RESPIRATORY CIRCULATION. 221 



through the capillaries of a warm-blooded animal, has been 

 determined by microscopic examination to be about 3-100ths 

 of an inch 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 j 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 replace that which has been withdrawn from it.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 



