EYE 



279 



Fishes. The eyes of all the true Fishes are, with few excep- 

 tions, of considerable size, and are formed on essentially the same 

 plan as that described in the introductory portion of this chapter. 



The eye-ball is almost always surrounded by a gelatinous sub- 

 stance, penetrated by connective tissue fibres, and in many 

 Elasmobranchs it is articulated on its inner circumference with a 

 rod of cartilage connected distally with the lateral wall of the 

 skull. The sclerotic is usually extensively chondrified, and not 

 infrequently becomes calcified or ossified towards its junction with 

 the cornea. 



The lens of Fishes is globular, or nearly so, and possesses there- 

 fore a high refractive index. It touches the cornea and fills up the 

 greater part of the eye- 

 ball, so that only a small 

 space is left for the vitreous 

 humour. It differs from 

 that of other Vertebrates 

 in the fact that, in the 

 condition of rest, it is ac- 

 commodated for seeing 

 near objects. Fishes pos- T 



sess no ciliary muscle, and /\^ 

 in many of them (most L$ .9.... 

 Teleostomi) accommoda- 

 tion takes place by means 

 of a process of the choroid, 

 the processus falciformis. 

 This extends through the 

 embryonic choroid fissure 

 into the vitreous humour 

 towards the lens, around 

 which it expands to form 

 the so-called campanula 

 HaUcri, which is often 

 pigmented (Fig. 205). In 

 the interior of this struc- 

 ture are nerves, vessels, 

 and smooth muscle-fibres, 

 and the latter exert an in- 

 fluence on the lens, drawing it towards the retina. The processus 

 falciformis is small in Ganoids and certain Teleosts, is apparently 

 absent in Cyclostomes and Dipnoans, and is probably represented 

 in Elasmobranchs, at any rate in the embryo: how accommodation 

 is effected in these Fishes is not known. 



The iris of Fishes takes no part in accommodation, and only 

 appreciably reacts to light or electric stimuli in a few species living 

 in shallow water in which the eyes are directed upwards. 



Externally to the choroid proper, and internally to the supra- 



FKJ. :205. DIAGRAM OF THE EYE IN A 

 TYPICAL TKLEOST. 



Ay, argentea ; Co, cornea ; Cp, campanula 

 Halleri ; CV, vitreous body ; Ir, iris ; L, 

 lens ; L-s, lamina suprachoroidea ; Lr, 

 lamina vasculosa ; Op, optic nerve ; OS, 

 sheath of optic nerve ; PE, pigment epi- 

 thelium ; Pr, processus falciformis ; Ri , 

 retina ; Sc, sclerotic, with cartilaginous 

 and osseous (I) portions; Tp, tapetum ; 

 VK, aqueous chamber. 



