574 



OF RESPIRATION. 



Fishes, breathing by gills during their tadpole state ; but at the time that the 

 legs are developed and the tail has decreased, the pulmonary organs also are 

 evolved, and the course of the blood is altered, so that it is no longer trans- 

 mitted through the gills, which speedily shrivel and disappear ( 31). There 

 are some species, however, whose metamorphosis is checked, so that in their 

 permanent condition both lungs and gills are present ; but the former are then 

 present in a very rudimentary form, not being more developed than the air- 

 sacs of many Fishes. The lungs of Reptiles are, for the most part, simple 

 sacs ; into which the bronchial tubes open freely ; and on the walls of which, 

 the pulmonary vessels are distributed. The extent of surface is considerably 

 increased, however, by the formation of a number of little pits or sacculi on 

 the inner wall of the cavity, especially at its tipper part ; and between these, 

 we observe a sort of cartilaginous frame-work, which is continuous with the 

 cartilage of the bronchus on either side. The Turtles and their allies are the 

 only Reptiles, in which the cavity of the lung is itself divided by membranous 

 partitions ; and thus it happens that, excepting in these, the net-work of pul- 

 monary capillaries, in the class of Reptiles, is exposed only on one side to 

 the influence of the air. The general distribution of these vessels is shown 

 in the accompanying figures. It will be seen that the trunk of the pulmonary 

 artery runs along one side of the sac, and that of the pulmonary vein along 

 the other (Fig. 215) ; and that numerous branches arise from the former, 

 which subdivide into capillaries that ramify over the whole surface, and then 

 reunite into small veins which terminate in the latter. The islets of paren- 

 chyma left between the capillary vessels, are seen to be much smaller than 

 those which are usually to be observed in the systemic circulation (Figs. 

 216, 217); so that the membrane is more copiously traversed by vessels, than 



Fig. 215. 



[Fig. 216. 



Lung of Triton cristatxs, magnified 

 about 3 diameters ; a, pulmonary artery; 

 b, pulmonary vein. 



Portion of the Inner of the same animal, 

 more highly magnified ; the vessels, finely- 

 injected with size and vermilion, form a 

 net-work so minute, that the parenchyma is 

 only seen in small islets in its interstices. 



