72 PHYSIOLOGY CHAP. 



Very little work has been done on the determination of the 

 limits between which the total pressure of the air and water, and 

 the partial pressure of the oxygen and carbonic acid which these 

 contain, condition the life of the organisms which inhabit them. 



The experiments of Paul Bert (1873) bring out the interesting 

 fact that pure oxygen under a pressure of three atmospheres is 

 fatal to warm-blooded animals, while ordinary air only produces 

 the same effect at a pressure of 15-20 atmospheres. The same 

 fatal effect ensues when the partial pressure of the oxygen of the 

 air is reduced below a certain limit. 



In order to determine how great a fall in the barometric 

 pressure is compatible with life, we may utilise certain data 

 furnished not so much by ascents of the highest mountains as 

 from aerostatic ascents, in which the effects of fall of barometric 

 pressure are not complicated by muscular fatigue. The famous 

 ascent by Croce-Spinelli, Sivel, and Tissandier in 1875 was fatal 

 to the two former. When the balloon reached 8000 metres 

 Tissandier, the sole survivor, lost consciousness, and only came to 

 his senses when the balloon had dropped to 7059 metres. 



We know hardly anything of the effect of aqueous pressures 

 upon sea animals. Contrary to former conceptions, it has within 

 the last few decades been ascertained that there exists a special 

 flora and fauna at the lowest depths of the ocean, in regions where 

 there is a pressure of several hundred atmospheres, and where no 

 light can ever penetrate. The fishes caught at the greatest 

 depths are, when first brought to the surface, so distended in 

 consequence of the sudden reduction of pressure, which allows the 

 gases in their bladder to expand, that the viscera protrude from 

 their mouths and the scales stand up (Keller). 



In regard to the pressure exerted by water upon marine 

 animals, the fact must be insisted on that it exercises a great 

 influence only upon such organs as, like the fish's swim-bladder, 

 contain gas in the gaseous state, and do not communicate with the 

 exterior. The tissues of these animals, which may be considered 

 as liquids, only feel the effects of the high pressure in a negligible 

 degree, since, as we know from physics, several hundredths of 

 atmospheric pressure are necessary in order to obtain any marked 

 diminution in volume of fluids these being practically incom- 

 pressible. This is confirmed by the fact that marine animals, such 

 as Echinoderms, Molluscs, Crabs, and Selachians or Teleosteans, 

 which have no swim-bladder, and normally live at a great pelagic 

 depth, can be transported to the surface without any danger, and 

 continue to live for a long time in ordinary aquaria when the 

 pressure of the water is from |-1 metre. 



VI. With the exception of those above enumerated, none of 

 the general external conditions are essential to life. Other 

 external physical or chemical factors may indeed influence vital 



