MICROSCOPICAL STUDIES. 67 



beneath. Another advance is made by the formation of a gelatinous 

 substance in the cavity of the capsule. This is a vitreous humour 

 and is a prelude to the appearance of a lens. The eyes of the Roman 

 Snail ( Helix pomatia) and of Tritonium are typical examples. 



The third type, found solely among the Cephalopoda, is formed in 

 direct sequence with the form last mentioned, but before treating of 

 its origin, we will detail the chief points in its structure, taking for 

 our text the eye of the Cuttlefish (Sepia officinalis). In this species, 

 as is usual among the Cephalopods generally, the eyes are placed 

 prominently upon either side of the head. Each consists of a hollow 

 bulb sunk in a deep orbit in large measure hollowed out of the 

 cephalic cartilages. This orbit becomes a closed chamber, the optic 

 capsule, by the extension across it, and in front of the optic bulb, of 

 a transparent fold of skin, which functions as a cornea. 



If we now bisect the optic bulb we find it contains but a single 

 chamber filled with gelatinous vitreous humour, bounded in front by 

 a very large bipartite crystalline lens, and elsewhere by thin walls 

 that are stiffened, and thus prevented from collapse, by the presence 

 of delicate plates of cartilage in their middle subtance. External to 

 these cartilages, and obvious to the naked eye as a brilliant bronze- 

 hued coating, are two layers of pigmented membrane, the argentea 

 externa and interna. 



Internal to the cartilaginous and fibrous layers of the bulb is the 

 retina, lining the hinder part of the ocular cavity, the front being 

 occupied in large measure by the lens. The latter is almost globular 

 in shape, the longest diameter coinciding with the visual axis. It is 

 made up of the junction along a transverse plane of two unequal 

 plano-convex lenses. The anterior is the smaller. As a consequence, 

 the posterior has much greater convexity, and projects boldly into 

 the ocular chamber. After hardening in spirit or otherwise, each 

 portion of the lens can readily be split into a large number of con- 

 centric layers, whose curvature coincides with the external convexity 

 of that half of the lens to which they respectively belong. A fine 

 membrane stretches across the lens at the junction of the two halves, 

 and passes at the edge into a fibrous and muscular sphincter-like 

 organ, known as the ciliary body. This, in turn, merges with the 

 fibrous wall of the ocular bull. The use of this ciliary body is the 

 regulation of the convexity of the lens, to permit of its adaptation to 

 near or to distant vision. 



A fold of the external coat of the bulb is carried part way over 

 the outer aspect of the lens, and is the iris entrusted with the im- 

 portant duty of regulating the quantity of light passing through the 

 lens. It is strengthened internally with thin plates of [cartilage. In 

 Sepia, it has an upper and a lower fold that have much superficial 

 resemblance to eyelids and give the eye a slit-like pupil. In passing, 

 it may be mentioned that Sepia possesses a true eyelid, consisting of 



