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CIRCULATION IN REPTILES. 317 



ich breathe permanently by gills like Fishes, besides possessing 

 imperfectly-developed lungs, the apparatus exhibits a blending of both 

 plans ; for a small portion of the blood, which is propelled by each 

 contraction of the ventricle, passes directly to the lungs ; the principal 

 part of it being at once distributed to the gills, as in Fishes. After 

 passing through these, it is transmitted to the general system ; and on 

 returning thence, in a completely venous state, it is mingled with the 

 blood which has been arterialized in the lungs. This latter, however, 

 bears so small a proportion to the rest, that, if the aeration were not 

 partly effected by the gills, it would be insufficient for the wants of the 

 animal. The tadpoles of the common Frog and Water Newt, as well 

 as of other species which, like them, begin life in the general condition 

 of Fish, present a similar condition at one period of their change. At 

 first, the whole aeration is effected by means of gills, the lungs being 

 in a rudimentary or undeveloped state ; and the entire circulation is 

 carried on as in Fishes, the pulmonary vessels being scarcely traceable. 

 As the lungs begin to be developed, however, a portion of the blood is 

 sent to them ; and at the same time, communicating passages which 

 previously existed, between the vessels that convey blood to the gills, 

 and those that return it from them, are increased in size ; so that a 

 certain proportion of the blood is transmitted to the system, without 

 having passed through the gills at all. By a further increase in the 

 diameter of these, the whole current of blood takes this direction, the 

 gills being no longer serviceable ; and as, at the same time, the lungs 

 are attaining their full development, the aeration which they effect in 

 the blood transmitted to them becomes sufficient, and the whole circula- 

 tion is thus permanently established on the Reptilian type. 



563. On the other hand, among the higher Reptiles, we find the 

 circulating apparatus presenting approaches to the form it possesses 

 in Birds and Mammals. For the ventricle is divided, more or less 

 completely, into two cavities, one of which propels aerated blood to 

 the system, whilst the other transmits venous blood to the lungs. A 

 certain amount of mixture of arterial and venous blood always takes 

 place, however, either in the heart itself, or in the vessels; so that 

 the blood which the body receives is never purely arterial. But this 

 mixture is sometimes effected in such a manner, that pure arterial 

 blood is sent to the head and anterior extremities ; though the re- 

 mainder of the body receives a half-aerated fluid. ; This is accomplished 

 in the Crocodile, by a provision very similar to 'that which exists in 

 the foetus of warm-blooded animals. (CHAP, xi.) The portal circulation 

 in Reptiles is carried on nearly upon the same plan as in Fishes. It 

 receives the blood from the posterior extremities and from the tail, 

 as well as from the abdominal viscera ; and this blood is distributed by 

 the portal capillaries, not only through the liver, but also through the 

 kidneys, although the latter also receive arterial branches from the 

 aorta. The fact that the kidneys are supplied from the general portal 

 circulation in Fishes and Reptiles, has an important bearing on the 

 difference in the arrangement of their own vessels, which will be here- 

 after shown to exist, between the kidneys of these animals and those of 

 Birds and Mammals ( 728). 



