344 BIOLOGY. 



tegument is prolonged into mucous tubes or absorbent 

 vessels ; so also, with a more vigorous formation, the 

 branchial membrane is drawn out into tubes, in order to 

 conduct the air or oxygen towards the intestinal vessels, 

 just as the intestine conveys through its absorbent 

 ducts the water to the vessels of the skin. 



1915. This saccular inversion of the skin constitutes 

 the tegumentary lymphatic vessels, whose original func- 

 tion has been to transport the oxygen, combined with 

 the water, to the intestine. They are the original 

 respiratory vessels, which in the higher animals become, 

 through the pure influence of air, true Air-tubes, like the 

 spiral vessels. 



1916. The air-vessels traverse the thickness of the 

 body toward the intestinal membrane, like the mucous 

 vessels do toward the branchial membrane. 



1917. Thus an infinite number of air-vessels will and 

 must originate. 



1918. The air- tubes are consequently the formation 

 which is properly opposed to the lymphatic vessels. 

 They are for the air or for the skin, what these are for 

 the water or the intestine. Air-vessels are first displayed 

 in insects, then in fishes, reptiles, birds, and Thricozoa 

 or Mammalia. 



1919. If the infinitely numerous air-vessels concur to 

 form one stem, they are then called lungs, as in the 

 higher animals. 



1920. The pulmonary vesicles are nothing but ramified 

 air-tubes, such as the insect has. 



1921. The formation of air- tubes is one of a higher 

 character than that of the gills. For in it, indeed, the 

 function has been separated from all other functions. 

 It is simply destined to convey the air without water. 



1922. As they pass into the dark, light does not ope- 

 rate upon them; and they will therefore less promote 

 evaporation than mediate unto combustion. 



1923. The anatomical idea ofHEhe air-vessels, or of 

 the lung, is a saccular inversion of the skin. The skin 

 is prolonged into, and ramifies towards, the body. The 



