196 THE AMERICAN NATURALIST. . [Vor. XXXIV. 
It is well known that the perforation of a vein by an artery 
or nerve is not, by itself, of unusual occurrence, but that it 
should occur with such frequency in the domestic cat, and in 
connection with so many other abnormalities, is deemed by the 
writer as worthy of mention, as it further emphasizes the 
unstable condition of the veins in the lumbar and pelvic 
regions. 
Whatever the cause may be which has produced this type of 
abnormality, it is probably the same for both nerve and artery, 
and, Treadwell says, ‘‘may be referred back to the origin of 
the vessels in the embryo, where an interference between the 
formative cells of the artery and those of the vein has resulted 
in a penetration of one by the other." 
It seems worthy of emphasis to note, in this connection, 
that in each instance it is the vein which is perforated and 
never the artery. 
5. DOUBLE VEINS WHERE ONE IS NORMALLY PRESENT. — 
This type of abnormality is, likewise, of fairly common occur- 
rence, and is only mentioned to further emphasize the frequency 
of venous abnormalities in the regions in question. 
In seven instances veins which are usually single were paired, 
and with one exception, Fig. 1 (left double ilio-lumbar veins, 6), 
this duplicity occurred in connection with the veins of the right 
side. 
Four of the above-mentioned cases refer to the renal veins 
and three to the ilio-lumbar. 
In one case, Fig. 8 (6), the right ilio-lumbar vein (V. z/ze- 
lumbalis dextra) opened into the postcava (1) by two veins, 
but laterad of the postcava the two veins were fused together, 
and through the cleft thus formed the right ilio-lumbar artery 
(15) passed. 
The question might be asked, —Is the manner in which this 
cleft is formed, in any sense, comparable with that of the 
foramina mentioned above in connection with the postcava 
and common iliac veins? 
It seems probable to the writer, since the foramina in the 
common iliac veins are located near the point of union of two 
veins, that these foramina may have been formed as the result 
