60 DISEASE IN WILD MAMMALS AND BIRDS 
while dilatation occurs nearly ten times more often 
among the mammals than among the birds. Hypertrophy 
is accompanied by myocardial change in 44 per cent, of 
the cases, whereas muscular degeneration was only seen 
in 24 per cent, of the dilatations ; this change is conspicu- 
ously lacking in the Primates, Ungulates and Marsupials. 
The usual teaching has been that dilatation, which means 
enlargement of chambers and thinning of walls or at least 
no tliickening thereof, implied an inability on the part of 
the heart to keep up with increased demand — a decom- 
pensation. If Starling be correct that dilatation is not a 
degeneration of pump value but merely one of adapta- 
tions to increased demand, then this method is more 
characteristic of mammals than of birds. There is, 
however, the reserve power to increase the muscle 
bulk inherent in the mammalian, not possessed or 
needed by the avian heart. The large-hearted class 
Aves certainly dilate their blood pump less frequently 
than mammals and indeed have less cardiac disease. 
An analysis of the incidence of hypertrophy versus 
dilatation shows that hypertrophying power resides 
in the Primates, Accipitres and Struthiones, their 
hearts relatively seldom dilatating. Lack of such power 
and consequent dilatation resides in Rodentia, Ungulata, 
]\Iarsupialia and Anseres. Hypertrophying power lies 
therefore chiefly in the heart of average size for its class, 
dilatation occurring in the small heart. (See page 63.) 
Avian Hypertrophy. 
There is little to be learned from the nature and 
anatomy of the hypertrophies and dilatations except per- 
haps their character among the birds, in which the 
physics of the circulation is somewhat peculiar. In this 
class both the hypertrophy and distention are predomi- 
natingly left-sided, a state probably explained by the 
pressure against which the pump must work in flight 
because then the lungs and the viscera are somewhat 
