AMPHIBIANS 



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divided into two, a right half to receive the impure blood and 

 a left to receive the pure, and although these two halves possess 

 a common opening into the undivided ventricle, yet the prolonga- 

 tion of the party-wall between them into that opening prevents 

 the blood from mixing there. Arrived in the cavity of the 

 ventricle, which is transversely elongated, mixture does not take 

 place so much as would be anticipated, for there is a system 

 of ingrowths from the ventricular wall which largely prevents 

 this, so that the blood passed into the right side of the ventricle 

 remains impure and that on the left pure, while between the two 

 comes a zone of mixed blood. The arterial cone to which blood 

 from the ventricle passes on is placed on the right, and when 

 the ventricle contracts it receives first impure, then mixed, 

 and lastly pure blood, as the result of the arrangement just de- 

 scribed. Next, as to the way in which the four pairs of aortic 

 arches coming off from the arterial cone are modified for their 

 new purpose. The first pair, which supplied the first gills, are 

 converted into carotid arches running to the head; the second 

 and third pairs, which supplied the corresponding gills, unite 

 to form the dorsal aorta, and supply most of the body except 

 the head; while the fourth aortic arches, branches of which 

 have all along supplied the lungs, continue to do so, having, 

 however, enlarged. And it is interesting to notice that the parts 

 of the fourth arches which connect them with those in front 

 become converted into fibrous cords through which blood is 

 not able to pass. It is clear, therefore, that the blood received 

 by the cone must go either to the head, general body, or to 

 the lungs, its course for any given moment being in the direction 

 of least resistance. Now, when the ventricle begins to contract, 

 forcing out blood which is quite impure, the easiest course is to 

 the lungs, and this impure blood takes that course and flows 

 to the lungs for purification. As that is being accomplished 

 mixed blood begins to flow into the cone, and by this time the 

 easiest course is through the second and third arches to the 

 general body, since the fourth arches have just been filled and 

 can take no more. Meanwhile the ventricle has begun to squeeze 

 out its remaining blood, which is pure, and the easiest course 

 this can take is through the first arches to the head. Containing 

 as it does the brain and highest sense organs, it is important for 

 the head to receive, as in fact it does, the purest blood-supply. 



