204 LECTURE VIII. 



is laid upon the central surface, not the periphery, of the choroid : 

 (Prep. No. 1669.) 



The formation of the iris by the production of all these membranes 

 is well shown in this preparation of the eye of the Sword-fish {Xiphias, 

 No. 1661.), where its thick base or 'ciliary ligament' (h) overlaps the 

 convex border of the bony sclerotic. The pupil ii) is large and usu- 

 ally round : in many Plagiostomes it is elliptic : in the flat-bodied 

 Skates and Pleuronectidse, that grovel at the bottom and receive 

 the rays of light from above, a fringed process descends from the 

 upper margin of the pupil, and regulates the quantities of admitted 

 light by being let down or drawn up like a blind. The muscular 

 structure of the iris is very feebly developed in most fishes : it is 

 best seen in the pupillary curtain of the Skate. The preparations 

 of the Sword-fish's eye (Nos. 1661 and 1662.), and these of the eyes 

 of the Grey-Shark (Galeus, No. 1670), and the Basking-Shark 

 {Selache, No. 1670. A.), demonstrate the plicated anterior border of 

 the uvea, forming the so-called ' ciliary zone, or processes ' (A) : they 

 are the most complicated in the great Shark, where each process 

 " consists of two or three minute folds, which, as they run forward, 

 unite into one, and terminate in a point at the circumference of the 

 iris* : " but they do not, as yet, project freely imvards and forwards 

 from the surface of the uvea. 



The subordinate and accessory character of the sclerotic capsule (/, /,) 

 is illustrated in most Osseous Fishes by its deviation from the sub- 

 spherical form of the true eyeball Avhich it protects, and by the great 

 quantity of cellular, and often also of adipose tissue (/«), which fills the 

 wide interspace between the sclerotic and the choroid. In the fibrous 

 tissue of the sclerotic are usually developed the two cartilaginous 

 or osseous hemispheroid cups already described (p. 103. Jig. 30. 17) ; 

 but in place of these, in the Oi'thagoriscus, as in the Plagiostomes, the 

 capsule is strengthened by a single hollow, cartilaginous, perforated 

 spheroidal globe. The anterior aperture is closedby the cornea (?z), 

 which is essentially a modified portion of the corium (o), adhering to, 

 as it passes over, the usually thickened borders of that aperture. In 

 this specimen of the eye of the Xiphias (No. 1661.) you may trace an 

 accession to the cornea from the outer fibrous layer of the sclerotic, 

 which undergoes the same change of tissue, and forms the posterior 

 layer of the cornea. This transparent window of the eye-capsule is 

 quite flat : its laminated structure is well displayed in the prepa- 

 ration of the cornea of the Orthagoriscus (No. 1665.), and a dark- 



* Lxvi. ili, p. 147. 



