396 RESPIRATION. 



the capacity of the vessels to hold blood for the time being. In the 

 second place, in consequence of the difference of resistance, occasioned by 

 the widening or narrowing, they alter the rate of flow through the pul- 

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

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

 charge 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 prominent 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 infla- 

 tion, that is in inspiration, the blood-pressure curve stimulates that of a 

 slowly repeated natural respiration, where the pressure also rises in in- 

 spiration, 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 illustra- 

 tions of a like kind could be given. 



328. 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 vasomotor 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 inhib- 

 itory and augmentor cardiac nerves. They may also, probably, affect the 

 peripheral vessels, not only through vasomotor 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 undulation and becomes slower during the fall. (See Fig. 107.) 

 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 in- 

 creased flow of blood which causes the rise of pressure, at the same time 

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

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

 the undulations, by section of both vagus nerves. Evidently the slower pulse 

 during the fall is caused by a coincident stimulation of the cardio-inhibitory 

 centre in the medulla oblongata, the quicker pulse during the rise being due 

 to the fact that, during that interval, the centre is comparatively at rest. 

 We have here indications that, while the respiratory centre in the medulla . 

 oblongata is at work, sending out rhythmic impulses of inspiration and ex- 

 piration, the neighboring cardio-inhibitory centre is, as it were, by sympathy, 

 thrown into an activity of such a kind that its influence over the heart waxes 

 with each expiration and wanes with each inspiration. We cannot as yet 

 explain exactly the manner in which the activity of the one centre influ- 

 ences that of the other ; it may be that during the expiratory phase the 



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

 difference of pulse-rate in the rise and fall. 



