220 RESPIRATION 



In the arcella, bubbles, probably consisting largely of oxygen, 

 appear and disappear within the cell body, according to the ex- 

 isting physiological conditions. It seems probable that the bubbles, 

 for the development of which a high internal oxygen pressure 

 will be needed, occur in interstices of the living substance, due to 

 the presence of inclosed liquid or solid substances. In these inter- 

 stices the gas pressure can rise up to the point at which it pro- 

 duces disruption and bubble formation. Gas bubbles have not 

 hitherto been observed in the cells of oxygen-secreting glands, 

 although certain microscopic appearances have been taken for 

 such bubbles. 



The well-known transparent larva of Corethra possesses two 

 gas floats : one near the anterior, and the other near the posterior 

 end of the larva. The gas is enclosed in chitinous bladders de- 

 veloped from the tracheal system and partially rigid, with cells 

 on their external walls. If the pressure of the water is increased 

 the larva begins to sink owing to diminution in the capacity of 

 the bladders, but regains its equilibrium in two or three minutes; 

 and conversely if the pressure is diminished. This looks, therefore, 

 like a case of gas secretion. Krogh showed, however, in a beautiful 

 series of experiments 10A that there is no gas secretion, but secretion 

 of liquid out of or into the bladders, so as to compensate for the 

 alteration in their capacity. The larva can equilibrate itself in 

 this way since the bladders are partially rigid. In deep water, for 

 instance, the gas pressure is kept the same as that of the atmos- 

 phere, and hence much less than that of the surrounding water. 

 The gas pressures inside and outside the bladders are thus the 

 same, and simple diffusion of gases is not modified by gas secretion. 



Having to some extent cleared our ideas by the consideration 

 of undoubted cases of gas secretion, we can now proceed to dis- 

 cuss the evidence as to gas secretion by the lungs. As mentioned 

 already, Ludwig had the idea (in which he was right) that prob- 

 ably something occurs in the lungs to facilitate the escape of CO 2 , 

 and possibly the absorption of oxygen ; and this idea appeared in 

 the work of some of his pupils. It was a time when physiological 

 research was very active in Germany; and friendly, or some- 

 times anything but friendly, shots were often exchanged between 

 the leading laboratories. The Leipzig idea was accordingly put 

 to the test by Pfliiger and his pupils at Bonn, and for the purpose 

 Pfliiger devised an instrument which he called the aerotonometer, 

 its object being to measure the partial pressures or tensions of the 



10A Krogh, Skand. Archiv. /. Physiol., XXV. p. 183, 



