23 
Locality. 
Peculiarities of external form and of internal 
structure manifest the destination of the greater 
part of several distinct classes of organized beings 
to live wholly in atmospheric air. Animals of these 
classes have lungs through which the circulating 
blood finds continual passage to the double or four- 
chambered heart; the principal chambers of which 
have no common or direct communication with each 
other, but send blood from one to the other through 
valvular passages or tubes. The right ventricle 
opens into a minor chamber, the right auricle, and 
this into the trunk of the pulmonary artery, which 
conveys the blood into the left ventricle, from whence 
it is impelled into the left auricle, and into the 
great tube or trunk of the aorta. Valves opening 
only in one direction admit the progress of the 
blood from veins and lungs, through the heart, to 
arteries, but, closing against all impulse in an oppo- 
site direction, wholly prevent its regress. 
But in the fcetal state all these animals live and 
grow without access to atmospheric air. Their lungs, 
though fully prepared for future use, are then use- 
less, as the eye, so excellently instanced by Paley, 
is prepared in darkness for future light. During 
this period, however, the communication between 
two of the chambers of the heart is direct through 
the foramen ovale. 
This opening between the auricles is closed in 
almost all mammalia as soon as the lungs begin to 
exercise their functions: but not in al]. Such as 
c 4 
