126 INVERSION OF THE RODS. 



receptive part, which arises from the central nervous 

 system ; and the inverted arrangement of the rods is, 

 "vve can hardly doubt, connected with the develop- 

 ment of the eye, though it is not yet, I think, 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 fragilis) there is a dark spot surrounding 

 a small unpigmented body immediately over the pineal 

 gland. Eabl-Kuckhard,t in 1884, again called atten- 

 tion to this structure, and suggested that it might 

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

 same year, was the first to suggest that it was a 

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



* " Die Arten der Saurier." 



t " Eutw. des Knochenfiscligebirn," Bericht der Sitz. nafurf, Freunde, 

 Berliu: 1882. 



t "Ueber d. Bedeiitung der Zirbeldrlise," Zeit. fiir Wis?. Zool, 1884. 



§ " Zur. Aiiat. uud Ent. der Epi. b. Arapbibieu uud Reptilien," Zool. 

 Anz., 1886. 



