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bring forth their young, for the following reafons, 
viz. It is well known that the only effential difference 
(as to the general flrudture of the heart) between 
amphibious and meer land animals, or fuch as never 
go into the water, is that in the former the oval hole 
remains always open. Now, in fuch as are without 
this hole, if they were to be immerfed in water for 
but a little time, refpiration would ceafe, and the ani- 
mal muftdie; becaufe a great part of the mafs of 
blood paffes from the heart, by the pulmonary artery, 
through the lungs, and by the pulmonary veins returns 
to the heart 3 while the aorta is carrying the greater 
part of the mafs to the head and extremities, &c. 
Now the blood paffes through the lungs in a conti- 
nual uninterrupted ftream, while refpiration is gentle 
and moderate 3 but when it is violent, then the circu- 
lation is interrupted, for infpiration and expiration 
are now carried to their extent 3 and in this hate the 
blood cannot pals through the lungs either during the 
total infpiration or total expiration of the air in 
breathing 3 for in the former cafe the inflation com- 
prefles the returning veins, and in the latter, by the 
collapfion of the lungs, thefe veins are interrupted 
alfo, fo that it is only between thefe two violent a&ions 
that the blood can pafs : and hence it is that the lives 
of animals are fhortened, and their health impaired, 
when they are fubjedted to frequent violent refpiration 3 
and thus it is that in animals who have once breathed, 
they mull continue to refpire ever after 3 for life is at 
an end when that ceafes. 
There are three neceflary and principal ufes of 
refpiration in all land animals, and in thefe kinds that 
are counted amphibious 3 the firfl: is that of promoting 
C c 2 the 
