524 PISCES FISHES. 



the internal concavity of tlie 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. 



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 eye-ball, 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 (Jig* 231, 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 net-work in the choroid. The nature of this organ 

 it is not very easy to determine. Some have believed it muscular ; 

 but the strise perceptible in it are vascular, and not fibrous : others 

 have thought it to be glandular, but it has no excretory 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 surround- 

 ing medium.* 



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 

 motionless. 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. 



(562.) 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 cartilaginous tribes, such as the Sharks and Rays, each eye-ball 

 is moveably articulated to the extremity of a cartilaginous pedicle 

 fixed to the bottom of the orbital cavity (Jigs. 232, t, and 231, c). 



(563.) 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 represented in Jig.%3l 9 wherein the reader will observe 

 that the superior oblique (g) does not pass through a pulley, as is 

 the case in the human subject. 



(564.) It is extremely remarkable, that even in fishes the muscles 



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



