372 



RESPIRATION. 



ing from the respiratory centre are not simply afferent impulses reaching the 

 centre along afferent nerves and transformed by reflex action in that centre. 

 They evidently start de novo from the centre itself, however much their 

 characters may be affected by afferent impulses, reaching that centre at 

 the time of their being generated. The action of the centre is automatic, 

 not simply reflex. 



307. We find, on inquiry, that the activity of the centre is profoundly 

 influenced by two classes of events. These, as we might expect, are, on the 

 one hand, events producing changes in the quality of the blood distributed 

 to the medulla by the left ventricle, especially as regards its gases, that is to 

 say, events modifying the interchange taking place in the lungs ; and, on the 

 other hand, nervous impulses started in various ways and reaching the centre 

 along various nerves or nervous tracts. It will be convenient to consider the 

 latter first. 



Afferent nervous impulses may affect the centre in many various ways. 

 The whole act of breathing or of taking a breath is a double act, consist- 

 ing of an inspiration and an expiration, and nervous impulses may especially 

 affect the one or the other. One mode of breathing may differ from another 

 in the depth of the individual breath, in the volume of air taken in and given 

 out ; and nervous impulses may increase or may diminish the depth of a 

 breath, the volume of air respired. One mode of breathing again differs 

 from another in the rapidity with which one breath succeeds another, that 

 is, in the rate of rhythm ; and nervous impulses may slow or may quicken 

 the rate of rhythm. Then, again, combination of affects so numerous and 

 varied as almost to baffle description, may result from the influence of vari- 

 ous nervous impulses. Emotions may affect a single breath or a long 

 series of breaths, may quicken the rhythm while making each breath 



FIG. 99. 



Effect on Respiration of Section of One Vagus. The vagus was divided at the point marked 

 x. The curve was obtained by means of a tambour connected with a receiver into which the 

 animal (rabbit) breathed, as shown in Fig. 90, the lever falling in inspiration as air is sucked out 

 of the tambour and rising in expiration as the air returns. Inspiration begins at a and ends at 

 b. Expiration begins at b and ends at c. The lever gradually falls between c and a, owing to the 

 escape of air from the apparatus. 



more shallow, or may, at the same time, make each breath deeper, or may 

 slow the rhythm in either the one or the other manner, and may bear chiefly 

 on inspiration or on expiration. Moreover, there is not an afferent nerve 

 in the body which by means of afferent impulses passing along it may 

 not be the instrument of influencing the respiratory centre. Of all the 

 automatic centres in the body the respiratory centre is the one whose in- 

 dependence is most obscured by the repeated effects of afferent nervous 

 impulses. 



