37 2 RESPIRATION. [CH. xxiv. 



burning charcoal stoves in small, close rooms), are due to its 

 entering into combination with the hsemoglobin of the blood- 

 corpuscles and thus expelling the oxygen. Hydrogen may take 

 the place of nitrogen if the oxygen is in the usual proportion, 

 with no marked ill effect. Sulphuretted hydrogen interferes 

 with the oxygenation of blood. Nitrous oxide acts directly 

 on the nervous system as a narcotic. Certain gases, such as 

 carbon dioxide in more than a certain proportion ; sulphurous 

 and other acid gases, ammonia, and chlorine produce spas- 

 modic closure of the glottis, and are irrespirable. 



Alterations in the Atmospheric Pressure. 



The normal condition of breathing is that the oxygen of the air 

 breathed should be at the pressure of ^ of the atmosphere, viz., -^ of 

 760 mm. of mercury, or 152 mm., but considerable variations may 

 occur without producing ill effects. This is due to the fact that 

 the blood gases are mostly in a state of chemical combination, not 

 of simple solution. Variations beyond certain limits are, however, 

 fatal. When the tension of oxygen exceeds 3^ atmospheres (i.e., in 

 air at a pressure of 1 7 atmospheres), slow but powerful poisonous 

 (narcotic) effects are produced on all living matter. (Bert.) The 

 excised sartorius is paralysed by about half an hour's exposure to 

 80 atmospheres of oxygen ; and the excised frog's heart ceases 

 to beat in about two hours under the same conditions. It is 

 dangerous for men to work in caissons where the atmospheric 

 pressure is greater than 4 atmospheres. Even lower pressures 

 may be followed on " decompression " (i.e., on coming out of the 

 increased pressure), by what are called "bends," that is, pains in 

 the joints and muscles, by paralysis, and auditory symptoms such 

 as deafness and vertigo. The cause of such symptoms is probably 

 the setting free of bubbles of nitrogen in the lymph spaces and 

 capillaries ; any oxygen set free is rapidly re-absorbed by the 

 blood. Capillary embolism from gas bubbles in the central 

 nervous system is the most probable cause of the paralysis. 

 (Bert.) Oxygen poisoning may be a secondary cause of the 

 symptoms. Short shifts are essential for caisson workers, for 

 then the body has not time to become saturated with gas at the 

 caisson pressure. 



A toad was but slightly affected by 5 minutes exposure to 

 20 atmospheres of oxygen, but after 40 minutes on "decom- 

 pression " it went into tetanic convulsions and died ; the heart 

 was distended with frothed blood ; bubbles of gas were in all the 

 lymph spaces, in the anterior chamber of the eye, and other parts. 



