SECTION III 
DISEASES OF THE BLOOD VESSELS 
The gross anatomy of the blood vascular system is 
constructed upon the same general scheme throughout 
mammals and upon a comparable basis in birds. Micro- 
scopically there is little variation throughout the orders 
unless it be in the relative proportion of muscular and 
connective tissues. The origin of the great vessels at 
their cardiac base and their distribution to the pul- 
monary and to the greater circulations in no way differ in 
these two classes in that it always consists of an efferent 
pathway to the lung and a root vessel above the aortic 
orifice. The fomier has usually quite a distinct origin 
on the right side, but in some birds the posterior wall of 
the pulmonary artery may overlie the entire aortic base ; 
this, however, is not the rule for birds. The aorta in most 
mammals remains a separate and distinct vessel for some 
distance, after which it gives off the innominate and sub- 
clavians. In the birds on the other hand, the stretch 
immediately above the aortic valve is usually ballooned 
out somewhat, into a sort of sac or ampulla from which 
the subclavians and descending aorta arise. This forms 
a structure of rather trident shape, the lateral prongs 
being the subclavians, the middle and posterior being the 
aorta proper. In some birds the aorta may have the 
length of a centimetre or more then dividing into the left 
subcla\ian and right aorta from which the right sub- 
cla\aan comes off. 
There is definitely more support to the heart and 
vascular roots in mammals than in birds, in the latter 
class these structures lying quite free between the lateral 
air sacs and well in front of the lungs. Nor is there the 
richness of mediastinal areolar and fatty tissue in the 
mnged creatures. 
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