294 ANATOMY. 



segment of a circle, or which are hemispherical, it is regulated by the 

 size of the entire sphere : larger and consequently flatter spheres, receive 

 longer ones, and smaller, and, therefore, more convex ones, receive 

 shorter cones. In one of the Noctuee, Joh. Miiller found the proportions 

 of length to the breadth of the base to be as 5 to 1. In CEschna, these 

 relations, according to Diiges' figure, are as 10 to 1 ; the base itself 

 also rises so much, that it even appears conical. 



As we have mentioned above, a filament of the optic nerve stands in 

 connexion with the apex of each cone. These filaments are thin, 

 extremely delicate nerves, which, like the rays of a sphere, originate 

 from the exterior surface of the optic nerve, and spread themselves to 

 the circumference, one passing to each cone. Nothing further can be 

 remarked of them ; a separation or radiating division of them has never 

 been observed. They bring the external portion of the eye into con- 

 nexion with the cerebrum, and may be therefore considered as the most 

 important conductors of the sense of sight. According to the figure of 

 Straus, this nerve somewhat distends where it joins the crystalline body, 

 and encompasses its apex, there forming a kind of retina ; but MUller 

 and Diiges never detected this distension of the filaments of the optic 

 nerve. 



The dark pigment spreads all over the entire eye between the filaments 

 of the optic nerve. It is a variously coloured, generally a dark purple 

 red, sometimes brighter (Mantis), thickish fluid, which is transpierced 

 throughout by fine tracheal branches, which proceed from a trachea 

 surrounding the inner circumference of the eye like a ring. This layer 

 of colour consequently corresponds with the choroidea of the higher 

 animals, which is both colouring matter and a vascular tunic. The 

 pigment in the majority of insects admits of being divided into two 

 layers, from its difference of colour. The external brighter pigment 

 displays very various colours, as is proved by the mere appearance of 

 the eyes. All bright, glittering metallic eyes, or such as are ornamented 

 Avith stripes and spots, derive their painting and markings from this 

 superficial pigment. I will cite here merely the green yellow eyes of 

 the butterflies of the genus Poniia and the banded metallic eyes of the 

 Tabani, the brassy coloured ones of the Hemerobia, and the beautifully 

 coloured eyes of so many other insects. The internal pigment is uni- 

 formly dark, but, likewise, it is not entirely similar in all insects, but 

 varies according to the families and genera. Mantis exhibits it bright 

 red, the moths violet, many butterflies of a blue violet, and other butter- 



