336 Vertehrata. 



muscles {musculi recti), superior, inferior, internal, and external, and 

 two oblique muscles {musculi obliqid). The recti, whicli are 

 usually attached to the skull, close to the point where the optic 

 nerve perforates it, and to the eyeball, in a circle a short distance 

 from the cornea, move the eye upwards, downwards, inwards, or 

 outwards; the oblique muscles, which are usually attached to the 

 anterior or nasal wall of the orbit, and above or below to the optic 

 bulb, cause it to rotate on its axis which runs approximately 

 through the middle of ^he cornea. Besides these, there is, in not 

 a few animals (Amphibia, Eeptilia, Mammalia), a muscle which with- 

 draws the eye-ball [retractor hulbi) ; this muscle surrounds the optic 

 nerve, and springs from a point close to the optic foramen. 



Excepting in Pisces, a circular fold of skin, which can be drawn 

 over the cornea, arises a short distance from, and almost parallel 

 to, it. This fold consists of upper and lower portions, the upper 

 and lower eyelids, whose edges meet when they are drawn 

 over the eyes ; in the Mammalia the upper lid is best developed ; 

 in other animals, the lower. Just below the portion of skin covered 

 by the eyelids there is usually a thin and soft membrane, which 

 is termed the conjunctiva hulhi.* In many Reptiles, in Birds, and 

 in many Mammals there is a nictitating membrane, a fold 

 of skin developed in the inner corner of the eye, within the true 

 eyelids. In the two first-mentioned groups it is large, semi-trans- 

 parent, and moved by a special muscle ; in the Mammalia it is not 

 so well-developed. t Connected with the eyes are various glands, 

 opening below the eyelid or nictitating membrane, and serving to 

 keep the cornea and the inner side of the eyelid damp and smooth. 

 In Pisces such glands are altogether absent, whilst in all other 

 Vertebrates one or more are present. There is generally a 

 lachrymal gland which opens behind (at the outer corner 

 of the eye), usually by several apertures, on the inner side of the 

 lower or of the upper eyelid, J and a Harderian gland, which 

 opens anteriorly in the inner corner of the eye (usually within the 

 nictitating membrane if this is, present) ; the secretion of the former 

 is usually more watery, of the latter more fatty in composition. 

 Part of the secretion from these glands runs through a tube, the 

 lachrymal canal, which has several openings in the inner 

 corner of the eye and discharges into the nose. (The lachrymal 

 canal is primitively a trough-like epiblastic invagination which 

 becomes constricted to form a canal; see Fig. 283 ANr). 



* The soft inner covering' of the eyelids is termed conjunctiva palpebrarum. 



+ In many Fish immovable folds of the skin, otherwise like eyelids, lie round 

 the eye. In some Sharks there is a movable nictitating membrane. 



t Only in Mammalia some (often the majority) of the openings of the lachrymal 

 glands are on the upper eyelid, 



