RESPIRATION 255 



curve is ascending in the carotid, it is descending in the 

 jugular. 



The respiratory variations in the volume of the brain, 

 which are so striking a phenomenon when a trephine hole is 

 made in the skull, have by some been attributed to inter- 

 ference 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 question turns largely upon the time-relations of the 

 movements. The swelling of the brain is usually syn- 

 chronous with expiration, and the shrinking with inspiration ; 

 and this is in favour of the first view. But sometimes the 

 dura mater bulges into the trephine hole in inspiration and 

 sinks down in expiration. This is in favour of the second. 

 The truth appears to be that both factors may be involved. 



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 con- 

 nected 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 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 (Einbrodt). 



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

 to diminish the intra-thoracic 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 diminu- 

 tion in it to lessen, the pressure inside the thorax, which 

 always remains equal to the intra-alveolar pressure, minus 



