356 HISTORY OF THE HUMAN BODY 



into the pulmonary artery, a condition which continues as a 

 permanent one from this point on. The relation of the two 

 aortic arches, however, is not so perfect, for they cross in 

 such a way that while the right one contains mainly pure 

 blood, the left one, in company with the pulmonary artery, 

 collects, in part, impure blood from the right side. This 

 causes a mixture of blood in this arch as well as in the main 

 aorta, and the result is that the blood is never wholly aerated, 

 a condition which does not allow the establishment of a con- 

 stant bodily temperature, but compels the animal to be " cold- 

 blooded," or more correctly, poikilothermous, that is, change- 

 able in temperature in more or less accordance with its sur- 

 roundings. 



The heart of reptiles (Fig. 102, D) is similar to the last 

 save that the ventricular partition is more extensive, though 

 still incomplete, therefore the relation of these animals to ex- 

 ternal temperature is the same as in amphibians; in birds 

 (Fig. 1 02, E), however, the opening between the ventricles 

 closes, thus, for the first time, completely separating the two 

 kinds of blood. Probably correlated with this is the complete 

 atrophy of the left aortic arch, leaving the right one as the 

 only connection between heart and median aorta. 



Mammals (Fig. 102, F), although but indirectly related to 

 the birds, have accomplished the same separation of pure and 

 impure blood, corresponding to the left and right halves of the 

 heart, respectively, a relation which is still further emphasized 

 by the main blood-vessels, the systemic venous trunks as- 

 suming a position oh the right, in connection with the atrium 

 of that side, and the main aorta being on the left, instead of 

 the right. In both birds and mammals, then, the tissues are 

 supplied with pure blood and are thus enabled to maintain a 

 constant body temperature, which fluctuates but a few degrees, 

 and is usually higher than the external temperature. Ti.ere 

 thus comes to be developed in the two most highly specialized 

 Classes of vertebrates, birds and mammals, a complete and 

 almost symmetrical double heart, of which the right half is 

 associated with the venous, the left with the arterial, blood. 



