PULMONARY NEKVES. 237 



that tliis function is normally influenced through these nerves, 

 by impressions conveyed to the centres ; but precisely what 

 this influence is, or what is the mechanism of its action, we 

 do not know. 



The positive statement that the sense of want of air is 

 not conveyed to the nerve-centres through the pneumogas- 

 trics is based, to a great extent, upon our own experiments, 

 which have been fully detailed in another volume,- 1 and we 

 will here give simply their results and the conclusions to 

 which they lead. 



The acts of respiration are involuntary, though they may 

 be modified, within certain limits, through the will ; and 

 they are reflex, due to an impression conveyed to the re- 

 spiratory nervous centre, the medulla oblongata, which gives 

 rise to the stimulus that excites the action of the inspira- 

 tory muscles. It has been conclusively shown by experi- 

 ments, the first being those of Robert Hook, 3 that if artifi- 

 cial respiration be efficiently carried on in a living animal, 

 so as to supply air fully to the system, the sense of want of 

 air is not appreciated, and the animal makes no effort to 

 breathe; but if respiration be imperfectly performed, the 

 animal almost immediately feels the want of air, and, in our 

 experiments, the exposed respiratory muscles were thrown 

 into violent but ineffectual contraction. 



The principal points with reference to the location of the 

 sense of want of air and its transmission to the nerve-centres, 

 developed by our own experiments, are the following : 



A dog was etherized, the chest was opened, exposing the 

 heart and lungs, and artificial respiration was carried on by 

 means of a bellows secured in the trachea. So long as the 

 supply of air was sufficient, the animal made no effort to 

 breathe, even when allowed to come from under the influ- 

 ence of the anaesthetic. 



1 See vol. i., Respiration, p. 479, et scq. 



2 An Account of an Experiment made by Mr. Hook, of Preserving Animals 

 alive by Blowing through their Lungs vrith BeUows. Philosophical Transactions, 

 London, 1667, voL ii., p. 539. 



