206 HAKDKOOR OF PHYSIOLOGY. 



and the air in the lung tissue by the blocking of one or more branches 

 of the pulmonary artery, may be mentioned the following : 



Alteration in the atmospheric pressure. The normal condition of 

 breathing is that the oxygen of the air breathed should be at the pres- 

 sure of \ of the atmosphere, viz., ^ of 760 mm. of mercury, or 152 mm., 

 but it is found that life may be carried on by gradual diminution of 

 the oyxgen pressure to considerably less than one half of this, viz., to 

 76 mm., or ^ partial pressure, which is reached at an altitude above 

 15,000 feet.* Any pressure less than this may begin to produce altera- 

 tions in the relations of the gases in the blood, and if an animal is sub- 

 jected suddenly to a marked decrease of barometric pressure, and so of 

 oxygen pressure (below 7 per cent), it is thrown into convulsions, and it 

 is found that the gases are set free in the blood-vessels, no doubt carbon 

 dioxide and oyxgen as well as nitrogen, although the latter is the xmly 

 one of the three gases the presence of which in the vessels in death 

 from this condition of affairs has been proved; the others are said to 

 be reabsorbed. Other derangements may precede this, e.g., bleeding 

 from the nose, dyspnoea, and vascular derangement. On the other hand, 

 the oygxen may be gradually increased to a considerable extent without 

 marked effect, even to the extent of 8 or 10 atmospheres, but when the 

 oxygen pressure is increased up to 20 atmospheres the animals experi- 

 mented upon by Paul Bert died with severe tetanic convulsions. The 

 alteration of pressure above or below a certain average affects primarily 

 the gaseous interchange in the lungs, and then that in the tissues gene- 

 rally, but signs of dyspnoea may be produced as well either by cutting 

 off the supply of blood to the medullary centres, or by warming the blood 

 of the carotid arteries which supply them. The cause in the former 

 case being the deprivation of oxygen and the accumulation of the car- 

 bon dioxide, and of the latter, the increased metabolism of the centre 

 set up by the warmed blood. 



* For an interesting account of the symptoms produced by diminished atmos- 

 pheric pressure in those mounting to very high altitudes, Whymper's "Travels 

 amongst the Andes of the Equator" may be consulted. 



