242 LIFE : OUTLINES OF GENERAL BIOLOGY 



arbitrary' or irrelevant. Yet we have found that tame mice in 

 Aberdeen do not require more than fifty lessons to learn to congre- 

 gate for a meal or a tit-bit when an electric bell is sounded. They 

 then continue doing so for a considerable time, although there is 

 no reward; and this is a "conditioned refiex". 



In one of Jean Paul Richter's books there is a story of a humorist 

 who put his teeth into a lemon just as the village whistlers were 

 beginning to whistle on some joyous occasion. The result was 

 inhibitory; yet the trick would not have worked if the members of 

 the band had had no exi>erience of lemons. A conditioned reflex 

 differs from an unconditioned reflex in presupposing individual 

 experience. There is no doubt that conditioned reflexes play a part 

 in the behaviour of animals and also in our human affairs, but there 

 is much more besides. There is often downright intelligence in 

 animals, and to that man adds Reason. 



Let us give an instance illustrating the subtlety of some of the 

 reflexes. Physiologists have analysed what to many of us is a com- 

 mon experience, that emotion or mental effort alters the state of 

 the skin. R. J. S. McDowall and H. M. Wells have shown that there 

 is an elementary skin-constrictor-reflex or "psycho-galvanic" reflex, 

 which may be brought about without co-operation of the higher 

 nervous centres. The most noteworthy change is a fall in the elec- 

 trical resistance of the skin, and this is shown to be the result of the 

 constriction of the peripheral blood-vessels. This may be considered 

 as part of the reflex arrangement by which an animal normally 

 adapts itself to the anticipation of muscular exercise and defence. 

 In man it may be instigated by a pin-prick, but also by a threatening 

 movement. In other words, the stimulus may be emotional, not 

 sensory. As the change in the blood flow affects the temperature 

 sensations, there is a justification of such common sayings as "my 

 blood ran cold" or "cold hands and a warm heart". The investigators 

 notice that the reflex is associated with a diminution in the volume 

 of a limb, as was previously demonstrated by Mosso and by Golla. 

 Mosso pointed out that the volume of the limb changed in a student 

 who passed from "seen" Greek translation to the greater mental 

 effort of tackling the "unseen". 



THE EYES.— In all the vertebrates, the structure of the eye is on 

 the same general plan as the structure of an ordinary camera. It 

 is a little dark box, into which light is admitted only from one side, 

 through a system of lenses; these lenses focus the light rays upon a 

 sensitive membrane at the back, the retina of the eye, which may 

 be compared to the sensitive plate or film of the camera. The 

 aperture of the lens, in the eye as in the camera, can be reduced 

 when the light is especially bright; the eye, like the camera, can be 

 focused for near or distant objects. But in sensitiveness, in adapta- 



