186 AIR AND LIFE. 



and dies, even if the final pressure is one which, if brought on slowly, 

 would not be injurious to life. The danger lies only in the rapidity of 

 the change. 



Post-mortem examination of the victim affords a clue to the cause of 

 death, and makes all symptoms clear and intelligible. We find gas or 

 air in free condition under the skin, in the tissues, in the blood vessels; 

 this we never observe under normal conditions. These gases are the 

 cause of death. All tissues, and the blood, of course, contain at all 

 times gaseous matters — oxygen, nitrogen, carbonic acid — either dis- 

 solved in the liquids or combined with hemoglobin in the blood, and 

 the amount of these gases varies according to external pressure, 

 according to the tension of atmosphere. jSTow, if the atmospheric 

 pressure decreases gradually, the tension of the gases of the organism 

 decreases accordingly; and they escape gradually into the atmosphere 

 without making any trouble. But if the decrease is sudden, this grad- 

 ual escape can not be effected; the liberated gases have no time to 

 escape; the result is that they accumulate in all parts of the body, 

 and in the circulatory system they obstruct small vessels and paralyze 

 the heart. 1 Such accidents are not uncommon among the workmen 

 referred to, and this is the reason why they are always advised to 

 come up slowly to the surface, and the deeper they have been the 

 slower the change should be. They have little to fear from working in 

 compressed air at 2, 3, or 4 atmospheres; the danger lies in the 

 decrease of pressure, which, if sudden, is generally fatal. As they say 

 in their own language, li You have to pay only when you come out." 



So much for decrease of pressure, rapid or slow. In the one case 

 it injures by a deficiency of oxygen, by anoxyhremia, and the only 

 way to counteract its effects is to be x>rovided with a supply of oxygen 

 of which small amounts may be inhaled now and then. Aeronauts 

 intending to attain very high altitudes can not do without such a pro- 

 vision, and it is their custom now to always take with them a supply 

 of oxygen. In the other, the injury is the result of a quite different 

 process, purely mechanical, the sudden liberation of gases in the tis- 

 sues and especially in the blood, where they immediately interfere with 

 the circulation, and stop the heart's action. When moderate pressure 

 suddenly follows high pressure, anoxyhseniia plays no part, and only 

 the mechanical effects occur; if low pressure follows moderate aud 

 sufficient pressure, anoxylmemia alone occurs if the passage is slow; 

 anoxyh*mia and the mechanical liberation of gases ensue if the pas- 

 sage is rapid and sudden. In both cases, decrease of pressure inter- 

 feres with life. 



Let us now consider the reverse case, that of an increase of pressure. 



Under normal circumstances such increase is always unimportant. 



'Just as air, even in very small quantity, drawn in the circulatory system through 

 some lesion of the venous system near the heart induces death in a few secouds, as 

 all physiologists know. 



