146 Mammalia. Organ of 8ight, 



E. Organ of Sight. 



It is only in Man and the Apes that the eye is situated in 

 a complete bony cavity ; in all the rest of the Mammalia the 

 Orbital fossae are completed, and separated from the Temporal 

 fossae by membrane only. Eyes are always present in this 

 Class, but they are occasionally very rudimentary, and covered 

 by the skin, which passes over the eyeball without any palpe- 

 bral opening or loss of hair. They frequently hold a lateral 

 position, and if, when this is the case, the Cornea is also pro- 

 minent, they are susceptible of receiving the image of a pur- 

 suer without turning the head. 



The Eye is commonly moved by six Muscles, as in Man ; 

 of these, the ' Superior oblique,' (which arises from the back 

 part of the rim of the orbit with the Recti, advances to the 

 upper part of the rim, glides there through a tendinous pulley, 

 returns towards the eyeball, is reflected backward and out- 

 ward beneath the Hectus superior, and is inserted into the 

 Sclerotic between this muscle and the Rectus Extemus), de- 

 mands special notice inasmuch as the trochlear arrangement 

 thereof is peculiar to the present Class. Besides these there 

 exists in all Quadrupeds up to the Quadrumana another 

 muscle which runs from the margin of the Optic Foramen, 

 surrounds the Optic nerve, and expands like a funnel as it 

 approaches the back part of the eyeball to which it is fixed ; 

 this is termed the * Choanoid Muscle,' or Suspensorius Occult; 

 it is generally single, but may be bifid, or even quadrifid ; 

 and occasionally, though very rarely, is the only muscle 

 present. The lower eyelid has occasionally a special depressor, 

 or apertor muscle. And the Third eyelid a special ' nictitator* 

 muscle, which however has not the same arrangement as in 

 Birds. Lastly, an ' Orbicular' muscle, to close the circular 

 eyelid, may be present. 



The longitudinal axis of the eye is generally of less extent 



