SECTION II 
DISEASES OF THE HEART 
The heart is an organ whose duty, throughout the two 
classes considered in this study, remains entirely identic, 
purely a physical one in dri\'ing the blood through the 
corresponding vascular system. The physics involved 
naturally differs between mammals and birds, but energy 
is derived from the automatic power lodged in the cardiac 
musculature. Whether this be neuromyogenic, as seems 
to be the case in all mammals, or purely myogenic, as is 
probably the case for the birds in which MacKenzie and 
Robertson (1) say there is no atrioventricular bundle, the 
result is the same, since in both classes there is some 
continuity of muscle fibres from auricle to ventricle. The 
gross anatomy varies little if any more than the physi- 
ology, albeit there is proportionately greater auricular 
capacity in the mammals than in the birds, and indeed 
there are differences within the classes which cannot now 
be readily explained ; certain minor variations of valvu- 
lar arrangement exist, such as the absence of the 
membranous right tricuspid in Aves. 
When, however, one considers the cardiac power 
available for various animals, the subject becomes one of 
greater breadth and complexity, for no consistency 
obtains even wdthin families, since the demand for 
cardiac strength will vary more with habits than with 
zoological relationships. Thus for example the domestic 
rabbit has a small heart volume while the wild hare has a 
great one. Although, of course, the size of an organ may 
not be an absolute measure of its efficiency (a flea's leg 
muscle has relatively greater power than a man's), yet 
size is the only physical gauge one has for estimating 
nature's preparation for expected demand. Perhaps this 
(1) Br. Med. Jour., 1910, 2, 1161. 
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