212 



COMPARATIYE ANATOMY 



Xe Vis: Co 



Fig. 169. 



Op 



-Eye or a Teleostean. 



campanula Halleri (Fig. 169). In the interior of this structure 

 are nerves, vessels, and smooth muscle- fibres, and the latter possibly 



exert an influence on the lens, draw- 

 ing it towards the retina. The pro- 

 cessus falciformis is never large in 

 Ganoids and is absent in Cyclo- 

 stomes, Elasmobranchs, and Dip- 

 noans : the question of accommoda- 

 tion in these Fishes is not under- 

 stood. 



Externally to the choroid proper, 

 that is, between it and the lamina 

 fusca, lies a silvery or greenish-gold 

 iridescent membrane, the argentea. 

 It extends either over the whole 

 interior of the eye (Teleosts), or is 

 limited to the iris (Elasmobranchs). 

 A second layer with a metallic 

 lustre, the tapetum lucidum, is- 

 present internally to the iridescent 

 portion, and within this again is the 

 chorio-capillaris of the choroid. No 

 tapetum appears to be present in 

 Teleostei or Petromyzon. 



The so-called choroid gland, pre- 

 sent only in Teleostei and Amia, 

 consists of a network of blood- 

 vessels (rete mirabile) which has the form of a cushion, lying near 

 the entrance of the optic nerve, between the argentea and pigment 

 epithelium of the retina : it thus has nothing to do with a " gland." 

 The sclerotic is usually extensively chondrified, and not unfre- 

 quently becomes calcified or ossified towards its junction with the 

 cornea. 



The eyeball is almost always surrounded by a gelatinous tissue, penetrated 

 by connective-tissue fibres, and in Elasmobranchs it is usually articulated on its 

 inner circumference witli a rod of cartilage connected distally with the 

 lateral wall of the skull. 



Amphibia. — The eyes of Amphibians are proportionately 

 smaller, and their form rounder than those of Fishes, but there are 

 many points of close correspondence between them. This is true, 

 for instance, as regards the more or less distinctly chondrified 

 sclerotic, the slightly convex cornea, and the globular lens. In other 

 important respects, however, the Amphibian eye is simpler than 

 that of Fishes ; thus it is wanting in an argentea, a tapetum, a 

 choroid gland, and a processus falciformis and campanula Halleri. 

 The iris contains smooth muscle-fibres, and a true ciliary muscle 

 is present in the whole series of animals from this point onwards, 

 though not strongly developed in Amphibians. The pupil is usually 

 round, but may be angular. 



Op, optic nerve ; OS, sheath of optic 

 nerve ; St, retina ; PE, pigment 

 epithelium ; 'fp, tapetum ; Lv, 

 lamina vasciilosa ; Ag, argentea ; 

 Ls, lamina supra-ohoroidea ; Sc, 

 sclerotic, enclosing cartilage or 

 bone (t) ; Co, cornea ; Ir, Iris ; 

 V-K, anterior chamber ; L, Lens ; 

 GV, vitreous humour ; Pr, pro- 

 cessus falciformis ; Cp, campanula 

 Halleri. 



