374 RESPIRATION [CH. XXIV. 



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 17 atmos- 

 pheres), 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. Decompres- 

 sion must also be gradual and slow. 



A toad was but slightly effected by 5 minutes' exposure to 20 atmospheres of 

 oxygen, but after 40 minutes on "decompression" 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. A mouse 

 in a similar high pressure is narcotised, and on ** decompression " convulsions and 

 death ensue. (L. Hill.) Prolonged exposure to 2 atmospheres of oxygon 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 4800 metres, the summit of Mt Blanc. Here the pressure of oxygen 

 is only 1 1 '53 per cent of an atmosphere. 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 increased. Croce-Spinelli, the balloonist, 

 perished at an altitude of 8600 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. 



CHEMISTRY OF EESPIRATION. 



The air in the air vesicles and the blood in the capillaries are 

 separated only by the thin capillary and alveolar walls. The blood 

 parts with its excess of carbonic 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 respiration. 

 The gaseous interchange in the lungs is often called external respira- 

 tion. The actual combustion processes take place all over the body 

 and constitute what is known as internal or tissue-respiration. The 



