272 A MANUAL OF PHYSIOLOGY . 



displacement of cerebro-spinal fluid (p. 160), 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 question 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 expiratory rise of 

 arterial pressure or is more than enough to counterbalance an 

 expiratory fall of pressure in the cerebral arteries if the respiratory 

 conditions are such as to lead to an expiratory fall. But some- 

 times the dura mater bulges into the trephine hole in inspiration 

 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 counter- 

 balance the quickened escape of blood from the cerebral veins. 



The effects of breathing condensed and rarefied air are 

 (i) mechanical, shown chiefly by changes in the circulation, 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 respiration is increased above that of the atmo- 

 sphere. But a maximum is soon reached ; and when respira- 

 tion 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 

 dimmish 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 un- 

 favourable to the venous return to the heart and to the filling of 

 the arteries, and the arterial pressure should fall ; while breathing 

 rarefied air should have the opposite effect. But a very great 

 diminution of the intrathoracic pressure is not necessarily favourable 

 to the circulation, since the auricles are then unable to contract" 

 perfectly. 



