38 A TYPICAL VERTEBRATE EYE: THE HUMAN 



oblique may be accompanied between the eyeball and the pulley by an 

 extra muscular slip which has a common insertion with it upon the eye- 

 ball. An additional and interesting atavism in an occasional human is a 

 'retractor bulbi' muscle, which in other mammals serves to hold the eye- 

 ball tightly back in the orbit regardless of the relaxations and contrac- 

 tions of the eye-rotating muscles. It ordinarily has four parts in mammals, 

 alternating with the recti and originating with them at the apex of the 

 orbit. The anomalous human retractor bulbi may exhibit this complete 

 arrangement. The two oblique muscles, approaching the eyeball from the 

 nasal side, might seem to give the muscular apparatus extra power for 

 converging the two eyes — convergent movements being more frequent 

 than any others — but since they do not pass in front of the center of 

 rotation of the eye, their chief actions are to tilt the eyeball upward and 

 downward. Their original purpose was, however, very different (p. 303). 

 The six normal muscles are supplied by three different cranial nerves, 

 one of which cares for four of them. Their bilaterally cooperative actions 

 and the elaborate central-nervous control thereof are beyond the scope 

 of this elementary description. 



The Lids — The eyelids are essentially folds of skin, which were devel- 

 oped by land animals primarily for cleaning and moistening the cornea, 

 and which incidentally protect the eye from small foreign objects such 

 as insects, wind-blown sand, and the like. The cornea of an aquatic 

 animal is kept clean and succulent by the water itself, through which 

 no natural particle can travel with sufficient velocity to injure or embed 

 in the cornea. It is a mistake to suppose that the chief purpose of the 

 lids is to protect the eye — from blows, and so on; for they are no real 

 protection against such insults. That function, in man, is taken care of 

 by the supraorbital ridges of the skull which overhang the orbits and 

 bear the eyebrows, whose purpose appears to be to divert sweat from 

 the eyes. 



The opening between the lids, which reveals a portion of the eyeball, 

 is the 'palpebral fissure'. Its temporal and nasal angles are respectively 

 the (sharper) outer and (broader) inner 'canthi'. In the inner canthus 

 can be seen the plica semilunaris, a crescentic fold of conjunctiva which 

 is a vestige of a third, sidewise-sweeping eyelid present in many animals, 

 the nictitating membrane. Neither the special muscles nor the special 

 gland (Harder's) of the third eyelid are present, even as vestiges, in 

 man. Overlying the base of the plica is a pink mass, the caruncle, which 

 is really a bit of the margin of the lower lid which becomes isolated 



