On the Structure and Functions of the Organs of Respiration. 283 



two principal types, according as the process is carried on in air or water ; 

 but the changes effected by them are essentially the same in all. Animals 

 whose respiration is aquatic do not decompose the water which they 

 breathe, but the air contained in it j for if one of this class be placed in a 

 limited quantity of water, from which it soon exhausts the air, or in water 

 from which the air has been expelled by boiling, it dies almost as soon as 

 an animal whose respiration is atmospheric, when placed iu a vacuum. If, 

 however, the surface of the water be in contact with the atmosphere, it 

 will absorb air from it ; and the life of the animal will be longer, the more 

 fully the quantity thus obtained compensates for that which is consumed. 

 When a fish, in a limited quantity of aerated water, has reduced the pro- 

 portion of air until its respiration has become difficult, it rises to the surface 

 and takes in air from the atmosphere : but if prevented from doing so, it 

 will die much sooner ; a fact which proves that animals whose respiration 

 is aquatic can breathe air, and that this mode of respiration tends to pro- 

 long their life in water. What then, it will be asked, is the reason of the 

 death of aquatic animals, when entirely exposed to the atmosphere? Dr. 

 Edwards has shown, by a series of masterly experiments, that it is partly 

 due to the very rapid evaporation which takes place from the surface of the 

 body, and especially from the gills, (about -^-^ of the whole weight of a fish 

 being lost previously to its death,) and partly to the collapse of the bran- 

 chial apparatus, which prevents the air from having access to the surface 

 of the filaments. Many fishes are provided with a special apparatus, for 

 keeping the gills moist and free when exposed to the air, and such are able 

 to live a considerable time out of water ; and if we raise the gills of other 

 fishes, in which this power does not exist, we may protract life nearly as 

 long as in those cases, where the respiratory movement is continued spon- 

 taneously. 



It has been mentioned that the external surface of the batrachian reptiles 

 plays a very important part in the aeration of the blood j and Dr. Edwards 

 has shown that cutaneous respiration is sufficient to maintain the life of a 

 frog, for a certain time, without the action of the lungs, not only in the 

 air, but even in water, provided the temperature be low. The air in the 

 latter case must have a very feeble vivifying effect, on account of the very 

 small proportion of it contained in the water; but it suffices to maintain 

 the life of the animal, as long as the temperature is between 32° and 50°. 

 If, however, a slight increase of heat take place, pulmonary respiration is 

 necessary, and the animal takes in air at the surface of the water. During 

 the heat of summer, pulmonary respiration, <iided by cutaneous respiration 

 in water, is unable to counteract the effect of the high temperature, and 

 cutaneous respiration in air becomes necessary ; and we find that frogs 

 confined to the water at this time almost certainly die. The same in- 

 fluence of temperature is seen on the existence of fishes in limited quantities 

 of water ; and the degree of heat which causes frogs to increase their respi- 



