454 PHYSIOLOGY CHAI-. 



The complete nervous system on which the respiratory 

 mechanism depends may thus be divided into three sections : the 

 bulbar, the spinal, and the cerebral respiratory centres. From the 

 first emanate the rhythmical impulses to thoracic -abdominal 

 movements on which the activity of the spinal respiratory centres 

 which pass the impulses on to the muscles depends ; but the 

 rhythmical activity of the bulbar centres is in its turn regulated, 

 and may be modified in a number of ways, by the cerebral 

 respiratory centres. 



V. In order to form a more adequate conception of the mode 

 in which these highly complicated systems function, it must be 

 remembered that we understand by the term respiratory centres 

 an accumulation of central mechanisms, which are capable of a 

 double and antagonistic action one inspiratory, which dilates the 

 thorax, the other expiratory, which contracts it. As we have seen 

 in the preceding chapter, both forms of movement are active and 

 are brought about under normal conditions by the contraction of 

 antagonist muscles ; we cannot regard expiration as the simple 

 effect of inhibition acting upon the inspiratory centres, but must 

 further assume the existence of true expiratory centres. The 

 concept respiratory centre thus implies the association of two 

 centres, which have an opposite function in respect of pulmonary 

 ventilation, and can, therefore, under normal conditions, only 

 function rhythmically and alternately. 



Not only must we recognise the existence of expiratory centres 

 as distinct from inspiratory centres ; on the ground of irrefutable 

 observations we must further admit that these centres, while 

 normally associated in their rhythmical and alternating functions, 

 are yet capable under abnormal conditions, particularly the 

 influence of certain poisons, of functioning independent of each 

 other. That the inspiratory centres may, under special circum- 

 stances, alone be active will readily be admitted by all (and these 

 form the vast majority) who hold that, normally, inspirations. alone 

 are active. But it is also possible to demonstrate that under 

 various other conditions the converse holds good, the expirations 

 only being active, while the inspirations are passive. In these 

 cases the inspiratory centres do not function, and the expiratory 

 centres alone perform their work. Inspiration is effected by the 

 elastic recoil of the thoracic walls, which are pushed below their 

 resting position. 



Aducco succeeded in collecting certain observations and 

 recording them graphically enough to establish this fact, which as 

 a contribution to the theory of the centres that control the 

 mechanics of respiration is of no small importance. The curves of 

 Fig. 209 were recorded by two tambours with exploring buttons 

 (applied to the sternum and the linea alba abdominis respectively, 

 their movements being transmitted to two other tambours writing 



