AMPHIBIANS 345 



Amf)liibians, all of which are aquatic, and in those adult frogs which 

 do not leave tlie water. In the majority, however, which live their adult 

 life on land, a short njaper and lo\A'er lid develop during metamorphosis 

 (Maggiore, 1912) ; the upper lid is immobile, but associated with the 

 lower an elastic translucent fold forms n false nictitating meinbrane, the 

 free border of which is usually spotted with a brilliant bronze pigment 

 (green in some Hylidae, as Hyla coerulea). Normally the lid lies as a 

 Z-shaped fold in the loAver fornix and its thickened upper border is 

 continued as a cord which runs around the posterior part of the eyeball 

 slinging itself around the retractor bulbi muscle (Fig. 410) : when this 

 muscle contracts the eye is pulled into the orbit and the tug on the 

 cord draws the membrane upwards over the cornea completely covering 

 it. The membrane is thus entirely passive in its action and. forming 

 part of the lower lid itself, differs funda- 

 mentally from the pseudo-nictitating mem- 

 brane seen in some Teleosteans and also from 

 the true nictitating membrane of the higher 

 Vertebrates. Lubrication is effected by a 

 development of glands in the margin of the 

 upper lid ; those on the nasal side hyper- 

 trophy to form the massive harderian gland ^ 



, 7 ,' . -Ill -1 'tUi. 410. ]MrSCUL.\TUKE OF 



which occupies a considerable sjjace m the the Nictitating MEM- 

 nasal half of the orbit, while those on the ^^-^^e of the Frog. 

 temporal side become the precursor of the R-'etmctor bulbi muscle ; 



^ ^ \, tendon oi nictitatuig 



lacrimal gland ; two puncta aj^pear on the membrane on the temporal 



free border of the lo\\er lid, the canaliculi side of the posterior aspect 



OI the globe (after Pranz). 



uniting into a subcutaneous naso-laerimal 



duct running horizontally into the middle fossa of the nose. 



Ocular movements, apart from retraction, are negligible. The 

 usual extra-ocular muscles, however, are present with, in adchtion. a 

 jDowerful EETRACTOR BULBI MT7SCLE innervated by the Vltli nerve and 

 probably derived from the external rectus, and a second muscle behind 

 the eye, the levator bulbi, derived from the jaw-musculature and 

 supplied by the Vth nerve. If the eye is touched, retraction of the 

 globe is effected by the retractor muscle which at the same time pulls 

 the nictitating membrane over the cornea ; thereafter the levator 

 bulbi pulls the globe forAvard again and the membrane of the lower lid 

 falls back into its normal folded j^osition. This movement of retraction, 

 however, is possibly as useful as an aid to swallowing food as a protec- 

 tive device : the partition between the orbit and the mouth is merely 

 a thin membranous sheet and when the eyeball is jiulled into the head 



^ In most Vertebrates the lids are lubricated by a row of compound glands which 

 are frequently best developed temporally and nasally ; those on the temporal side 

 develop into the l.^crimal glands .secreting tears, tliose on the nasal side into the 

 GLAND OF HARDER {Actci eruditorum pub., Lij^siae, 1694) with a sebaceous oily secretion. 



