ATMOSPHERIC RESPIRATION. 221 



§ 3. •^tmosjiheric licsjjiration. 



The next series of structures which are to come under 

 our review, comprehends all those adapted to the respiration 

 of atmospheric air in its gaseous form; and their physiology 

 is no less diversified than that of the organs hy whicli water 

 is respired. 



Air may be respired by irachese, or by pulmonary cavi- 

 ties; the first mode is exemplified in insects; the second is 

 that adopted in the larger terrestrial animals. 



The greater part of the blood of insects being diffused by 

 transudation through every internal organ of their bodies, 

 and a small portion only being enclosed in vessels, and cir- 

 culating in them, the salutary influence of the air could not 

 have been generally extended to that fluid by any of the or- 

 dinary modes of respiration, where the function is carried on 

 in an organ of limited extent. As the blood could not be 

 brought to the air, it became necessary, therefore, that the 

 air should be brought to the blood. For this purpose, there 

 has been provided, in all insects, a system of continuous and 

 ramified vessels, called trachex, distributing air to every 

 part of the body. The external orifices, from which these 

 air tubes commence, are called spiracles, or stiginaia, and 



bladder of fishes inliabiting" the greatest depths of the ocean, the quantity of 

 oxyg-en is greater, while in those of fishes which come often to the surface, 

 the nitrog-en is more abundant; and De k lloche came to the same concUision 

 from his researches on the fishes of the Mediterranean. From the exjieriments 

 of Humboldt and Provencal, on the other hand, we may conclude, that the 

 quality of the air contained in the air-bladder is but remotely coniiected with 

 respiration. (Memoires de la Socicte d'Arcueil, ii. 359.) 



According- to Ehrmann, the Colitis, or Loche, occasionally swallows air, 

 which is decomposed in the alimentary canal, and effects a change in the 

 blood vessels, with which it is brought into contact, exactly similar to that 

 which occurs in ordinary respiration. It is also believed that in all fishes a 

 partial aeration of the blood is the result of a similar action, taking place at the 

 surface of the body under the scales of the integuments. Cuvicr, sur les 

 Poissons, I. 383. 



