FISHES 



277 



X 2-55 = Matthiessen's ratio). The lens has not only a light-refracting 

 function, but in the absence of an optically effective cornea, it must 

 also assume the onus of light -gathering. It is therefore typically 

 situated far forward in the globe, bulging through the pupil and 

 approximating the cornea. This large anteriorly-situated lens being 

 an optical necessity, all other considerations of general configuration 

 give place to it, and in cases wherein the globe would become too 

 large to accommodate a lens of the required dimensions, as in some 

 dec; _?a types frequenting an almost lightless habitat, the shape of 

 the eye is changed from the ellipsoidal to a tubular form so that 

 the large lens can remain at the required distance from the retina 

 (Fig. 380). 



The large spherical lens makes accommodation by its deformation 

 impossible, so that where an accommodative mechanism exists the 

 expedient is adopted of moving the lens forwards or backwards — 

 towards the cornea in the hypermetropic Selachians so that they 

 accommodate for near vision, towards the retina in myopic Teleosts 

 so that they accommodate for distance. The ciliary region is thus 

 more specialized than in C!yclostomes giving rise to a suspensory 

 apparatus for the lens and different tj'jjes of muscular structures to 

 effect these changes in its position. 



The necessity of making as much use of the relatively small 

 amount of light available in most watery habitats (apart from abyssal 

 depths where no light is available) has 

 led to the jacketing of the uvea of most 

 pelagic and surface fishes with a mirror- 

 like arrangement of guanine crystals to 

 form the argentea, while the choroid of 

 Selachians is provided with a tapetum 

 lucidum ; an alternative seen in certain 

 Teleosteans, is a similar deposition of 

 guanine crystals in the jjigment epithe- 

 lium (the retinal tapetum). 



While these form the main struc- 

 tural characteristics of the eyes of 

 Fishes, other advances are seen in com- 

 parison with those of Cyclostomes, 

 particularly the presence of an iris 

 musculature so that the structure be- 

 comes mobile, a considerably greater 

 elaboration of the visual cells and the 

 retinal structure, and myelination of 

 the optic nerve fibres and the provision 

 of septa within the optic nerve itself. 



Ant 



ON 



-apex 



SIR 



Fig. 293. — The General Scheme 

 OF Fish Muscles (seen from the 

 DoRS.\L Aspect). 



Ant, anterior part of orbit ; apex, 

 apex of orbit ; LR, lateral rectus ; 

 MR, medial rectus ; O, superior 

 (and inferior) oblique ; ON, optic 

 nervp ; SIR, superior (and inferior) 

 rectus. 



