186 COMPARATIVE ANATOMY. 



(Fig. 153, Pr, Cp}. The processus falciformis is never large in 

 Elasmobranchs and Ganoids. 



In the interior of this structure lie nerves, vessels, and 

 smooth muscle-fibres, and the latter possibly exert an 

 influence on the lens, and thus serve as an apparatus for ac- 

 commodation. 



External to the choroid proper, that is, between it and the 

 lamina fusca s. suprachoroidea, lies a silvery or greenish-gold 

 iridescent membrane, the so-called argentea. It extends either 

 over the whole interior of the eye (Teleostei), or is limited to the 

 iris (Elasmobranchs). 



A second layer with a metallic lustre, the tapetum 

 cellulosum s. lucidum, lies internally to the iridescent portion, 

 and within this again there is the chorio-capillaris of the choroid. 

 No tapetum appears to be present in Teleostei or Petromyzon. 



The so-called choroid gland, present only in Teleostei and 

 Amia, consists of a rete mirabile (comp. p. 202), composed of 

 arteries and veins, which has the form of a cushion, lying near 

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

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

 it corresponds in position to the choroid. 



The sclerotic is usually extensively chondrified (Elasmobranchs, Sturgeons), 

 and not unfrequently becomes calcined or ossified towards its junction with the 

 cornea : this also holds good for Teleosteans. 



The eyeball is almost always surrounded by a gelatinous tissue, pene- 

 trated by simple and elastic connective-tissue fibres, and in Elasmobranchs 

 it is curiously articulated on its inner circumference with a rod of cartilage 

 arising from the lateral wall of the skull. 



The eyes are reduced or abortive in Amblyopsis spelaeus, a Fish living in 

 the caves of Kentucky : a similar abortion of the eyes takes place in many 

 Invertebrates which live in caverns or in the deep sea (Vermes, Mollusca, 

 Crustacea, Insecta). 



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, of the more or less strongly chondrined sclerotic, the 

 slight convexity of the cornea, and the globular lens. In other 

 important points, 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. A proper 

 ciliary muscle is present in the whole series of animals from 

 this point onwards. 



The eyes of Proteus and of the Gymnophiona always lie more or less 

 deeply beneath the skin ; they are very small, and are much degenerated 

 (Figs. MS, Bar, and 149, A). In Proteus the crystalline lens and vitreous 

 humour are both wanting. 



Reptiles and Birds. In these also, the sclerotic is in great 

 part cartilaginous, and in Lizards it is provided with a ring of 

 delicate bony plates around the external portion. Very many 



