302 
IOWA ACADEMY OF SCIENCE Voi,. XXVII, 1920 
branches being subject to “considerable variation” is correct, but 
it seems important that the point should also be made, that great 
differences occur in these vessels on the right and left sides in the 
same animal. 
Again, Parker and Has well ^ describe correctly the relation of 
these vessels as they occur in the majority of cases, but the figure 
shown (p. 465) represents the condition in an abnormal individual, 
where the left common carotid artery originates as a branch from 
the arch of the aorta, and thus constitutes the third vessel from 
the arch, the innominate and the left subclavian being the other 
two. Since these discrepancies exist in the descriptions of the 
blood vessels of the region in the various texts, and in view of 
the variability of both the arteries given off by the arch of the 
aorta, and their subsequent subdivisions, especially those of the 
subclavian, it seems of sufficient interest to record their frequency 
and extent. Accordingly the following description is based upon 
the study of over one hundred specimens. Such records, of 
course, have no immediate practical value from the surgical or 
pathological sides, but from the educational considerations, espec- 
ially from the standpoint of comparative anatomy, they are rather 
important. No doubt the variations which are described below 
are to be explained in part by the persistence of the foetal con- 
ditions, or in some cases by abnormalities of the vessels themselves, 
or to the development of extrinsic parts in their immediate region. 
Many of the changes brought about probably are due to different 
modes of transformation of the primary vessels of the branchial 
arches, especially the fourth, since both the aorta and the pul- 
monary artery are derivatives of this arch. Again, it is well 
known that the heart itself originally develops high up in the 
neck region of mammals, and is gradually shifted downwards, 
so that this gradual shifting might account for some of the 
variations noted. 
Of one hundred and six rabbits dissected,* nineteen individuals 
showed marked variations from the usual condition, either in the 
branches from the aorta, or in respect to the subclavian and its 
branches on either side. There were others (fifteen) which showed 
minor variations but which could easily be placed in some of those 
showing marked variations, so that their condition is represented, 
partly at least, in some one or in a composite of the subjoined 
figures. 
In what may be termed the usual condition the aorta (Fig. 1,A) 
* My thanks are here given to Mr, Ralph L,, Parker, for collaboration in the 
dissections. 
