Theories and Experiments 207 



needs only to be able to overcome the resistance which the contractile 

 power of these organs opposes to it, for there is no thoracic air to 

 increase this resistance, and this pressure hardly exceeds that of two 

 inches of mercury; whence it follows that an air, even when extremely 

 rarefied, still exerts sufficient pressure for the mechanism of 

 respiration. 



So he reaches this opinion that "the suffocation of animals kept 

 in closed vessels is the work of vapors". But following him in this 

 path would lead us astray from our subject. 



We shall return to travellers who have ascended lofty moun- 

 tains; but we should first report the interesting experiments of 

 the poet-naturalist Darwin 14 and the curious theoretical conclu- 

 sions which he draws from them; we shall return to these explana- 

 tions later. 



The author asks himself whether there really exist in the blood 

 elastic vapors of some sort or other, which could cause "lunar and 

 equinoctial maladies" to be attributed to variations of the atmos- 

 pheric pressure: 



The truth of this opinion (he says) seems to be demonstrated by 

 the following experiment: Four ounces of blood are drawn from the 

 vein of the arm and immediately placed in the reservoir of an airpump: 

 when the air is removed, the blood begins to froth and rise in bubbles 

 so as to occupy ten times its original volume. 



But that reasoning is mistaken, says Darwin. If, in an animal 

 which has just been killed, a certain length of a vessel full of blood 

 is isolated between two ligatures, and this fragment is placed in a 

 vessel full of water, under the receiver of the pump, it remains at 

 the bottom of the water when a vacuum has been made, without 

 rising or swelling, as it should do if it really contained air: 



So a great change is produced in the blood drawn from the vein 

 by the introduction of atmospheric air .... Therefore a cupping-glass 

 applied to a living animal brings out no froth, as happens in a 

 vacuum. 



It is, therefore, probable that animals can undergo great variations 

 in pressure without inconvenience .... Some persons who have 

 ascended lofty mountains report that they have spat blood; but that 

 has never been noted in animals placed in the pneumatic machine, 

 where the decrease in pressure was greater than occurs on the highest 

 mountains. This blood-spitting was therefore an incidental disturb- 

 ance, or was the result of the violent exercise of the ascent. 



We have seen, quoted above by Veratti, the explanation at first 

 given by Borelli of the symptoms of decompression, which he him- 

 self had experienced when he ascended Etna; he thought they were 

 the result of a sort "of effervescence which might occur in the 



