420 



ADAPTATIONS TO MEDIA AND SUBSTRATES 



rule by scales or feathers, its action results in a brief period of blindness. 

 A third lid, the vertical 'membrana nictitans', has consequently evolved 

 as a fold of the conjunctiva at the inner or nasal comer of the lid 

 opening (Fig. 142). Being transparent, it can sweep the cornea from 

 the nasal to the temporal side, to clean and moisten it, without shutting 

 out the light. This action is of paramount importance to the scampering 

 lizard or to the bird in flight, exposed to a stream of air which would 

 quickly dry the cornea. There are many ornithologists who believe that 

 the nictitans is held over the eye most or all of the time that a bird is 

 in the air — the forerunner of the motorcyclist's goggles. The retractor 

 bulbi muscle remains important in the reptiles and persists into the 

 mammals. In mammals, as in the dog and cat for example, it is often 

 divided into four slips alternating with the rectus muscles. It is lacking 



Fig. 142 — The nirtitating membrane or third eyelid. From Wolff, after Sutton. 



a, b, front and rear views of turkey eyeball with nirtitans, its tendon, and the muscles 

 which operate it. x 1. c, the mechanism in situ in a disserted head. 



in birds — the bird orbit hardly ever affords enough room for the retrac- 

 tion of the large avian eyeball, and the flexibility of the bird's neck is 

 adequate compensation. In man, it is the heavy bony rim of the orbit, 

 particularly the ridge bearing the eyebrow, which makes a retractor bulbi 

 unnecessary. We may be 'hit in the eye' by a swift baseball, without the 

 eyeball necessarily being harmed. Our erect posture may also have some- 

 thing to do with the loss of the retractor, which obviously is of greatest 

 value to those large-eyed forms which, like the horse, hang their heads 

 for a good part of the time, when feeding. 



Adnexa in Sphenodon — In Sphenodon, the most generalized of liv- 

 ing reptiles, the lacrimal gland is lacking; but a large Harderian gland 

 moistens the cornea and lids adequately with its oily secretion. The 



