306 
IOWA ACADEMY OF SCIENCE Voi.. XXVII, 1920 
variation is interesting since it closely simulates the normal con- 
dition found in the horse. It may be explained by the fusion of 
the two aortic stems and the shortening of the fourth arch so 
that the left subclavian artery joins with the common stem during 
the transformation of the primary vessels. In one instance the 
left vertebral (Fig. 3, V) takes its origin well down on the left 
subclavian vessel so that it is almost in a position to be considered 
a separate branch from the arch of the aorta and could be inter- 
preted as an additional vessel from the latter as has been recorded 
as a variation in the human (McMurrich, p. 511). It is easy 
to see how a slight displacement caudad of the left common 
carotid artery in this case would produce four distinct vessels 
originating from the arch of the aorta instead of the usual two. 
SUMMARY 
Although the usual number of blood vessels arising from the 
arch of the aorta in the rabbit is two — a so-called innominate 
or brachio-cephalic artery and a left subclavian artery — the 
variations from this condition herein described indicate the pos- 
sibility of a considerable departure. In one individual (Fig. 11) 
a single vessel leaves the aortic arch and after passing a short 
distance forward subdivides successively to form the left sub- 
clavian, the left common carotid and the right subclavian arteries. 
In a number of cases as shown in figures 6, 8 and 10 three 
vessels have their origin on the arch and in these the order is 
the brachio-cephalic, the left common carotid and the left sub- 
clavian arteries. In one individual (Fig. 3) the left vertebral 
replaces the left common carotid artery in the series, the carotid 
in this case having its origin on the innominate as normally. 
This case suggests the possibility of four vessels forming the 
series. 
Conspicuous differences in the order and sequence of the 
vessels from the subclavian arteries of the two sides are noted. 
On the left side the vessels in a number of cases show a tendency 
to group themselves either proximally or distally in the form of 
a sort of corona as indicated in figures 6, 9 and 10. The forma- 
tion of various innominate stalks common to certain arteries are 
found in some cases, while in others the intervals between certain 
arteries are rather noticeable. Less marked variations are noted 
in the vessels of the right side. The vertebral artery in one in- 
stance (Fig. 2) is displaced from its usual place to the lateral 
side of the right common carotid artery. The transverse scapular 
