248 FROGS, TOADS, AND NEWTS [CH. VIII 



The head of the frog is bluntly triangular. From its 

 otherwise flat dorsal surface the eyes project for some 

 distance, and the nostrils but slightly, above the general 

 level. The animal is thus enabled both to breathe and to 

 see above the water while keeping the rest of its head and 

 body completely hidden beneath the surface. The upper 

 eyelid is attached to the eyeball and moves with it. The 

 lower eyelid (nictitating membrane) alone is capable of 

 winking, in which act it is raised as a transparent curtain 

 over the eyeball by a special muscle, while the eyeball 

 itself is retracted. When frogs are beneath the surface of 

 water, and especially when hibernating in this situation, 

 the lower eyelid is frequently kept half or completely drawn 

 across the front of the eyeball. The eye sockets have no 

 bony floor but are separated from the mouth cavity by soft 

 tissues only. It is thus possible for the eyes to be drawn 

 down into the mouth. The muscle that brings this move- 

 ment about is known as the retractor bulbi : it arises from 

 the parasphenoid bone and forms a fairly complete sheath 

 surrounding the hinder hemisphere of the eyeball. The 

 tendons of the nictitating membrane are attached to the 

 retractor bulbi in such a way that when the muscle contracts 

 the membrane of necessity rises across the front of the 

 cornea. Elevation of the eyeball is caused by a muscle, 

 the levator bulbi, which arises from the fronto-parietal, 

 sphenethmoid and other bones, and passes obliquely back- 

 wards and outwards as a sheet below the eyeball to be 

 inserted into the upper border of the maxilla. A few fibres 

 pass from the muscle into the lower eyelid and serve to 

 pull it down again as the levator pushes the eyeball up. 



