THE PRESIDENTIAL ADDRESS. 3 



must slacken,' or conversely. There also play a part organs whose 

 stimulation changes with change of their relation to the line of gravity. 

 Thus, a pair of tiny water-filled bags set one in each side of the skull. In 

 each of these a patch of cells endowed with a special nerve. Attached 

 to hairlets of these cells a tiny crystalline stone whose pi'essure acts as 

 a stimulus through them to the nerve. The nerve of each gravity-bag 

 connects, through chains of nerve-oentres, with the muscles of all the 

 limbs and of one side of the neck. In the ordinary erect posture of the 

 head the stimulation by the two bags right and left is equal, because the 

 two gravity-stones then lie symmetrically. The result, then, is a 

 symmetrical muscular effect on the two sides of the body, namely, the 

 normal erect posture. But the right and left bags are mirror pictures 

 of each other. If the head incline to one side the resulting slip, 

 microscopic though it be, of the two stones on their nerve-patches makes 

 the stimulation unequal. And fi'om that slip there results exactly the 

 right unsymmetrical action of the muscles to give the unsymmetrical pose 

 of limbs and neck required for stability. That is the mechanism dealing 

 with limbs and trunk and neck. An additional one postures the head 

 itself on the neck; a second pair of tiny gravity-bags, in which the 

 stones hang rather than press. These, when any cause inchning the 

 head has passed, bring the head back at once to the normal symmetry 

 of the erect posture. And these same bags manage the posturing of 

 the eyes. The eye contributes to our orientation in space; for instance, 

 to perception of the vertical. And for this the eyeball, that is the 

 retina, has to be postured normally. The pair of little gravity-bags 

 in the skull, which act to restore the head posture, act also on the 

 eyeball muscles. Whichever way the head turns, slopes, or is tilted, 

 these adjust the eyeball's posture compensatingly, so that the retina 

 still looks out upon its world from an approximately noi-mal posture, 

 retaining its old verticals and horizontals. As the head tvsdsts to the 

 right the eyeball's visual axis untwists from the right. These reactions 

 of head and eyes and body unconsciously take place when a bird wheels 

 or slants in flight or a pilot stalls or banks his aeroplane. And all 

 this works itself involuntarily as a pure mechanism, whose analysis we 

 owe mainly to Prol. Magnus and Dr. de Kleijn, of Utrecht. 



True, in such a glimpse of mechanism v/hat we see mainly 

 is how the machinery starts and what finally comes out of it ; 

 the intermediate elements of the process we know less of. Each 

 insight into mechanism reveals more mechanism still to know. 

 Thus, hardly was the animal's energy balance in its bearing 

 upon food intake shown comfoi'tably to confomi with thermodynamics 

 than came evidence of the so-called 'vitamines.' Unsuspected 

 influence on nutrition by elements of diet taken in quantities so 

 small as to make their mere calorie value quite negligible ; thus, for 



