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RESPIRATORY CIRCULATION. 221 
through the capillaries of a warm-blooded animal, has been 
determined by microscopic examination to be about 3-100ths 
of aninch per second. From the comparison of this rate with 
that of the flow of blood through the larger arteries, which has 
been found by experiment to be nearly 12 inches per second, 
it appears that the area of the capillary system must be 
nearly four hundred times as great as that of the vessels 
which supply it with blood. 
252. Thus the Arterial and Venous systems communicate 
with each other at their opposite extremities; their large 
trunks through the medium of the heart; and their ultimate 
subdivisions through the capillaries. Hence we may consider 
this double apparatus of vessels as forming a complete circle, 
through which the blood flows in an uninterrupted stream, 
returning continually to its point of departure ; and the term 
circulation is therefore strictly applicable. 
253. But the conveyance of the nutritive fluid to the several 
organs of the body, for their support and maintenance, is not 
the only object to which its circulation has to minister. It is 
requisite that the blood should be continually exposed to the 
influence of the air, by which it may get rid of the carbonic 
acid with which it has become charged during its circulation 
in the system, and may take-in a fresh supply of oxygen to 
replace that which has been withdrawn from it. In order to 
effect this exposure, the blood is conveyed to a particular organ, 
in which it is made to pass through a special set of capillary 
vessels, that bring it into almost immediate contact with 
air. In the lower tribes, in which this aeration is (from 
various causes hereafter to be explained) much less constantly 
necessary than in the higher, we find the respiratory organ 
supplied by a branch from the general circulation ; and the 
blood which has passed through it, and which has been sub- 
jected to the invigorating influence of the air, is mingled in 
the heart with that which has been deteriorated by circulating 
through the system, which is again supplied with this mixed, 
half-aerated blood. But in the highest classes, there is a dis- 
tinct circle of vessels subservient to the respiratory function : 
namely, an arterial trunk issuing directly from the heart, and 
subdividing into branches which terminate in the capillary 
system of the respiratory organ ; a set of capillaries, in which 
the aeration of the blood takes place ; and a system of veins 
