 
responding vein of the opposite side, a circumstance character- 
istic of the left posterior cardinal vein.! 
The “Connecting branch" is not usually present when the 
left posterior cardinal persists. Its absence, under these condi- 
tions, is undoubtedly due to the circumstance that the left pos- 
terior cardinal vein, during all stages of development, continues 
to collect the blood from the left hind extremity, which makes 
uncalled for the development of a vessel to share in its function. 
When for some reason the ** Connecting branch " does persist, 
together with the left posterior cardinal, so far as the writer's 
experience extends, one vessel is usually developed at the 
expense of the other. This idea is well borne out by a com- 
parison of Figs. 1, 3, and 4. 
In the type of abnormality represented by Fig. 1, the “ Con- 
necting branch ” is absent, and the left posterior cardinal per- 
sists as far forward as the kidneys. 
In Fig. 3 the left posterior cardinal is likewise persistent as 
far forward as the kidneys, but the ‘Connecting branch," 
though present, collects little, if any, of the blood from the left 
hind extremity. 
In Fig. 4 both vessels persist, but the left posterior cardinal is 
insignificant, while the “ Connecting branch " is normal in every 
respect, and undoubtedly, on account of its large relative size, 
is the chief collector of the blood from the left hind extremity. 
1 In the preparation represented by Fig. 4 the common internal iliac artery (10) 
is unusually long. Compare with corresponding arteries in Figs. 1, 3, 6. 
