1000 



PISCES. 



iris, is gathered into numerous beautiful ra- 

 diating folds (ciliary plicie ) ; these in very 

 large eyes, as in the Moon-fish (Orthagoriscus) 

 for example, are seen each of them to consist of 

 two or three minute folds, which, as they run 

 forwards, unite into one and terminate in a 

 point at the circumference of the iris, but in no 

 instance do they project freely inwards as dis- 

 tinct processes, so as to resemble the ciliary 

 processes of Mammiferous Vertebrata. The 

 ciliary plicae, as indeed most of the posterior 

 surface of the iris, is in immediate contact with 

 the membrane of the vitreous humour, to which 

 it is intimately adherent; for in Fishes there is 

 no posterior chamber of the aqueous humour, 

 the anterior segment of the crystalline lens pro- 

 jecting in many instances quite through the 

 pupillary aperture. 



In a space enclosed between the proper 

 choroid and the membrana argentea is a struc- 

 ture quite peculiar to the osseous Fishes, for it 

 is not met with even in the Chondropterygious 

 races.* This consists of a spongy mass of 

 irregular form, which partially surrounds the 

 entrance of the optic nerve (fig. 532, h\ and 

 extends for some distance towards the front 



Fig. 532. 



Coats of the eye of the Perch. (After Cuvier.) 



Fig. 1, muscles of eye- ball ; a, superior oblique ; 

 b, inferior oblique ; 1,2, 3, 4, rccti muscles ; i, optic 

 nerve. Figs. 2 and 3, f.f,f, fatty matter; g, cho- 

 roid ; h, " choroid gland." 



of the eyeball. This body, which has been 

 absurdly called the choroid gland, is some- 

 times divided into two portions; at others 

 it assumes a somewhat crescentic form, but 

 it is always deficient towards the lower part 



of the eye. Its colour is always a deep red, 

 and its tissue is principally made up of 

 bloodvessels running transversely in close pa- 

 rallel lines. Other vessels issue from it which 

 are frequently very tortuous and always much 

 ramified ; these run into the choroid, where 

 they form so dense a network that it was de- 

 scribed by Haller as a distinct membrane, and 

 has been subsequently named membrana Hal- 

 leri. The use of the so-called choroid gland 

 has not been fully ascertained ; most probably, 

 however, it is essentially composed of erectile 

 tissue, which by its dilatation and contraction 

 may have some influence in accommodating the 

 form of the eye to the distance of objects, or 

 the varying density of the medium through 

 which they are seen. 



The optic nerve in many Fishes (at least 

 among the Acanthopterygii) is made up of a 

 broad layer of nervous matter folded upon it- 

 self like a fan (Jig. 532) and enclosed in a 

 fibrous envelope, which is continuous with the 

 sclerotic coat of the eye. The nerve enters the 

 eye at a point remote from the axis of vision, 

 penetrating for the most part by an oblique 

 course, so that after having pierced the sclerotic 

 it has still a considerable distance to pass 

 through the substratum of cellular tissue and 

 between the masses of the " choroid gland " 

 before it pierces the choroid and Ruyschian 

 tunics. Its diameter is much diminished at 

 the point where it shews itself in the interior of 

 the eye, where it appears sometimes as a mere 

 point, at others under the form of a round or 

 irregular spot, or sometimes represents a straight 

 line. It then expands into the retina, which, 

 when the nerve is folded, as above described, 

 has likewise a plicated appearance. The re- 

 tina, as in other Vertebrata, lines all the inter- 

 nal cavity of the eye as far as the ciliary plicae, 

 thus enveloping the vitreous humour. 



Another peculiarity in the structure of the 

 Fish's eye is the existence of an apparatus 

 apparently analogous to the marsupium of 

 Birds, which extends from the choroid to the 

 back of the lens, passing quite through the 

 vitreous humour, to which the name of falci- 

 form ligament has been given. This structure 

 arises by a broad origin from the inner surface 

 of the choroid at the back part of the eye, and 

 extending forwards, following the concavity of 

 the eyeball along its lower surface, arrives at 

 the ciliary zone and is connected with the back 

 of the capsule of the lens. Its shape is falci- 

 form, as the name indicates, the convexity of 

 the curve being attached along the floor of the 

 interior of the eye. In the recent eye it is a 

 delicate and almost imperceptible membrane, 

 but maceration in spirit by rendering it opaque 

 reveals it to consist of several layers of cellu- 

 losity, most probably enclosing numerous ves- 

 sels. According to Cuvier and the younger 

 Soemmering,* the falciform ligament passes 

 through the retina, which is fissured to let it 

 through ; but an examination of the large eyes 

 of the Moon-fish after long immersion in spirit 



* Cuvier et Valenciennes, Hi&t. Nat. des Pois- 

 sons, torn. i. p. 337. 



* De oculorum hominis animaliumque sectione 

 horizontal! commentatio. Fol. Goettinga;, 1818. 



