THE COMPOUND EYE OF THE BLOW -FLY. 533 



creased tension of this membrane must increase the length of 

 the great rods, hence I regard it as a muscle of accommodation 

 (PL XXXVIII. , Fig. 2). I shall hereafter explain that its 

 probable action differs from that of the ciliary muscle in 

 Vertebrates, although I have spoken of it as ' accommodation * 

 for want of a distinctive term. 



b. The Retina and Optic Nerve. 



The Retina is termed the periopticon by Hickson. It consists 

 of true bacilli or retinal rods, which extend from its inner to its 

 outer surface. The inner extremities of these rods rest upon a 

 fine membrane, the membrana limitans interna, and are con- 

 tinuous with the fibrils of the optic nerve. Their outer ends 

 are contiguous with the membrana basilaris of the dioptron. 

 These rods are arranged in fasciculi, retinulse, and each retinula 

 corresponds with an oinmateum, and consists of a central rod 

 surrounded by six or seven peripheral rods. Each retinula is 

 surrounded by a delicate membranous sheath, which extends 

 from the basilar membrane to the membrana limitans in- 

 terna. 



The bacilli or rods are formed of two segments, like the 

 bacilli of the retina of the simple eye, an outer cylindrical 

 transparent segment and an inner granular protoplasmic seg- 

 ment. The inner segments differ from the inner segments of 

 the rods of the simple eye in not having any nucleus; in this 

 respect they resemble the inner segments of the cones of the 

 vertebrate eye. 



The principal difference between these elements and the 

 rods and cones of the vertebrate retina is, that the transparent, 

 rod-like outer segments correspond with the outer segments of 

 the cones and rods of the Vertebrate; the rods are not reversed, 

 but are turned towards the light, not from it, as they are in 

 vertebrate retina. In all other respects size, form and struc- 

 ture the two are exceedingly similar. 



The outer ends of the retinulse are closely surrounded by 

 numerous small round cells, the chaplet cells of Viallanes ; the 

 existence of these cells renders the observation of the outer 



