EYE. 



185 



lens, iris, and of a differentiated sclerotic and cornea. Moreover, 

 the eye of Myxinoids and of Ammoccetes lies beneath the skin 

 and sub-dermal connective-tissue. In Petromyzon the skin covering 

 the eye becomes thinned out, and thus the animal, which was 

 blind in the larval state, can see on reaching the adult condition : 

 at the same time the eyeball increases in size, and becomes more 

 highly organised. 



The eyes of all the true Fishes are, with few exceptions (e.g. 

 Siluroids and Eels), of considerable size. They have but little 

 power of movement, and as the cornea is very flat, and the lens 

 lies almost directly against it, the eyeball always possesses a 

 hemispherical or ellipsoidal form, and the anterior chamber is very 

 small. In other points, the eye is formed on the same plan as that 

 described in the introductory portion of this chapter, but a few 

 other details concerning it must now be considered. 



FIG. 153. EYE OF A TELEOSTEAN. 



Op, optic nerve ; OS, sheath of optic nerve ; t, retina ; PE, pigment epithelium : 

 Tp, tapetum ; Lv, lamina vasculosa ; Ag, argentea ; La, lamina suprachoroidea ; 

 Se, sclerotic, enclosing cartilage or bone (t) ; Co, cornea ; IT, iris ; Lc, ciliary 

 ligament ; VK, anterior chamber ; L, lens ; C r , vitreons humour ; Pr, processus 

 falciformis, and Cp, campanula Halleri, here shown as if continuous with one 

 another. 



The lens of Fishes, as in all aquatic animals, is globular, and 

 possesses therefore a high refractive index. It fills up the greater 

 part of the eyeball, so that not much space is left for the vitreous 

 humour. It forms an exception to that of other Vertebrates in the 

 fact that, in the condition of rest, it is accommodated for 

 seeing near objects. In place of a ciliary muscle, there is 

 only a fibrous ciliary ligament. 



In the eye of Teleostei, a fold, the processus falciformis, 

 arises from the choroid and extends into the vitreous humour 

 towards the lens. The so-called campanula Halleri, which is 

 inserted round the periphery of the lens, and which is usually 

 described as a trumpet-shaped expansion of the processus falci- 

 formis, is, according to Virchow, entirely independent of the latter 



