RESPIRATION. 393 



Now, it is well known that the inhibitory power of the vagus in 

 man is very great, and its power varies in different individuals; this 

 would explain why the thirteen other glass-blowers showed no habitual 

 acceleration of the heart. As this performance is kept up many 

 hours daily for a series of years, it is easy to conceive that the eardio- 

 inhibitory power of the vagus center receives such a diminution of 

 irritability so often that it would at length remain constantly weak. 



Respiration of Different Gases. The usual and normal medium 

 inspired is ordinary atmospheric air, from which there is derived the 

 needful supply of oxygen. The open atmosphere is a mixture of gases 

 in the following approximate proportions : 



f Nitrogen, including argon, etc ... . 79.00 ""j 



Atmosphere J Oxygen 20.96 L n 100 partgi 



[^ Carbon dioxide , 0.04 J 



Argon, NH 3 , H 2 O, and organic matter in 

 small variable quantities. 



Though the quantity of water in the air is marked, over 1 per 

 cent., it is not customary to reckon it in the gaseous constituents. 



Some gases, as hydrogen and nitrogen, produce no specific effects 

 from any toxic powers in themselves when they are breathed; "they 

 produce results simply because they exclude the proper supply of 

 oxygen for the animal. On the other hand, gases such as carbon 

 dioxide, carbon monoxide, nitrous oxide, and hydrogen sulphide, 

 when respired in sufficient bulk, are absorbed and so produce specific, 

 toxic effects. A third class of gases, such as ammonia and nitric 

 oxide, are not respirable, because of their highly irritant action upon 

 the respiratory apparatus. Spasm of the glottis is produced. 



Carbon dioxide, when undiluted, is irrespirable by reason of the 

 spasm of the glottis produced by it. Properly diluted it can be 

 respired, but produces headache, dizziness, drowsiness, and dyspnoea 

 by an action on the nervous system. Nitrous oxide acts directly 

 upon the nervous system, partly by a special action and partly by 

 producing an excess of C0 2 in the blood. Nitrogen and hydrogen 

 gases produce their fatal effects by asphyxia, due to exclusion of the 

 oxygen and thereby preventing oxygenation of the blood-corpuscles. 

 Differing from these gases are the effects produced by inhalation of 

 carbon monoxide. It was long known that this gas was poisonous, 

 but it has only been within recent years that its mode of producing 

 asphyxia has been learned. Instead of excluding the oxygen, it 

 displaces the latter in the blood, forming a very stable compound 

 with the haemoglobin of the red corpuscles. It is interesting to note 



