CHAP. JL] RESPIRATION. 371 



artificial respiration effects exactly the reverse of those which we 

 find in normal respiration ; and indeed in many curves of blood- 

 pressure taken during artificial respiration this is the case; but 

 here as in natural respiration the features of the blood-pressure 

 curve vary according as the breathing is hurried or slow, shallow 

 or deep. 



We may add that another explanation than those given above 

 has been offered of these effects of the respiratory movements. 

 It has been suggested that when the lung is expanded, the 

 increase in the area of the wall of each pulmonary alveolus tends 

 to stretch and elongate the capillaries lying in the alveolar walls, 

 and in elongating them necessarily narrows them, just as an 

 india-rubber tube is narrowed when it is stretched lengthways. 

 This narrowing of the capillaries would present an obstacle to 

 the passage of blood through them ; and hence the expansion 

 of the alveoli in inspiration, other things being equal, would 

 be unfavourable to the flow of blood through the lungs. It has been 

 further suggested that the first effect of the expansion of the alveolus 

 and narrowing of the capillaries would be to drive out suddenly the 

 blood at the moment contained in them and thus for the instant to 

 produce a passing increase of flow; and conversely that the first effect 

 of the collapse of the alveolus and consequent widening of the 

 capillaries would be to find room for an extra quantity of blood, and 

 thus for a moment to check the flow. Whether in each case the 

 first or the second phase becomes predominant would depend on 

 the rate and depth of the breathing. There are difficulties however 

 in accepting this view and the one previously given seems to be 

 the more valid one. 



From what has been said it is clear that the influences of 

 the respiratory movements are not only many but conflicting, and 

 that the exact effect at any one moment will vary according 

 as circumstances render one or other factor predominant. It will 

 hardly be profitable to make any further attempt to unravel 

 the complexity of the several cases. 



The relations between respiration and circulation which we 

 have just discussed are of a mechanical nature, but there are also 

 ties of a nervous kind between the two systems. One striking 

 feature of the respiratory undulation in the blood-pressure curve 

 of the dog 1 is the fact that the pulse-rate is quickened during the 

 rise of the undulation and becomes slower during the fall. The 

 quickening of the beat might be considered as itself partly 

 accounting for the rise, or on the other hand it might be urged 

 that the increased flow of blood which causes the rise, at the same 

 time leads to the quickening, were it not for one fact, viz. that the 

 difference is at once done away with, without any other essential 



1 In the rabbit, the respiratory undulations, though well marked, present a very 

 small difference of pulse-rate in the rise and fall. 



