GENERAL STRUCTURE OF THE RESPIRATORY ORGANS. 



575 



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

 less distinct than those of the systemic circulation. These two conditions 

 are obviously favourable 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 pulmonary artery is in fact one of the smaller branches of the aorta, which 

 conveys a mixed fluid to the system at large. 



Fig. 217. 



Portion of the lung of a living Triton, as seen under the microscope with the power of 150 diam. ; a, 

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



757. In the warm-blooded Vertebrata, which have a complete double cir- 

 culation, namely, Birds and Mammalia, a much larger extent of surface is 

 provided for the aeration of the blood ; the whole current of which is trans- 

 mitted to the lungs, before circulating again through the system. This in- 

 crease is provided in Birds, partly by the greater extension of surface in the 

 lungs themselves, these cavities being subdivided by partitions into numerous 

 smaller chambers, each having pitted walls, and resembling the entire lung of 

 a Reptile ; and partly by the addition of a number of large air-sacs, which 

 are disposed in various parts of the body, and even in the interior of the long 

 bones. Hence it happens, that the amount of Respiration is greater in this 

 class than in any other ; although the form of the apparatus is not nearly so 

 concentrated as in the Mammalia ; nor is the mechanism of the chest so well 

 adapted to a constant exchange of the air contained in its cavities ( 37). In 

 Mammalia the lungs are proportionally smaller ; and the whole respiratory 



