Arterial Anomalies II 
SVE COMPARATIVE "ANATOMY: 
As already intimated, early observers were impressed with the 
resemblance of certain variations encountered in man to condi- 
tions which were type forms for the lower animals and this led 
to early group classification of these anomalies according to the 
arrangement of the trunks which sprung from the aortic arch. 
While many of the variations may be explained by the figures de- 
veloped in the preceding section, there are others relating par- 
ticularly to the arch which cannot be so explained. Among mam- 
mals in which the same primary arrangement of arches occurs and 
in which the same parts of the arches atrophy a difference in 
tvpes is found. For the sake of brevity we will speak here only 
of the general principles and leave details of type for the later 
sections. 
Turner (1862) believed that the modification of the early ves- 
sels brought about by the movement of the heart into the thorax 
was responsible for the various forms encountered. While more 
recent embryological observations have more fully explained the 
definitive arches and subclavians and have confirmed the migra- 
tion of the heart, they have failed to detect differences in migra- 
tion or other factors which would explain the various types ex- 
tant. Dr. Owen (1868) suggested that the best service of the 
body economy would determine the type, but in the light of re- 
cent biological studies we must look for a more tangible factor. 
Many exhaustive observations have been carried out in attempts 
te throw light on the subject but it will not be profitable to con- 
sider them here for they simply furnish morphological data. 
It seems to me that the point of particular interest to us in this 
study is the relation of human variation, if any, to type forms in 
the lower animals and the significance of variation per se. An 
idea which has been most popular is that the type of chest shape 
and the degree of descent of the heart are determining factors of 
the normal. Observation has failed so far to establish a rela- 
tion between chest shape and the anomalous arrangement of the 
branches of the arch, possibly because it is of a transient charac- 
ter. As to the significance of an animal type in man we may 
ZO) 
