388 RELATION OF RESPIRATION TO OTHER PROCESSES [GIL XXVII. 



becomes more marked with each inflation than it does when the 

 thorax is open. 



The last point of detail we have to consider is the cause of the 

 greater frequency of the heart during the inspiratory phase, a 

 phenomenon which is evidently due to lessening of vagus action, 

 since the inequality of the heart-rate disappears when the vagi are 

 cut. The question before us is, What is the cause of the rhythm in 

 the activity of the vagus centre ? There appear to be two factors 

 concerned in its causation : one is a reflex action, the other is what 

 may be termed a central overflow. We will consider these separately. 



1. The reflex. Stimulation of the pulmonary branches of the vagus 

 by electrical stimuli, or of their terminations in the alveoli by certain 

 irritating vapours such as bromine, causes a reflex inhibition of the 

 heart ; great distension of the alveoli has a similar effect, but moderate 

 distension, such as occurs in an ordinary inspiration, has the opposite 

 reflex effect, causing the heart to beat more rapidly. The afferent 

 fibres from the pulmonary alveoli enter the bulb by the upper set of 

 the rootlets of the combined glossopharyngeal- vagus-spinal accessory 

 nucleus (the a group, p. 249). Sometimes the rootlets of this group are 

 three in number, sometimes two. When there are two, the lower 

 rootlet, when there are three, the lower two rootlets, contain the fibres 

 in question (Cadman). 



2. The overflow. The respiratory centre exhibits alternate phases 

 of activity, or what is termed a rhythmical action. It is in close 

 anatomical connection with two other important centres in the bulb, 

 namely, the cardio-inhibitory and the vaso-motor centres. Consider- 

 ing how closely these three centres are connected by association 

 fibres, it is not surprising that the cells of the two latter centres 

 should be affected by the rhythm of the cells of the respiratory 

 centre, and the term overflow is an expression that roughly indicates 

 what occurs. This overflow from the respiratory centre affects its 

 two neighbours in the same way. During inspiration the activity 

 of both the cardio-inhibitory centre and of the vaso-motor centre is 

 diminished, hence the heart beats faster. The factor which we have 

 termed the overflow is more important than that which we have 

 described as the reflex. 



These facts show us that the parallelism of the respiratory and 

 arterial pressure -curves is not merely the result of the mechanical 

 conditions already described, though these are the most important. 

 But in the normal condition with the thorax closed, and the vagi 

 uncut, certain nervous factors come also into play. 



Valsalva's Experiment. In speaking of the effects of expiration, 

 we have considered only ordinary quiet expiration. With forced 

 expiration, there is considerable impediment to the circulation ; this 

 is markedly seen in what is called Yalsalva's experiment. This con- 



