658 PISCES. 



eye, therefore, generally contains two cartilaginous plates imbedded in 

 its tissue, which are sufficiently firm in their texture to prevent any 

 alteration in the shape of the eyeball ; and in some of the large fishes 

 the sclerotic is actually converted into a cup of bone presenting orifices 

 at the opposed extremities one for the insertion of the transparent 

 cornea, the other for the admission of the optic nerve. 



(1810.) The vitreous humour and crystalline lens in many fishes are 

 kept in situ by a ligament placed for the purpose. This is a delicate 

 falciform membrane derived from the retina (fig.321,B,c), which plunges 

 into the vitreous humour, and, being continued along the internal con- 

 cavity of the eye, is fixed to the capsule of the lens. In some fishes, as 

 the Salmon, this ligament is of a dark colour ; and in the Conger there 

 are two such bands, by which the crystalline is suspended as by its 

 opposite poles. 



(1811.) Another peculiarity in the structure of the visual apparatus 

 of osseous fishes is the existence of a vascular organ placed at the back 

 of the eyeball, and interposed between the choroid tunic and a brilliant 

 metallic-coloured membrane which invests the choroid externally. This 

 organ, generally called the " choroid gland " by the older anatomists 

 (fig. 321, A, g g), is of a crescentic form, and always of a deep-red colour. 

 It is principally made up of blood-vessels, which run parallel to each 

 other; and from it issue other vessels, frequently very tortuous, and 

 always much ramified, which form a vascular network in the choroid. 

 The nature of this organ it is not very easy to determine. Some have 

 believed it muscular ; but the striae perceptible in it are vascular, and 

 not fibrous : others have thought it to be glandular ; but it has no ex- 

 cretory duct. Most probably it is an erectile tissue analogous to that of 

 the corpus cavernosum, and has some influence in accommodating the 

 form of the eye to distances, or to the density of the surrounding 

 medium*. 



(1812.) The pupil of the eye in the animals we are describing is very 

 large, so as to take in as much light as possible, but generally motion- 

 less. In some genera the shape of the aperture is curious : thus in the 

 Rays a broad palmate veil hangs in front of the pupillary aperture ; and 

 in one case, the Anableps, there are two pupils to each eye. 



(1813.) The eyes of osseous fishes are lodged in the bony orbits of 

 the face, imbedded in a soft glairy cellulosity; but in many of the car- 

 tilaginous tribes, such as the Sharks and Rays, each eyeball is moveably 

 articulated to the extremity of a cartilaginous pedicle fixed to the bottom 

 of the orbital cavity (figs. 322, i, & 321, c). 



(1814.) Six muscles serve to turn the eye in different directions : 



namely, four recti, arising, as in Man, from the margin of the optic 



foramen ; and two oblique muscles, derived from the anterior part of the 



orbit, and inserted transversely into the globe. These muscles are well 



* Cuv. et Val. op. cit. p. 338. 



