NERVOUS SYSTEM 875 



The outer capsule surrounding the various layers just discussed, 

 consists of a sclera, which covers the proximal side of the eye, and the 

 cornea which is transparent, and through which light first passes before 

 reaching the lens. The sclera is usually white, and is covered externally 

 in part by the conjunctiva, which is modified epithelium. In some of the 

 extinct amphibia, and in many of the modern reptiles and birds, portions 

 of the sclera ossify and form a ring of sclerotic bones. Snakes and croco- 

 diles, however, do not develop sclerotic bones. In the sturgeon, and in 

 many teleosts, there are two or more dermal bones developed on the 

 sclera, and in some of the sharks and teleosts there are calcifications to 

 be found also, but these are not true sclerotic bones. Between the lens 

 and the cornea there is an opening, partly divided by the iris into an- 

 terior and posterior chambers. The two chambers are in direct connec- 

 tion through the pupil and are filled with a liquid, called the aqueous 

 humor. 



All that has been discussed so far in connection with the eye forms 

 the eyeball proper, also called the bulbus oculi. The eyeball is moved 

 in its socket by six muscles, best understood by studying Figure 466. 



Amphibia possess a distinct muscle which draws the eyeball back 

 into its socket. This is known as the retractor bulbi. Even some of the 

 jaw muscles may assist in elevating and depressing the ball. In the dog- 

 fish there is a cartilaginous rod, called the optic pedicle extending from 

 the eyeball to the skull, this being replaced in the bony fishes by a fibrous 

 band, the tenaculum. 



The eyelid varies in different groups. The upper and lower, as in 

 the higher vertebrates, and the third lid, called the nictitating membrane, 

 usually drawn horizontally across the front from the inner angle of the 

 eye, all form beneath the lower lid. The eyelids themselves have a lining 

 which lies next to the eye and which is a continuation of the conjunctiva 

 already mentioned. In the higher mammals the nictitating membrane 

 appears as a rudimentary fold called the plica semilunaris in the inner 

 angle of the eye (Fig. 4). 



There are no glands connected with the eye in cyclostomes or fishes. 

 In amphibia they are of the rudimentary type, but in both reptiles and 

 birds they are divided into two groups, Harder's glands (nictitating 

 glands), lying near the inner angle and the true lachrymal or tear glands, 

 lying in the outer angle. 



The tear glands in mammals ultimately come to lie beneath the 

 upper lid and lead, by many ducts, into the conjunctival sac, while 

 Harder's glands degenerate. The tears secreted by the lachrymal glands 

 pass over the conjunctiva and are collected at the inner angle of the eye, 

 where they then pass through the lachrymal duct into the cavity of the 

 nose. 



The eyes of the cyclostomes are of a very degenerate type. In the 



