288 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



Since each inspiration is followed by an expiration, which, though 

 passive during quiet breathing, may and frequently does involve the 

 co-ordinated activity of a number of muscles, we might, from a purely 

 physiological point of view, divide the respiratory centre into two parts, 

 one always active and presiding over inspiration, the other only active 

 when a special call is made on the respiratory mechanism and presiding 

 over expiration. Nevertheless we have no experimental evidence of any 

 anatomical separation between these two centres. Localised stimulation 

 applied to the medulla oblongata may call forth inspiration or expiration 

 according to the condition of the centre and the phase of respiration at 

 which the stimulus is applied ; inspiration, however, predominates in 

 most cases over expiration. No point of the respiratory centre has been 

 found as to which we can say with certainty that we shall obtain only 

 expiration or only inspiration as the result of the stimulus. We do 

 not know, therefore, whether some groups of cells originate or transmit 

 inspiratory and others expiratory impulses, or whether both sets of 

 muscles are innervated from the same cell-groups. 



We have now to inquire into the nature of the activity by virtue 

 of which this centre is able to excite rhythmic co-ordinated move- 

 ments of the respiratory muscles. This inquiry involves two distinct 

 questions 



1. How is the activity of the centre aroused? Is it aroused by 

 stimuli arriving at the centre from the periphery along afferent nerves, 

 or by changes occurring in the cells of the centre itself, the results of 

 their normal metabolism, or by changes in the fluids lymph and blood 

 surrounding the cells ? 



2. What is the cause of the natural rhythm of this activity ? 



In deciding the first point, i.e. whether the activity is automatic or 

 reflex, we must attempt to study the behaviour of the centre when 

 separated as much as possible from the influence of all afferent im- 

 pulses. E-ach, 1 who was the first to make this attempt, came to the 

 conclusion that the respiratory centre separated from all afferent 

 impulses was unable to originate any movements. His experiment con- 

 sisted in dividing the cord below the fifth cervical nerve, and the 

 medulla oblongata above the striae acusticee, and then cutting both vagi 

 and all the posterior roots of the upper cervical nerves. Subsequent 

 repetition of Kach's experiment by Kosenthal and Marckwald 2 failed 

 to confirm these results. We may consider here shortly the effects on 

 the respiratory movements of the various operative procedures employed 

 in this experiment. 



Section of the lower part of the cervical cord in the rabbit leaves 

 the diaphragmatic movements practically unaltered, since in this animal 

 the chief part of inspiration is carried out by the diaphragm. The 

 section through the upper part of the medulla oblongata, separating the 

 respiratory centre from the higher parts of the brain, is equally without 

 effect on the depth and rhythm of the respiratory movements. A great 

 change is observed, however, if the vagi are subsequently divided under 

 these conditions. The first effect is a long inspiratory spasm of the 

 diaphragm, which may last as long as a minute. Its relaxation then 

 follows, to be succeeded again by another inspiratory spasm many times 

 exceeding in duration the ordinary respiratory contraction of the 



1 "Inaug. Diss.," Kouigsberg, 1863. 



2 "The Movements of Respiration, " translated by Haig, London, 1888. 



