130 THE LUNGS AND RESPIRATION. 



traces of ammonia and certain organic matters, gen- 

 erally the results of decomposition, which give a bad 

 odor to the breath and are more dangerous in a close 

 room than the mere lack of oxygen or the presence of 

 carbon dioxide. Indeed, the amount of oxygen may 

 be very much diminished, being reduced even to 5 or 

 6 per cent, instead of the normal 21 per cent., without 

 being noticed or giving rise to any immediate bad re- 

 sults. Yet the import? nee of ventilation is very 

 evident. 



Effect on Blood. Respiration causes changes also in 

 the blood, the venous blood being purple and the ar- 

 terial bright red. This difference in color is due to the 

 absence or presence of oxygen, which is not absorbed 

 or dissolved by the blood but forms a rather unstable 

 compound, oxy hemoglobin, with the hemoglobin of 

 the blood. As the oxgen is removed in the passage 

 of the blood through the body, there results in venous 

 blood reduced hemoglobin, which is of a purplish 

 color. Upon exposure to the air, however, it absorbs 

 oxygen once more and resumes its scarlet color. If 

 carbon monoxide gets into the blood, as in cases of 

 gas poisoning, it drives off the oxygen and forms a 

 more stable compound with the hemoglobin, whence 

 the difficulty in restoring a person so poisoned. 



Nervous Mechanism. Nervously, respiration is con- 

 trolled in three ways: 1. by the phrenic nerve to 

 the diaphragm; 2. by some fibers of the vagus or 

 pneumogastric, and 3. by the respiratory center in the 

 bulbous portion of the spinal cord. Injury to the res- 

 piratory center means the ceasing of respiration and 

 death. Stimulation of the respiratory center seems 

 to depend upon the character of the blood. If it is 

 well oxygenized, the breathing is slow and quiet; if 

 there is a lack of oxygen, dyspnoea results. Probably 

 certain chemical substances in the blood, which are 

 ordinarily rapidly burned up by the oxygen but which 

 accumulate in its absence, serve to stimulate the 



