- 
510 Remarks on the Morphology of Arteries, ete. [May, 
the arteries of the limbs vary in convergence it is usually a de- 
fect, and the bifurcation is, therefore, nearer the heart. The 
brachial artery divides normally just below the elbow joint, cases 
of a lower division are so rare that Quinn found but one in 481 
cases, and that doubtful because complicated by other anomalies. 
The same law hold good for other arteries. Now, if the normal 
point of division is to be considered as the xoda/ point of the 
archetypal ray, a slight variation would carry it to the next higher 
ray, while the amount must be greater to carry it to a lower 
one. Whena high division takes place it is usually near the 
next higher node. The high division of the brachial is usually 
as far up as the origin of the superior profunda. 
Many examples of this law of convergence are seen in the 
lower vertebrates. In reviewing these it should be borne in mind 
that the original main branches of the aortic or spinal trunk are 
the hypogastric arteries, these being comparatively large vessels 
before the iliacs appear. 
In birds we see the external and internal iliacs derived sep® 
rately from the aorta—a case of defective convergence. In 
Ornithorhynchus the profunda femoris, the femoral proper and 
the internal iliac all come off together—another case of high 
division. A similar condition obtains in the frog, where two ves- 
sels, called the external and internal circumflex, whose homology 
is unknown to me, come off at the same point as the femoral and 
umbilical.. (Plate XI.) ce 
Another law creating diversity may be termed intersubstitutio® 
A trunk may be diverted from its usual situation and found in the 
line usually occupied by a smaller vessel. This, it is supposable, 
may be from some cause affecting the foetus, such as ager 
force of gravity, embolism, or those unexplained causes whi 
we call atavism or reversion to an ancestral type. A similar 
phenomenon takes place after the. ligation of vessels. The ge 
trunk is reduced or disappears and the channel of collateral cif 
culation becomes the main one. 
Another striking example of this is the case where 
artery becomes posterior, passing down in the situ pr 
arteriole known as the comes nervi ischiadici. This is the aa 
arrangement in birds. (Plate XI.) ndoubt- 
Functional activity of the parts to be supplied may ¥ wee 
edly considerably modify the size and arrangement of art 
the femora 
ation of t 
