334 RESPIRATION. 



being known that in suffocation two changes take place in the 

 chemical condition of the blood diminution of oxygen and 

 increase of carbonic acid gas it is obviously not unreasonable 

 to attribute the phenomena either to the one or the other of 

 these changes, or to the combination of both. In the preced- 

 ing pages it has been assumed that they are due to the dimi- 

 nution of ox3'gen. The chief proofs that this is so are as fol- 

 lows : In order to demonstrate in a striking way and in one 

 experiment that diminution of oxygen in the air breathed 

 does, and that excess of carbonic acid gas does not, produce 

 the phenomena of dyspnoea, the following method, devised by 

 Rosenthal, may be employed. The mercurial gosometer (fig. 

 251) is filled with oxygen. The animal is then allowed to 

 breathe the gas in the way described ( 95) until it may be 

 reasonably supposed that the air contained in the air-passages 

 is displaced by it. This occurs in the rabbit in about ten 

 respirations. The communication is then opened between the 

 valve B and the receiver, while the exit tube is clipped so that 

 the animal both inspires from the gasometer and expires into 

 it. As the experiment goes on, it is obvious that the propor- 

 tion of carbonic acid increases and must continue to increase, 

 until that gas attains such a tension in the gasometer that no 

 further escape from the blood is possible. At first the volume 

 of gas in the gasometer undergoes no sensible diminution, 

 for the animal expires nearly as much of carbonic acid as it 

 inspires of oxygen ; afterwards, as the quantity of carbonic 

 acid gas given off becomes less and less, the cylinder sinks in 

 each inspiration more and more. As soon as this is the case 

 it is of course absolutely certain that the animal is breathing 

 an atmosphere containing a large excess of carbonic acid gas, 

 yet notwithstanding, there is no sign of asphyxia, the reason 

 being that the oxygen still exists in the mixture in a propor- 

 tion exceeding that in which it exists in the atmosphere, or at 

 all events, not falling far short of it. When at a still later 

 period the breathing begins to be excessive, the dyspnoea can 

 at first be relieved by increasing the pressure to which the 

 gases contained in the gasometer are exposed. This, of course, 

 while it favors the absorption of oxygen, equally favors that of 

 carbonic acid gas ; that the latter has no physiological effect 

 cannot be maintained, but the experiment proves that its 

 effect is very inconsiderable. 



The direct proof that dyspnoea is dependent on defect of 

 oxygen, is obtained by the analysis of the gases of the blood 

 in an animal which has been asphyxiated by the inhalation of 

 pure nitrogen. Pfliiger has found that an animal (dog) breath- 

 ing nitrogen becomes hyperpnceic in 15 seconds. In 20 seconds 

 the struggle is at its height, the blood being already very dark. 

 In Pflliger's experiments, blood was allowed to flow from an 



