126 INVERSION OF THE RODS. 



receptive part, which arises from the central nervous 

 system ; and the inverted arrangement of the rods is, 

 we can hardly doubt, connected with the develop- 

 ment of the eye, though it is not yet, I thiuk, satis- 

 factorily explained. 



There is, however, another eye in vertebrates, with 

 reference to which I must say something, and which, 

 though now rudimentary, is most interesting. Our 

 brain contains a small organ, about as large as a hazel- 

 nut, known, from its being shaped somewhat like a cone 

 of a pine, as the pineal gland. Its function has long 

 been a puzzle to physiologists. Descartes suggested 

 that it was perhaps the seat of the soul ; and though 

 this idea, of course, could not be entertained, no 

 suggestion even plausible had been made. 



So matters stood until quite recently, when a most 

 unexpected light has been thrown upon the question. 

 As long ago as 1829, Brandt, describing the skull of 

 a lizard (Lacerta agilis), pointed out that in the 

 centre of the top of the head was a peculiar spot, one 

 of the scales being quite unlike the rest. Leydig* 

 subsequently observed that on the head of the slow- 

 worm {Anguis fragiJis) there is a dark spot surrounding 

 a small unpigmented body immediately over the pineal 

 gland. Rabl-Eiickhard,t in 1884, again called atten- 

 tion to this structure, and suggested that it might 

 serve for tiie perception of warmth. Ahlborn,:|: in the 

 same year, was the first to su^rgest that it was a 

 rudimentary eye. De Graaf § has the merit of dis- 



* " Die Arten dor Saurirr." 



t " Eutw. dos Kuocheufiscligehirn," Bericld der Sitz. natiirf.Freunde. 

 Berlin: 1882. 



} "Ueber d. Bedeutuiig der Zirbcldriise," Zeit.fiir Wis?. Zoot, 1884. 



§ "Zur. Auat. und Ent. der Epi.b. Amphiliieu uud RcptilieD," Zool. 

 Anz; 18S6. 



