164 ELEMENTARY PHYSIOLOGY less. 



becomes accelerated. Indeed, if the amount of acid, 

 of whatever kind it may be, in the arterial blood be 

 materially increased, the respiratory centre is stimulated 

 and respiration is quickened. Small changes in the 

 partial pressure of oxygen in the air breathed do not 

 affect the respiratory rhythm but great deprivation of 

 oxygen rapidly jjroduces the symptoms of aspliyxia. 

 The reason of this is that in the absence of a sufficient 

 supply of oxygen the combustion in the tissues is 

 incomplete, large quantities of acid substances are 

 formed as the result, these are thrust into the blood 

 where they accumulate, and stimulate the respiratory 

 centre to make the desperate efforts v, hich are character- 

 istic of asphyxia. The acids produced in the tissues, in 

 their stimulating action on the respiratory centre, act as 

 hormones (p. 27). 



Tliat the lack of oxygen is an important thing is 

 further shown by the asphyxiating effects of certain 

 poisonous gases. Thus sulphuretted hydrogen, so well 

 known by its offensive smell, has long had the repute of 

 being a positive poison. But its evil effects appear to 

 arise chiefly, if not wholly, from the circumstance that its 

 hydnjgen combines with the oxygen carried by the blood- 

 corpuscles, and thus gives rise, indirectly, to a form of 

 oxygen starvation. 



Carbonic oxide gas (carbon monoxide, CO) has a much 

 moi'e serious effect, as it turns out the oxygen from the 

 blood-corpuscles, and forms a very stable combination of 

 its own with the hjemoglobin. The compound thus 

 formed is only very gradually decomposed by fresh 

 oxygen, so that if any large proportion of the blood- 

 corpuscles be thus rendered useless, the animal dies 

 before restoration can be effected. Badly made common 

 coal gas sometimes contains 20 to 80 per cent, of carbon 

 monoxide ; and, under these circumstances, a leakage 



