200 THE VITAL FUNCTIONS. 



form ; and their physiology is no less diversified 

 than that of the organs by which water is respired. 



Air may be respired by tiacliece, or by pulmonary 

 cavities; 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 circulating in them, 

 the salutary influence of the air could not have 

 been generally extended to that fluid by any of the 

 ordinary 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 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 trachea, distributing, and, as it were, 

 circulating air through every part of the body. 

 The external orifices, from w hich these air tubes 

 commence, are called spiracles, or stigmata, and are 

 usually situated in roM s on each side of the body, 



of the Mediterranean. From the experiments of Humboldt and 

 Proven9al, on the other hand, we may conclude, that the quality of 

 the air contained in the air-bladder is but remotely connected with 

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



According to Ehrmann, the Cobitis, or Loche, occasionally swal- 

 lows air, which is decomposed in the aUmentary 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. Cuvier, sur les Poissons, 

 I, 383. 



