14 



THE PINEAL ORGAN 



ends of the rods and cones ; but whether a tapetal layer is present, or is 

 absent as in the human eye, it is universally held that the rods and cones 

 are the sensitive elements of the retina and that light reaches them by 

 passing through the transparent anterior layers which intervene between 

 them and the refractile elements — cornea, aqueous humour, lens, and 

 vitreous. The relation of the nerve fibres to the sensory cells of the retina 



Fig. 9. — Vertical Median Section through One of the Three Frontal 

 Ocelli, or Stemmata, of a Blow-fly Imago. (After T. B. Lowne.) 



c. : cuticle. hyp. : hypodermi. 



c.l. : cuticular lens. n.f. : nerve fibres. 



pg. : pigment cells forming fibrous sheath, continuous with the peri- 

 neural sheath. 



pr. sp. : preretinal space, r. : rods. ret. : retina. 



v. : vitreous cells, continuous with hypoderm cells. These cells, which 

 are present in the imago as a layer of tall, columnar cells, degenerate 

 and in the mature animal exist only as a very thin stratum which is 

 easily overlooked. 



is well seen in Fig. 9, which represents an invertebrate eye of the upright 

 type, and Fig. 10, A, B, the developing " inverted " lateral eye of a 

 vertebrate. 



The retina of both " upright " and " inverted " eyes may be either 

 " simple " or " compound." The simple retina consists of a single layer of 

 sensory cells, as in Fig. 9 ; the compound retina of two or more layers of 

 sensory or neuro-sensory cells, as in the compound faceted or non-faceted 



