EYE 211 



1. Eyelids (palpebrae). 



2. Glandular oi-gans. 



3. Muscles, serving to move the eye-ball. 



The eye-ball is thus formed of a series of concentric layers 

 which are called from within outwards retina, choroid and iris 

 (vascular layer), and sclerotic and cornea (skeletal layer). The first 

 corresponds with the nervous substance of the brain, the second with 

 the pia mater, and the third with the dura mater. The interior of 

 the eye contains refractive media, the lens and vitreous hicmour. 

 -To these, certain accessmy structures are added (pp. 216-220). 



The relative development of the eye varies considerably amongst Verte- 

 brates. It may reach a very high degree of perfection ; or may, on the other 

 hand, undergo more or less degeneration in those animals which live in caves 

 or burrows (e.g.. Fishes — Amblyopsis splelseus, Typhlogobius ; Amphibians 

 —Proteus, Gymnophiona ; Snakes — Typhlops : Mammuls — Talpa, &c.). 

 In Ammoooetes and Myxine the eye is hidden- beneath the integument (see 

 below), and in the Cetacean Platanista gangetica the eyes are extremely 

 minute. 



The retina will be dealt with after a description of the eyes 

 of the various classes of Vertebrates has been given (p. 214). 



In Amphioxus a simple pigment spot is present in the front' wall of the 

 "cerebral ventricle" (p. 157, and Fig. 219). 



Cyclostomes. — The eye of Cyclostomes remains at a very low 

 stage of development, not only as regards the structure of the retina, 

 but also — in Myxinoids, in the absence of a lens and iris and of a 

 differentiated sclerotic and cornea as well as of eye-muscles, and 

 in the persistence of the choroid fissure. Moreover, the eye in 

 Myxinoids and in the larval Ammocoete lies beneath the skin and 

 subdermal connective tissue. In Petromyzon the skin covering the 

 eye becomes thinned out at metamorphosis, and thus the animal, 

 which was blind, or nearly blind, in the larval state, can see on 

 reaching the adult condition : at the same time the eye becomes 

 more highly organised, though the primary lumen in the lens 

 (Fig. 167, b) does not entirely disappear. 



Fishes and Dipnoans. — The eyes of all the true Fishes are, 

 with few exceptions, of considerable relative size, and are formed 

 on essentially the same plan as that described in the intro- 

 ductory portion of this chapter. 



The lens of Fishes, like that of all aquatic animals, is globular, 

 and possesses therefore a high refractive index. It touches the 

 cornea and fills up the greater part of the eyeball, 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 

 accommodated for seeing near objects. In Teleosts accommodation 

 apparently takes place by means of a process of the choroid, the 

 processus falciformis. This extends into the vitreous humour 

 towards the lens, around which it expands to form the so-called 



P 2 



