652 RESPIRATORY UNDULATIONS. [BOOK n. 



are those discussed in 383. We may restate the conclusions of 

 that discussion by saying that the respiratory movements affect 

 the amount of flow of blood into the left ventricle, and so the 

 discharge of blood from the left ventricle into the aorta, in two 

 main ways. In the first place, through the widening or narrowing 

 of the pulmonary vessels they alter the capacity of the vessels to 

 hold blood for the time being. In the second place, in conse- 

 quence of the difference of resistance, occasioned by the widening 

 or narrowing, they alter the rate of flow through the pulmonary 

 vessels. The first factor is a brief and passing one ; the extra 

 room due to widening is soon filled up, the narrowed vessels soon 

 discharge the quantity which they can no longer hold. But the 

 second factor is a more lasting one ; so long as in the respiratory 

 movement the vessels remain widened or narrowed so long is the 

 rate of flow increased or diminished. These two factors produce 

 opposite effects, and hence the total result of any particular kind 

 of respiration will depend on their relative prominence. With 

 quickly repeated respiratory movements the first factor comes to 

 the front ; when the respiratory movements are more slowly re- 

 peated and more slowly carried out the second factor is the more 

 potent. Hence it comes about that in quickly repeated artificial 

 respiration where the first factor is predominant, and the promi- 

 nent effect of each inflation is to diminish the capacity of, and 

 so to empty the pulmonary vessels and to increase the flow into 

 the ventricle whereby the pressure rises in inflation, that is in 

 inspiration, the blood-pressure curve simulates that of a slowly 

 repeated natural respiration, where the pressure also rises in 

 inspiration, but where, the second factor being predominant, the 

 rise of pressure brought about by each inspiration is "due mainly 

 to the more rapid flow through the widened pulmonary vessels. 

 And other illustrations of a like kind could be given. 



385. Besides the mechanical effects of the respiratory 

 movements the vascular system is influenced by respiration 

 in other ways. 



We have indications of a connection between the respiratory 

 and the cardio-inhibitory systems, even in normal quiet respiration. 

 One striking feature of the respiratory undulation in the blood- 

 pressure curve of the dog 1 and certain other animals is the fact that 

 the pulse-rate is quickened during the rise of the undulation and 

 becomes slower during the fall ; see Fig. 85. A similar influence 

 may be seen in pulse-tracings taken from man. The quickening of 

 the beat might be considered as itself partly accounting for the 

 rise of pressure, or on the other hand it might be urged that the 

 increased flow of blood which causes the rise of pressure leads at 

 the same time to the quickening of the beat, were it not for one 

 fact, viz. that the difference is at once done away with, without 



1 In the rabbit and some other animals the respiratory undulations, though well 

 marked, present a very small difference of pulse-rate in the rise and fall. 



