2 THE PRESIDENTIAL ADDRESS. 



state of the blood supplied to itself that, as shown by Drs. Haldane 

 and Priestley some years ago, the very slightest increase in the partial 

 pressure of carbon dioxide at the bottom of the lungs at once suitably 

 increases the ventilation of the chest. And dovetailed in with this 

 mechanism is a further one working for adjustment in the same 

 direction. As the lung is stretched by each inbreath the respiratory 

 condition of the nervous centre, already attuned to the respiratory 

 quality of the air in the lungs, sets the degree to which inspiration 

 shall fill them ere there ensue the opposite movement of outbreath. 

 All this regulation, although the nervous system takes part in it, is a 

 mechanism outside our consciousness. Part of it is operated chemi- 

 cally ; part of it is reflex reaction to a stimulus of mechanical kind, 

 though, as sucli unperceived. The example taken has been nervous 

 mechanism. If in the short time at disposal we confine our examples 

 to' the nervous system, to do so will have the advantage that in one 

 respect that system presents our problem possibly at its fullest. 



To turn, therefore, to another instance, mainly nervous. Muscles 

 execute our movements ; they also maintain our postures. This postural 

 action of muscles is produced by nerve-centres which form a system 

 more or less their own. One posture of great importance thus main- 

 tained is that of standing, the erect posture. This involves due co- 

 operation of many separate muscles in many parts. Even in absence 

 of those portions of the brain to which consciousness is adjunct the 

 lower nerve-centres successfully bring about and maintain all this 

 co-operation of muscles which results in the erect posture. For 

 instance, the animal in this condition, if set on its feet, stands. It 

 stands reflexly. More than that, it adjusts its standing posture to 

 required conditions. If the pose of one of the limbs be shifted that 

 shift induces a compensatory shift in the other limbs, so that stability 

 is retained. A turn of the creature's neck sidewise and the body and 

 limbs of themselves take up a fresh attitude appropriate to the side- 

 turned head. Each particular pose of the neck telegraphs off to the 

 limbs and body a particular posture required from them, and that 

 posture is then maintained so long as the neck posture is maintained. 

 Stoop the creature's neck and the forelimbs bend down as if to seek 

 something on the floor. Tilt the muzzle upward and the forelimbs 

 straighten and the hind limbs crouch as if to look up at a shelf. Purely 

 reflex mechanism provides most kinds of ordinary postures. 



Mere reflex action provides these harmonies of posture. The nerve- 

 centres evoke for this purpose in the required muscles a mild, steady 

 contraction, with tension largely independent of the muscle length and 

 little susceptible to fatigue. Nerve-fibres run from muscle to nerve- 

 centre. By these each change in tension or length of the muscle 

 is reported to the activating nerve-centre. They say ' Tension rising, you 



