THE ORGANS OF SENSATION 



177 



In horses, oxen, sheep, pigs and goats, transversely oval. 



In cats and rabbits, a vertical slit. 



Passing to the accessory organs of the eye, we find six 

 muscles serving to move the eyeball, but in man one muscle, 

 the retractor, is absent. Man and apes are peculiar in having 

 eyes which look forwards only ; the distance between the 

 human eyeballs or separation of the axes averages 62 

 mm. 



FIG. 88. The eyeballs represented as lying imbedded in the orbital fat with the 

 attachments of the oculomotor muscles. (From Thome's Zoologie.} A, cor- 

 nea; n, external rectus ; i, internal rectus ; s, superior rectus muscle; the 

 inferior rectus does not appear ; o, superior oblique, with its pulley (D) ; m, 

 is the origin of the inferior oblique muscle, which is not visible. 



The protection of the eyelids is practically absent in fishes ; 

 the amphibia (salamander and tailless amphibians) have fairly 

 well-developed lids, while reptiles and birds possess a nictitating 

 membrane in addition to a movable upper and lower lid. 



The so-called epicanthus occurs only in man ; it consists of 

 a fold of skin extending from the upper lid to the lower across 

 the inner canthus, and is found normally among the Mongolian 

 races and occasionally among Western nations. It produces a 



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