ORGANS OF VISION. 221 



ORGANS OF THE SENSES. 



Organs of Vision. 



THE Eyes are generally developed in Fishes,' and provided with 

 all the parts that enter into their composition in the rest of the Ver- 

 tebrata. They are lodged in a cartilaginous or bony orbit, imper- 

 fectly closed. The external integument becoming thinner in texture 

 and transparent, forms usually around the eyes a shallow circular 

 fold, beneath which muscular fibres may be frequently seen to pass, 

 forming a kind of sphincter. In some instances the integument 

 forms a transparent lamina which is continued simply over the 

 surface of the eye, as in Fishes of the Eel kind, Ammocoetes and 

 others. Not unfrequently an anterior and posterior plica semilunaris 

 may be distinguished, as in Clupea, Scomber, Salmo, but particu- 

 larly in the Sharks, where a true nictitating membrane exists ; this, 

 however, being wanting in the genera Scyllium and Acanthias. The 

 Sharks possess also free eyelids, which in the Rays are soldered 

 together. 



The Sclerotic is usually dense and cartilaginous, or actually formed 

 of thin bony plates, which in many large Fishes, as in Xiphias, 

 coalesce to form an osseous shell, having an opening left posteriorly 

 for the passage of the optic nerve. In front of the sclerotic is 

 inserted the very flat Cornea, which is usually thickest at its margins. 

 The Choroid coat is usually separated from the sclerotic by a loose 

 mass of adipose cells, and consists frequently of two widely sepa- 

 rated layers ; the outer of these, or the pigmentary, is of a silvery 

 lustre in the Osseous Fishes, and consists of very delicate fibres, 

 which almost resemble under the microscope needle-shaped crystals ; 

 it is continued in front to form the iris, which encloses a pupil that 

 remains probably always round and immoveable. The inner layer 

 is very highly vascular, and covered upon the internal surface by 

 black or purple-colored pigment. Between the two layers of the 

 choroid is situated in many Osseous Fishes what is called the vas- 

 cular gland, which will be described more minutely further on. 

 In the choroid of the true Cartilaginous Fishes (Plagiostomi) a layer 

 of black pigment is placed externally, and internally a tapelum of 

 metallic splendor. The ciliary body is, for the most part, feebly 

 developed. In the* Sharks the ciliary processes are most distinctly 

 formed, but, though less so in many of the Osseous Fishes, they still 



