RESPIRATORY SENSE. 483 



tlie bellows every few moments, as long as this is continued, 

 the animal will make no respiratory effort ; showing that, for 

 the time, the respiratory sense is abolished. 



2. When the artificial respiration is interrupted, the respi- 

 ratory muscles are thrown into contraction, and the animal 

 makes regular, and at last violent efforts. If we now expose 

 an artery, and note the color of the blood as it flows, it will 

 be observed that the respiratory efforts only commence when 

 the blood in the vessel begins to be dark. "When artificial 

 respiration is resumed, the respiratory efforts cease only when 

 the blood becomes red in the arteries. The invariable result 

 of this experiment seems to show that the respiratory sense is 

 connected with a supply of blood containing little oxygen 

 and charged with carbonic acid to the systemic capillaries by 

 the arteries, and that it varies in intensity with the degree 

 of change in the blood. 



3. If, while artificial respiration is regularly performed, a 

 large artery be opened, and the system be thus drained of 

 blood, when the hemorrhage has proceeded to a certain ex- 

 tent, the animal makes respiratory efforts, which become 

 more and more violent, until they terminate, just before 

 death, in general convulsions. The same result follows when 

 the blood is prevented from getting to the system by applying 

 a ligature to the aorta. 



O 



These facts, which may be successively observed in a 

 single experiment, remain precisely the same if we previously 

 divide both pneumogastric nerves in the neck ; showing that 

 these are by no means the only nerves which convey the 

 respiratory sense to the medulla otilongata. 



The conclusions which may legitimately be drawn from 

 the above-mentioned facts are the following : 



The respiratory sense has its seat in the system, and is 

 transmitted to the medulla oblongata by the general sensory 

 nerves. It is not located in the lungs, for it operates when 

 the lungs are regularly filled with pure air, if the system be 

 drained of the oxygen-carrying fluid. 



