422 THE HUMAN BODY. 



tion; and so, through the pneumogastric nerves, the respira- 

 tory mechanism is largely self -regulating. 



Asphyxia. Asphyxia is death from suffocation, or want 

 of oxygen by the tissues. It may be brought about in 

 various ways; as by strangulation, which prevents the entry 

 of air into the lungs; or by exposure in an atmosphere con- 

 taining no oxygen; or by putting an animal in a vacuum; 

 or by making it breathe air containing a gas which has a 

 stronger affinity for haemoglobin than oxygen has, and which, 

 therefore, turns the oxygen out of the red corpuscles and 

 takes its place. The gases which do the latter are very in- 

 teresting since they serve to prove conclusively that the Body 

 can only live by the oxygen carried round by the haemoglobin 

 of the red corpuscles; that amount dissolved in the blood 

 plasma being insufficient for its needs. Of such gases carbon 

 monoxide is the most important and best studied; in the fre- 

 quent French mode of committing suicide by stopping up all 

 the ventilation holes of a room and burning charcoal in it, it 

 is poisoning by carbon monoxide which causes death. 



The Relations of Carbon Monoxide to Haemoglobin. 

 If aerated whipped blood, or a solution of oxyhyaemoglobin, 

 be exposed to a gaseous mixture containing carbon monoxide, 

 the liquid will absorb the latter gas and give off oxygen. 

 The amount of carbon monoxide taken up will (apart from 

 a small amount dissolved in the plasma) be independent of 

 the partial pressure of that gas in the gaseous mixture to 

 which the blood is exposed ; the quantity absorbed depends 

 on the quantity of haemoglobin in the liquid, and is replaced 

 by an equal volume of oxygen liberated. This equivalence of 

 volume, of itself, proves that the phenomenon is due to the 

 chemical replacement of oxygen in some compound, by the 

 carbon monoxide; for if the carbon monoxide were merely 

 dissolved in the liquid in proportion to its partial pressure on 

 the surface, it would turn out no oxygen; the quantity of 

 dissolved gases held by a liquid being dependent only on the 

 partial pressure of each individual gas on its surface, and 

 unaffected by that of all others. During the taking up of 

 carbon monoxide the blood changes color in a way that can 

 be recognized by a practised eye; it becomes cherry-red in- 

 stead of scarlet. This shows that some new chemical com- 

 pound has been formed in it; examination with the spectro- 

 scope confirms this, and shows the color change to be due to 



