DISEASES OF THE BLOOD VESSELS 81 
unwise to draw deductions as to their incidence or as to 
the possibilities of vascular dilatation in any given order. 
Horses have aneurysms occasionally, cows and dogs 
rarely, according to Rievel. I can, however, state that 
there has not occurred in our experience a large growing 
pulsating aneurysm in the aortic arch region comparable 
to the condition so well knowTi in man. The literature to 
which we have had access gives a definite impression that 
parasitism of vessel walls is the most important factor 
in the causation of ectasia, and that simple non-parasitic 
arteriosclerosis is relatively unimportant. Two of our 
seven cases seem to have been free of parasites but the 
notes cannot entirely assure one of this. The distribution 
of cases at this Garden is found in Table 5. 
A dilatation of the first part of the arch in a seal to a 
size which might be described as an aneurysm caused us 
to make such a diagnosis, correctly enough from the size 
and shape of the vessel but possibly worthy of reconsid- 
eration in light of the fact that no damage to the wall was 
found. At the heart and in the descending arch the 
diameter measured 4-5 cm., while the first part of the 
aorta measured 7.5 cm. This great irregularity in width 
could not be found in other seals albeit this section of the 
arch is usually a trifle larger than its origin and descend- 
ing portion. The cava in seals is also large, but in this 
particular animal it measured 6 cm. across at the liver 
where there is a normal dilatation. These two spaces are 
looked upon as normal reservoirs for blood during 
diving, but the case in question seemed to have excessive 
''aneurysmoid" enlargements without mural disease. 
London reports an aneurysm of the aorta in a seal. (2) 
Aneurysms are not so common in birds, the incidence 
being in comparison with mammals as 1 to 2.2 Two seats 
are prominent for their development : the sinuses above 
the aortic valves and the first part of the subclavian ves- 
sels ; two of the six cases occupied the first position, two 
(2) Proc. London Zool. /S'oc, 1916. 
