No. 399. ] FREQUENCY OF ABNORMALITIES. 197 
of a secondary union of the veins behind the artery or nerve 
which passes between them. 
As previously stated, the lumbar artery, which, in two cases, 
was found to penetrate the postcava, was the one situated just 
anterior to the ilio-lumbar veins. In the case represented by 
Fig. 7, this artery (19) lies between the common iliac veins, near 
their point of union. If in this instance the veins should fuse 
behind the artery, a foramen would be formed which resembles 
in every respect the two met with in the postcava. 
So far as could be ascertained by the writer, the number of 
abnormalities of the arterial system, in general, was small. 
This was also the case with the veins of the 
neck and fore-limb regions, which in no way 
approximated those found in connection with 
the postcava and its tributaries. 
. The arterial abnormalities were chiefly 
confined to the ilio-lumbar and iliac arter- 
ies, an example of the latter being figured 
below (Fig. 9). 
Oné conclusion which may be drawn from 
a perusal of the preceding pages is that, in 
the cat, the veins of the lumbar and pelvic 
regions appear to be more liable to variation 
than those of any other region of the body. 
Whether this variation may be the result, 
as some might maintain in virtue of the 
Shifting character of the lumbar vertebra, 
of a general instability of this region is an open question. 
The greater frequency, however, with which abnormalities 
of the postcava and its tributaries occur, whatever the causes 
may be which produce them, may possibly be explained on the 
following grounds: The veins of the lumbar and pelvic regions 
possess a marked primitive arrangement in the embryo, which 
differs from that of the adult. In the transition from the em- 
bryonic to the adult condition, the veins, on account of the 
changes which they have to undergo, thus readily lend them- 
Selves to variations when, for any cause, their normal develop- 
ment is interrupted. 
F 
numbers, see Fig. r. 
