Dilatations occur in mammals and birds as 2.4 is to .28.[13] In other words, mammals are much more susceptible than birds to degenerative and inflammatory processes, show an ability to increase the muscle bulk two and a half times as great and are liable to chamber distention nearly ten times as often. It might also be put that birds cannot or do not need to increase their muscle, and that the chamber and muscle balance is more perfectly arranged. While in the preceding pages hypertrophy has been discussed rather from the standpoint of its value as a compensating and reserve capacity, and dilatation as a degenerative or decompensatory process on the part of the cardiac mechanism, it may be that dilatation of the mammalian heart is the usual method employed by the class in response to increased demand. It seems certain, however, that the originally and normally larger heart, both mammalian and avian, more often uses an increase of its muscle to this purpose. Hypertrophy was accompanied by myocardial disease in 44 per cent. of the cases, while dilatation showed this change in only 24 per cent. This supports the theory that dilatation is a normal response of the myocardium under strain and the belief held in many quarters that the muscle increases its bulk because some of it is damaged. The differences between classes are not so conspicuous between orders. However, the large heart of the carnivores increases both its muscle and chambers, while the small heart of the rodents and marsupials more often dilates. Analysis of the avian orders is inconclusive and somewhat contradictory. Let it suffice to say that the birds which fly most, with exception of the ducks, have a relatively low vulnerability, and the soaring carnivorous Accipitres and the largest birds, Struthiones, apparently have a high susceptibility to damaging influences and enlarge their muscle bulk in response to increased work. Aneurysms of the heart are quite rare; only two have been seen. They were both located at the apex of the left ventricle in birds; they did not rupture. Myocardial damage is evident in both cases but the cause is not clear; parasites could not be demonstrated. Plimmer reports a case of cardiac aneurysm at the apex from infestation of the heart muscle by sarcosporidia. SECTION III DISEASES OF THE BLOOD VESSELS