206 PATHOLOGY: W. S. HALS TED 
(3) Vasomotor and vasa vasorum disturbances leading to modified nutri- 
tional activities in the wall of the artery. 
In casting about for an explanation of these aneurysms there constantly 
obtruded itself the picture of the dilated arterial trunks which, I find from the 
study of about 400 cases, has occasionally been noted on the cardiac side of 
arterio-venous fistulae. In our own clinical and experimental cases, dilation 
of the artery proximal to the fistula has occurred invariably. For this re- 
markable manifestation, likewise, no satisfactory cause has been assigned. 
There might, I thought, be a common cause for both — for the dilation of the 
subclavian artery distal to the cervical rib, and for the dilation, central to the 
arterio-venous fistula, of the artery concerned in its formation. Hence, for 
a number of years, in the course of various experiments in partial occlusion of 
the arteries, I had somewhat in view the possibility of the production, beyond 
the point of constriction, of a dilation of the artery, analogous to the dilations 
which have been observed in cases of cervical rib. 
Four years ago when after many trials I had altogether despaired of having 
the hope realized, I was startled, on examining the abdomen of a dog whose 
aorta had been constricted for about six months to see that each of the branches 
of trifurcation had become dilated almost to the size of the main aortic trunk. 
With this observation as incentive, Dr. Mont Reid and I, the following win- 
ter, constricted the abdominal aorta just above its trifurcation, in many 
dogs and at intervals explored and reexplored the abdominal cavities, but with 
negative result. Finally, on investigating the abdomen of the last dog we 
found the hoped-for dilation. The degree of obturation of the aorta was ac- 
curately determined on sacrificing the animal, and the following year the experi- 
ments were more advantageously repeated because of the data obtained from 
this case. Now, that we have apparently determined the relative amount 
of constriction required to give the most pronounced results we are able in 
almost every instance to produce the dilation. 
As regards the cause of the dilation produced experimentally we may I 
think, conclude that it is not to be found in any of the three factors which 
have been proposed as responsible for the dilation observed in cases of 
cervical rib, viz., (1) vasomotor paralysis, (2) trauma and (3) variable blood 
pressure. 
Ad. 1. Vasomotor Paralysis, (a) The vasomotor nerves and the vasa 
vasorum are destroyed by the moderately constricting and totally occluding 
bands quite as surely as by those which, occluding almost totally, have produced 
the greatest amount of dilation, (b) Only a portion of the circumference of 
the subclavian artery is exposed to the pressure of the cervical rib and the 
scalenus anticus muscle and hence only a fraction of the vasomotor nerves or 
vasa vasorum could be pressed upon. 
Ad. 2. Trauma, (a) The dilation is usually fusiform and distal to the rib. 
(b) Trauma is excluded as a factor in the experimental dilations. 
Ad. 3. Variable Blood Pressure, (a) Patients suffering from the pressure- 
