6 THE PINEAL ORGAN 



Rabl-Ruckhard, in 1886, put forward the interesting suggestion that 

 the function of the organ was to estimate the heat of the sun's rays, and 

 that it was a thermal sense-organ rather than visual. Spencer, who in 

 '87 experimented on the sensitivity of the parietal organ to light, stated 

 that " In lizards, whose paired eyes are closed, no result is obtained by 

 focusing a strong beam of light on to the modified eye scale, and thus on 

 to the pineal eye ; in fact, strong light focused into one of the paired eyes 

 merely causes the lid to be drawn down, without any further apparent 

 result, whilst in the pineal eye there is no protecting lid, and no movement 

 whatever takes place to remove the eye from the direction in which the 

 light is coming." Nowikoff experimented similarly with electric lights 

 and magnesium wire on lizards without producing movements. Francotte 

 in 1887 experimented with Lacerta muralis and Anguis fragilis. He con- 

 structed a cage consisting of six boxes arranged round a central rectangular 

 space. Each box communicated with the central space by a small opening. 

 In one box he placed an electric lamp ; and in the central space a lizard 

 was placed, with its lateral eyes covered by a red material. The top was 

 then closed in by a covering lid. The experiment, which was repeated 

 several times, showed that in eight cases out of ten, after a quarter of an 

 hour the lizard was found in the lighted box. The same experiment was 

 also tried with a " blind worm," Anguis fragilis, under similar conditions, 

 and the animal was found in the lighted chamber three times out of 

 ten. 



Francotte considered that the experiment, without being absolutely 

 conclusive, allowed one to think that in the lizard, at any rate, the unpaired 

 eye is still capable of perceiving light. He thought, however, that the 

 experiment hardly proved that the lizard was attracted by the light only,, 

 but that it was also attracted by the warmth of the electric light. He 

 accordingly modified the experiment by darkening one half of the terrarium 

 and allowing a diffuse light to fall on the other half. After allowing a 

 considerable period of time to elapse he found that the animals had shown 

 no tendency to collect in the lighted half. 



Experiments were also conducted with the object of ascertaining the 

 influence of light in producing movement of pigment in the retina of the 

 median eyes of both vertebrates and invertebrates. Thus Nowikoff in 

 1 910 investigated the action of light on the median eyes of Lacerta agilis 

 and Anguis fragilis . He divided the experimental animals into two groups, 

 one of which he kept for 2 to 3 hours in absolute darkness, while the other 

 group was exposed for a similar period to full sunlight. In the subsequent 

 histological examination of the retina of those animals which had been 

 exposed to the light, he found the pigment granules tended to accumulate 

 in the inner ends of the cells nearer the source of light and central cavity ; 



