DESIG.V L\ THE ORGANIC WORLD. 461 



development into an organ for atmospheric respiration ; and is 

 unmeaning if not so viewed. 



So, again, we may trace a remarkable uniformity in the line 

 of progress from the lower to the higher forms of pulmonary 

 apparatus. The purpose which the lung has to serve being the 

 exposure of the blood to the air over an extended surface, that 

 extension must be proportionate to the demand for aerition set 

 up by the muscular activity and temperature-standard of the 

 animal. The swimming-bladder of the Fish, even when used for 

 atmospheric respiration, is a simple, undivided sac, or, as in the 

 Ganoids, a pair of such sacs. The lung of the Frog has its internal 

 surface increased by its extension into a number of little pockets 

 in the upper part of the principal cavity. The same is the case 

 in the Snake, and in many other Reptiles ; each lung having a 

 large undivided cavity, with diverticula in its walls, over the 

 extended surface of which the blood-vessels are minutely dis 

 tributed. In some of the higher Reptiles, as the Crocodile, the 

 cavity of the lung exhibits an incipient subdivision. In the lung 

 of Man, as of Mammals generally, an extraordinary increase is 

 given to the extent of aerating surface, by the excessively minute 

 subdivision of the cavity into air-cells ; of which thousands are 

 clustered round the end of each terminal twig of the bronchial 

 tree, lint this increase would be without effect, if there were not 

 at the same time a most elaborate provision in the skeleton of 

 the trunk, in the disposition of its muscles, and in the mode in 

 which these are acted on by the nervous apparatus, for alter 

 nately filling and emptying the lungs, so as to take in fresh sup 

 plies of oxygen for the aeration of the blood, and to get rid of the 

 carbonic acid which it gives off. The chief feature in this pro 

 vision is the enclosure of the lungs in a distinct cavity (that of 

 the chest) cut off from the abdomen by a muscular partition the 

 diaphragm ; the contraction of which, by increasing the capacity 

 of the chest, produces an in-rush of air down the air-passages, 

 which penetrates to the remotest parts of the minutely-subdivided 

 cavity of the lungs. By no other action could the air contained 

 in that cavity be so effectually renewed. Thus the pulmonary 

 apparatus of the Mammal is the most perfect form that could be 

 devised lor obtaining the highest amount of respiratory power 

 within the smallest compass. 



