158 



RESPIRATION OF FISHES AND REPTILES. 



complished in the following manner : The mouth is periodically filled 

 with water, which is driven past the gills by muscular compression, and 

 thereby the carbonic acid is removed from the blood which circulates in 

 those organs, and oxygen is obtained in return. For this reason, a fish 

 dies very quickly when its mouth is kept open. The angler knows that 

 it is not owing to any loss of blood, nor to any injurious lesion that the 

 hook may cause, but simply to suffocation, the water no longer lifting the 

 gill covers, but merely passing out through the open mouth. 



The experiments of Humboldt and Prove^al clearly demonstrate the 

 analogy between aquatic and aerial respirations ; for water is not de- 

 composed by the breathing of fishes : it is the air dissolved in it that is 

 used. In the sample examined by these chemists, there was 20.3 per 

 cent, of its volume of air, consisting of oxygen 29.8, nitrogen 66.2, and 

 carbonic acid 4.0, in the hundred parts. After the fishes had remained 

 in it for a due time, it still contained 17.6 per cent, of its volume of air, 

 but this in 100 parts now consisted of oxygen 2.3, nitrogen 63.9, and 

 carbonic acict 33.8. There had therefore been a consumption of oxygen 

 and evolution of carbonic acid, together with a slight removal of nitrogen, 

 this being the general result witnessed in aerial respiration. In a sim- 

 ilar course of experiments on the breathing of gold fishes, 

 made by myself, the result corresponds to the preceding 

 statement, only the water I used was richer in oxygen gas, 

 and the transposition into carbonic acid did not seem by 

 any means to be so complete. I also remarked the same 

 diminution in the quantity of nitrogen, but am disposed to 

 attribute it not so much to the consumption of that gas by 

 the fishes as to its diffusion from the water into the atmo- 

 sphere, the solvent power having changed by the substitu- 

 tion of carbonic acid for oxygen. 



In reptiles the lung presents the sac-like form, as in Fig. 

 Respiration of 75, a pulmonary artery passing on one side, 

 reptiles. an( j a pulmonary vein returning on the other : 



a is the trachea ; #, its bifurcation ; <?, pulmonary artery ; 

 d, d, pulmonary vein. It often occurs that. '"the two lungs 

 are not equally developed, one of them, B, being rudiment- 

 ary as compared with the other, A. Into such a sac in ser- 

 pents the air is forced by muscular contraction, a kind of 

 swallowing. It is expelled from them by the contraction 

 of the abdominal muscles, and hence the hissing sound 

 which it emits during its expulsion. From the simple sac 

 to the cellular lung the advance is made by degrees, a de- 

 velopment of parietal cells upon the inner surface taking 

 place. At the intermediate stage, between the simple sac 



