CH. xxiv.] CHEMISTRY OF RESPIRATION. 373 



A mouse in a similar high pressure is narcotised, and on " decom- 

 pression" convulsions and death ensue. (L. Hill.) Prolonged 

 exposure to 2 atmospheres of oxygen is followed by pneumonia. 

 (Lorrain Smith.) Mechanical pressure by itself has little or no 

 influence ; thus frog's muscle is not injured by exposure to fluid 

 pressure in salt solution equal to 400 atmospheres. Crustacea 

 are found alive on marine telegraph cables at a depth where the 

 pressure is as great. 



Turning now to diminution of pressure, we find that "mountain 

 sickness" occurs at the height of 4,800 metres, the summit of 

 Mt. Hlanc. Here the atmospheric pressure is 418 mm. of mercury, 

 and the pressure of oxygen is only 11*53 per cent, of an atmo- 

 sphere. The malady is increased by muscular effort, and is due 

 to want of oxygen. In those who habitually live in high altitudes, 

 the number of red blood corpuscles is said to be increased. Croce- 

 Spinelli, the balloonist, perished at an altitude of 8,600 metres, 

 where the tension of oxygen would be 7 per cent, of an atmosphere. 

 His companion Tissandier recovered. In such cases muscular 

 paralysis occurs before loss of consciousness. Higher ascents 

 could be made by aeronauts if they breathed oxygen from a gas 

 cylinder. (Bert.) That death is due to want of oxygen and not 

 to the setting free of gas bubbles in the blood is shown by the 

 following fact : a sparrow lived in pure oxygen at 95 mm. of 

 mercury pressure. Haldane has shown that animals can live in 

 two atmospheres of oxygen after all the haemoglobin is taken up 

 by carbonic oxide, for then sufficient oxygen is dissolved in the 

 blood-plasma. 



The foregoing paragraph is written largely from notes kindly provided me 

 by Dr. Leonard Hill. F.R.S., who has recently been working at the subject 

 in connection with ''caisson disease." I have to thank him for allowing me 

 to quote some of his unpublished experiments. 



CHBMISTRY OF RESPIRATION. 



The atmospheric air does not to any great extent penetrate beyond 

 the bronchial tubes ; the gases which get into the smaller tubes 

 and air-vesicles do so principally by diffusion. The most vigorous 

 expiratory effort is unable to expel the alveolar air. This air and 

 the blood in the capillaries are separated only by the thin capil- 

 lary ;ind alveolar walls. The blood parts with its excess of car- 

 bonic acid and watery vapour to the alveolar air ; the blood at 

 the same time receives from the alveolar air a supply of oxygen 

 which renders it arterial. 



The intake of oxygen is the commencement, and the output 

 of carbonic acid is the end of the series of changes known as 



