620 RESPIRATORY UNDULATIONS. [BOOK 11. 



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

 that discussion by saying that the respiratory movements affect 

 $he 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 

 through the changes in the gases of the blood. 



Changes in the blood may affect on the one hand the v^so- 

 motor system and on the other hand the heart. They may 

 further affect the heart either directly by acting on the cardiac 

 tissues or indirectly by means of the inhibitory and augmentor 

 cardiac nerves. They may also probably affect the peripheral 

 vessels, not only through vaso-motor nerves but by acting directly 

 on the walls of the smaller vessels. We have indications of an 

 action of respiration on the cardio-inhibitory system, even in 

 normal quiet respiration. 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 undu- 



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

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



