R-ESPIRATIOtf. 133 



18. History furnishes many painful instances of the ill 

 effects of overcrowding. In 1756, of one hundred and 

 forty-six Englishmen imprisoned in the Black Hole of 

 Calcutta, only twenty-three, at the end of eight hours, 

 survived. After the battle of Austerlitz, three hundred 

 prisoners were crowded into a cavern, where, in a few 

 hours, two-thirds of their number died. On board a 

 steam-ship, during a stormy night, one hundred and 

 fifty passengers were confined in a small cabin, but 

 when morning came, only eighty remained alive. 



19. Changes in the Blood from Respiration. 

 The most striking change which the blood undergoes by 

 its passage through the lungs, is the change of color from 

 a dark blue to bright red. That this change is dependent 

 upon respiration has been fully proved by experiment. 

 If the trachea, or windpipe, of a living animal be so com- 

 pressed as to exclude the air from, the lungs, the blood in 

 the arteries will gradually "grow darker, until its color is 

 thelrame as that of the venous blood. When the pressure 

 is removed the^ blood speedily resumes its bright hue. 

 Again, if the animal be made to. breathe an atmosphere 

 containing more oxygen than atmospheric air, the color 

 changes from scarlet to vermilion, and becomes even 

 brighter than arterial blood. This change of color is not 

 of itself a very important matter, but it indicates a most 

 important change of composition. 



20. The air, as we have seen, by respiration loses oxy- 

 gen and gains carbonic acid : the blood, on the contrary, 

 gains oxygen and loses carbonic acid. The oxygen is the 

 food of the blood corpuscles; while the articles we eat and 

 drink belong more particularly to the plasma of the blood. 

 The air, then, it is plain, is a sort of food, and we should 



18. Give some of the instances furnished by history. 



If). Change in the blood from blue to red. Upon what does the change depend ? 

 How shown ? 



2O. What does the air lose and gain by respiration ? What, the blood ? Air as 

 food? 



