68 DISEASE IN WILD MAMMALS AND BIRDS 
upon orders. The strands of muscle do not seem 
arranged so regularly as in mammals; the pulmonary- 
artery of the cat, for example, has a muscle arranged like 
an oblique band in waves or festoons along the length. 
The mammals as a class seem more richly supplied with 
arteries and veins than do the birds, and the square area 
of the vascular system is likewise larger. This is dis- 
tinctly different from the amount of heart bulk as given 
in the discussion of kilogram-heart ratios so that one 
might say that the birds are "overhearted and 
undervesseled. " 
In so far as the physiology of the two classes is con- 
cerned it is obvious that a different regulatory system is 
necessary because, aside from the variations of pressure 
incidental to pulmonary, muscular and visceral work, 
there remains the altering pressure within the air sacs of 
Aves, a force different under states of rest, of running, of 
deep water swimming and of flying with or against the 
wind. Part of the internal air pressure variation is cared 
for by the ability a bird has to respire the air in its sacs 
and bones, but in prolonged exposure to the pressure 
under water or during protracted flight some compen- 
satory mechanism doubtless exists. This seems to reside 
in part in the heavy elastic quality of the arterial stalk 
and the very rich venous supply of the abdomen, includ- 
ing the renal-portal system and the distensible pelvic 
veins. Just where the governing power for this mech- 
anism resides is as much a matter of debate as in the case 
of the human being, but certain researches would place it 
in the caudate lobe and pituitary body. 
Having discussed these general comparative data we 
can now pass to a consideration of the pathology seen at 
this Garden. The subject will be studied from the stand- 
point of the vessels as a system and the changes peculiar 
to it. Naturally the most important lesions affect the 
great stalks and the principal trunks, from which the 
processes may continue into the smaller vessels. The 
