REPTILES 393 



usually effected before birth, but in the uropeltid snake, Rhinophis, a 

 small slit-like palpebral aperture is still present at that time. The 

 spectacle is quite insensitive so that in time it gets scratched and dull ; 

 Johnson (1927) found that it could be touched and even polished with 

 a cloth in order to get a view of the fundus without any signs of 

 inconvenience or resistance on the part of the animal, even in resentful 

 species like the cobra or python. 



When the snake sheds its skin the milky layer which forms under the stratum 

 corneum throughout the body is very obvious through the transparent spectacle ; 

 and with the skin the spectacle is also shed, leaving a free ragged border on its 

 inner surface where it was attached at the sclero- 

 corneal junction. So tough is this thin layer of 

 skin (0-1 mm. thick) that it still retains its 

 hemispherical form after it has been discarded ; 



meantime, the snake lies sluggish and irritable /^ J_^ ^..rJ^^*,^^^ / •' 



and seeks no food. 



Fig. 481.— The Harderian 



It is curious that in snakes the 

 lacrimal gland (associated with the lids) is 



absent, but the harderian gland (usually Duct of" a" Snake! 



associated with the nictitating membrane) E, the eye ; H, harderian 

 is present. The latter is very large and its f^^^^ Be/a:irsf """""^^""^ °'^^" 

 oily secretion flows into the closed con- 

 junctival sac and from its nasal corner drains into the nose through a 

 single naso-lacrimal duct which empties (as in lizards) inside the 

 vomero-nasal organ of Jacobson (Bellairs and Boyd. 1947-50) ; thence 

 it flows into the mouth where it acts as an accessory salivary secretion, 

 lubricating the unchewed prey as an aid to the difiicult act of swallow- 

 ing the enormous mouthfuls of food habitual to the snake (Fig. 481). 



Underneath the spectacle the eyes of snakes are freely movable, 

 but spontaneous movements are not marked. The bursalis and 

 retractor bulbi are absent (Nishi, 1938). The movements of the two 

 eyes are independent except for convergence,^ and as a general rule 

 in order to obtain a view of an object reliance is placed on the pendulum- 

 like movements of the head as it is swmig from side to side rather 

 than upon movements of the eyes. 



Apart from the primitive boas and pythons, the orbit of snakes is 

 open and fenestrated, in keeping with the general lightness of the 

 architecture of the skull ; in contrast to Lacertilians there is, however, 

 a well-formed optic foramen. Temporal arches and a zygomatic bone 

 are absent, probably to facilitate the wide gape of the jaws. 



Abelsdorff. Arch. Anat. Physiol, Physiol. AneHi. i?/c. Mor/o?., 15, 233 (1936). 



Abt., 155 (1898). Armstrong. J. Anat., 85, 275 (1951). 



Albers. Denkschriften K. Akad. Wiss. Bage. Quart. J. micr. Sci., 57, 305 



Miinchen, 81 (1808). (1912). 



1 See p. 695. 



