EFFECTS OF BREA THING CONDENSED AND RAREFIED AIR 289 



The respiratory variations in the volume of the brain, which are 

 so striking a phenomenon when a trephine hole is made in the 

 skull, but which can also take place, thanks to the displacement of 

 cerebro- spinal fluid (p. 174), when the cranium is intact, have by 

 some been attributed to interference with the venous outflow from 

 the cranial cavity during expiration, and by others to those changes 

 in the arterial pressure whose causes we have just been discussing. ' 

 The truth is that neither factor is exclusively concerned. The ques- 

 tion turns largely upon the time-relations of the movements. The 

 swelling of the brain is sometimes synchronous with expiration, and 

 the shrinking with inspiration. Here the damming back of the 

 blood in the sinuses when the outflow is checked by the expiratory 

 rise of pressure in the thoracic veins either conspires with an expira- 

 tory rise of arterial pressure or is more than enough to counter- 

 balance an expiratory fall of pressure in the cerebral arteries if the 

 respiratory conditions are such as to lead to an expiratory fall. But 

 sometimes the dura mater bulges into the trephine hole in inspira- 

 tion and sinks down in expiration. Here the increase in the volume 

 of the brain produced by the increased pressure in the arteries and 

 capillaries in inspiration is more than sufficient to counterbalance 

 the quickened escape of blood from the cerebral veins. 



SECTION VIII. THE EFFECTS OF BREATHING CONDENSED AND 



RAREFIED AIR. 



These are (i) mechanical, shown chiefly by changes in the cir- 

 culation, in the blood- pressure, for instance ; (2) chemical. 



The mechanical effects differ according to whether the whole body, 

 or only the respiratory tract, is exposed to the altered pressure. 

 When the trachea of an animal is connected with a chamber in 

 which the pressure can be raised or lowered, it is found that at first 

 the arterial blood-pressure rises as the pressure of the air of respira- 

 tion is increased above that of the atmosphere. But a maximum 

 is soon reached; and when respiration begins to be impeded, the 

 pressure falls in the arteries and increases in the veins. When the 

 pressure of the air in the chamber is diminished a little below that 

 of the atmosphere, there is a slight sinking of the arterial blood- 

 pressure, which rises if the air-pressure is further diminished. 



It is clear that any change of the air-pressure which tends to diminish 

 the intrathoracic pressure will favour the venous return to the heart, 

 and therefore, if the exit of blood from the thorax is not proportionally 

 impeded, the filling of the arteries. An increase in the intra-alveolar 

 pressure must tend on the whole to increase, and a diminution in it to 

 lessen, the pressure inside the thorax, which always remains equal to 

 the intra-alveolar pressure, minus the elastic tension of the lungs. 

 Breathing compressed air should, therefore, under the conditions 

 described, be upon the whole unfavourable to the venous return to the 



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