200 ANATOMY OF VERTEBRATES. 



eyelid, and is reflected upon the eyeball, near its middle. At 

 the line of reflection are the orifices of a zone of ' Meibomiaii ' 

 follicles : an aggregate of somewhat more complex ones at the 

 inner side of the eyeball represents a f Harderian ' gland. There 

 is no true lacrymal gland, nor any ( third ' or nictitating lid. 

 The presence of this eyelid distinguishes the Sirenia from the 

 Cetacea ; l and the Harderian gland is more distinctly developed. 

 In Seals the circular eyelid is supplied by four dilators 

 and a sphincter, as in Whales ; but an external groove 

 at the inner can thus indicates the division of the horizontal 

 eyelids : the nictitating membrane is well developed and the 

 Harderian gland at its base is large. In the Elephant the 

 ' third ' or vertical eyelid is supported by a flat, slightly curved 

 cartilage, which becomes thinner as it is attached to the concave 

 free margin : the Harderian is continued as in Cetacea, from 

 a group of smaller mucous glands, which have many excretory 

 orifices upon the margin of the third eyelid, but its principal 

 duct terminates upon the inner surface near the base of that lid. 

 There is a special f nictitator ' muscle, the fibres of which pass at 

 first over the base of the membrane in a curve, then form an 

 angle to include the extremity of the nictitating cartilage, which 

 is consequently moved in the diagonal of the contracting forces, 

 and pushed forward and outward over the front of the eyeball. 

 In the Rhinoceros the lower eyelid has a depressor muscle. The 

 Harderian gland is large in the Hog-tribe ; its duct opens upon 

 the lower part of the inner surface of the membrane : it co-exists 

 with a ( caruncula lacrymalis.' There is a small lacrymal gland 

 the duct of which opens upon the inner surface of the upper 

 eyelid : the margin of this is provided with a row of stiff, unequal 

 cilia, beneath which are orifices of the ( Meibomian glands.' In 

 most Ungulates the base of the third eyelid is buried in a fatty 

 and fibrous substance. In the Sheep a large 4 caruncula ' co- 

 exists with the Harderian and lacrymal glands. The upper eyelid 

 has cilia in all Ruminants. The margins of the lids and the 

 conjunctiva are charged with black pigment in the Giraffe ; and 

 the cilia of the upper lid are very long. 



The eye is protected, in the Ornithorhynchus, by a cartila- 

 ginous plate continued from the upper part of the orbit, com- 

 parable with the palpebral plates in the crocodile. Both the water 

 Monotreme and the Echidna have a well developed membrana 

 nictitans : there are also an upper and a lower eyelid, each 

 of which has its proper apertor muscle. In Marsupials, the 



1 CXTIT". p. 28. 



