502 PHYSIOLOGY CHAP. 



Mosso has pointed out other yet more convincing analogies from 

 clinical observation. In sleep the pupils contract, the eyes rotate 

 inward and upward; in the waking state the pupils dilate, the 

 eyes rotate outward (Fontana). Just so in periodic respiration, 

 when the pause commences, the pupils retract, the eyes converge 

 inwards and upwards; when the group commences, the pupil 

 dilates, the eyes look forward (Leube). At the beginning of the 

 pause some patients are drowsy and become insensible ; at the 

 beginning of the group they are restless, and again suffer pain 

 (Leube, Merkel). Certain subjects close their eyes at the termina- 

 tion of the group, at the commencement of the pause, and in the 

 midst of it, and open them again at the commencement of the 

 group or soon after (Frantzel, Hein, Kaufniann). In more serious 

 cases the stupor and unconsciousness are continuous throughout 

 the Cheyne- Stokes breathing ; in other cases consciousness 

 returns, partly at any rate during the groups ; in others, lastly, the 

 respiratory phenomenon takes place in the awakening state. 

 These are differences of intensity, shades and gradations of one 

 fundamentally identical process, which explains why the patient 

 now reacts and now fails to respond to external stimuli during the 

 periodic pause. "The pause," writes Mosso, "always implies a 

 more or less serious drowsiness of the nerve centres," which is as 

 much to say, in more exact and strictly physiological language, 

 that during the pauses the respiratory centres suffer a negative 

 variation in their excitability. Reflex excitability, automatic 

 excitability, or reflex and automatic excitability together ? The 

 answer to this important question necessitates some consideration. 

 Let us consider the extremes. In many cases, whether clinical 

 or experimental, of periodic respiration, the reflex excitability of 

 the centres is maintained. Sometimes it is sufficient to invite the 

 patient to breathe, or to excite him with acoustic, luminous, thermal 

 or painful stimuli, during the pause, in order immediately to cut it 

 short, and obtain respiratory movements (Biot, Saloz, Murri, 

 Bordone). In rabbits with divided bulb which breathe periodically, 

 faradisation of the centres, with strong and infrequent break 

 shocks, will produce respiratory movements, either during the 

 groups or during the pauses (Kronecker and Marckwald). These 

 facts do not, as some maintain, contradict the theory that periodic 

 respiration depends essentially upon periodic oscillations in the 

 excitability of the centres, because the substitution of artificial for 

 natural stimuli probably involves disturbance of the whole of the 

 intimate and delicate metabolic process on which the periodic 

 grouping of the central impulses that determine Cheyne-Stokes 

 respiration depends ; but it has certainly been demonstrated that 

 the reflex excitability of the centres, though reduced, is not 

 suspended, during the pauses in these cases. Accordingly, it must 

 be assumed that the periodic quantitative variation in the external 



