GENERAL STRUCTURE OF THE RESPIRATORY ORGANS. 



505 



Fig. 142. 



other (Fig. 141f) ; 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 termi- 

 nate in the latter. The islets of parenchyma left be- 

 tween the capillary vessels are seen to be much smaller 

 than those which are usually to be observed in the sys- 

 temic circulation (Figs. 142, 143) ; so that the mem- 

 brane is more copiously traversed by vessels, than almost 

 any other that is known. The walls of the capillaries, 

 moreover, are much less distinct than those of the sys- 

 temic circulation. These two conditions are obviously 

 favorable to the exposure of the largest possible quantity 

 of blood to the influence of the air ; but as the surface 

 is not an extensive one, the amount which can be thus 

 exposed at any one time is very limited ; and the pul- 

 monary artery is often, in fact, like one of the smaller 

 branches of the aorta, which trunk conveys a mixed 

 fluid to the system at large. The lungs of Reptiles are 

 not, like those of Mammals, enclosed in a distinct cavity 

 partitioned off from the abdominal by the interposition 

 of a diaphragm ; but they lie in immediate contact with 

 the other viscera ; and the mechanism of inspiration and 

 expiration is consequently far less complete, than it is in animals which possess 

 a muscular diaphragm closing in the floor of the thoracic cavity, and capable, 



Fig. 143. 



Portion of the Lung of Tri- 

 ton, more highly magnified ; 

 the vessels, finely injected 

 with size and vermilion, form 

 a network so minute, that the 

 parenchyma is only seen in 

 small islets in its interstices. 



Portion of the Lung of a living Triton, as seen under the microscope with the power of 150 diameters 

 a, b, pulmonary vein, receiving blood from the large trunk c, and a smaller vessel d. 



