XII 



PHYLUM MOLLUSC 1 A 



r-r- -i 

 I I 1 



portion of the integument covering the exposed face of the eye is 

 termed the false cornea (corn}. The eye-ball has a firm wall, or 

 sclerotic, strengthened by plates of cartillage (scl. cart). Externally, 

 i.e., on the side turned towards the surface of the head, this presents a 

 large opening the pupil. The part of the sclerotic which imme- 

 diately bounds the pupil is termed the iris (ir) ; it contains muscu- 

 lar fibres by whose action the size of the pupil can, to a limited 

 extent, be increased or diminished. Just internal to the iris and 

 projecting slightly through the pupil is the lens a dense glassy- 

 looking body of a spherical shape. The lens consists of two plano- 

 convex lenses in close apposition ; it is supported by an annular 



scl.cart 



set. carl 



orb.ccLrt 



Fio. 668. Sepia, section of eye. til. pror. ciliary processes ; coin, false cornea; ir. iris; lens, 

 lens ; ojit. </. optic ganglion ; orl>. rtu't. orbital cartilage ; rds. rods ; /< /. retina ; xrl. cart. sckT<>t i>- 

 cartilage. (From Vogt and Jung, after Henscn.) 



process the ciliary process (cil. prcc.) projecting inwards from the 

 sclerotic. Between the two parts of the lens lies a thin layer of 

 cells the cornea. The lens with the ciliary process divides the 

 cavity of the eye into two portions, a smaller outer the cavity of 

 the aqueous humour, containing water, and a larger inner, contain- 

 ing a gelatinous substance the vitreous humour. Over the wall 

 of this inner chamber extends the retina (ret), the sensitive part of 

 the eye, in which the optic nerve- fibres derived from the optic 

 ganglion terminate. The retina is of somewhat complicated 

 structure, consisting of a number of layers: of these that which 



