390 RESPIRATION. 



bloodvessels more directly exposed to the diminution of pressure are pro- 

 foundly affected by it ; and what is true of them is, probably, in various 

 ways and to different degrees, true of the whole vascular system. The 

 breathlessness which is so marked a feature on these occasions seems due 

 not so much to the fact that the blood which reaches the respiratory nervous 

 centres is deficient in oxygen, as to the fact that the troubled vascular sys- 

 tem fails to deliver to those centres their blood in an adequate fashion. 



It is a feature of the vascular system, and indeed of the other mechan- 

 isms of the body in which nervous factors intervene, that they possess the 

 power of adapting themselves to changed conditions ; and as is well known, 

 the human organism somewhat rapidly becomes accustomed to these mode- 

 rate altitudes. Practice and custom have far less effect, though they have 

 some, on the more fundamental processes depending on the actual supply of 

 oxygen ; and it is at the extreme altitudes, where in addition to the other 

 troubles a deficiency of oxygen definitely makes itself felt, that the body 

 seems to fail in adapting itself to the new circumstances. 



The addition of these troubles not directly respiratory in nature, when 

 the supply of oxygen is diminished by a diminution of the total pressure, 

 perhaps explains why though an adequate lowering of pressure will produce 

 asphyxia, that asphyxia is somewhat different from the ordinary asphyxia 

 due to deprivation of air or oxygen. Convulsions which are essential to 

 ordinary asphyxia are at times wholly absent ; the nervous system under 

 the peculiar conditions does not respond to the stimulus of the lack of 

 oxygen ; and other nervous symptoms, such as a rapid onset of feebleness 

 amounting almost to paralysis, are apt to make their appearance. 



323. The effects of increase of atmospheric pressure. These are in 

 many ways remarkable. Up to a pressure of several atmospheres of air, the 

 only symptoms which present themselves are those somewhat resembling 

 narcotic poisoning. The animal becomes sleepy and stupid, the result pro- 

 bably not so much of respiratory changes, as of the effects of the increased 

 pressure on the whole organism to which we have just alluded. At a pres- 

 sure, however, of 15 atmospheres of air, or what amounts to the same thing, 

 of 3 atmospheres of oxygen, and upward, a very remarkable phenomenon 

 presents itself. The animals die of asphyxia and convulsions, exactly in the 

 same way as when oxygen is deficient. Corresponding with this it is found 

 that the production of carbonic acid is diminished. That is to say, when 

 the pressure of the oxygen is increased beyond a certain limit, the oxida- 

 tions of the body are diminished, and with a still further increase of oxygen 

 are arrested altogether. The oxidation of phosphorus is perhaps analo- 

 gous ; at a high pressure of oxygen phosphorus will not burn. Not only 

 animals, but plants, bacteria, and organized ferments, are similarly killed by 

 too great a pressure of oxygen. 



THE RELATIONS OF THE RESPIRATORY SYSTEM TO THE VASCULAR AND 



OTHER SYSTEMS. 



324. Many events in the body show the influence which the respiratory 

 movements exert on the circulation. When the brain of a living mammal 

 is exposed by the removal of the skull, a rhythmic rise and fall of the cere- 

 bral mass, a pulsation of the brain, quite distinct from the movements caused 

 by the pulse in the arteries of the brain, is observed ; and upon examination 

 it will be found that these movements are synchronous with the respiratory 

 movements, the brain rising up during expiration and sinking during inspira- 

 tion. They disappear when the arteries going to the brain are ligatured, or 

 when the venous sinuses of the dura mater are laid open so as to admit of a 



