DISEASES OF THE HEART 65 
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 carni- 
vores 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. 
