340 



ANATOMY OF VERTEBRATES. 



223 



In the Turtle the sclerotic is cartilaginous, thickest behind, 

 and thicker at the temporal than at the nasal side of the globe. 

 The cornea is flatter than in the Emys or Land-Tortoise. 

 The optic nerve penetrates the sclerotic, as in other reptiles, exter- 

 nally to the axis of vision, fig. 222, b, and makes a conical projection 



iu the interior of the eyeball, from which 

 the thick retina expands and extends to the 

 ciliary circle, fig. 223 : there is no falciform 

 ligament. The choroid is thick, and 

 coloured by a deep-brown pigment. The 

 ciliary plica? are neatly denned, but do not 

 project freely from the surface. The pupil 

 is round ; the crystalline is more convex in 

 the Turtle than in the fresh-water or land- 

 Tortoises. The short ciliary arteries form a 

 plexus round the optic nerve in Chelone, 

 The cornea is more convex in the Tortoise, 

 fig. 222, than in the Turtle. The lacrymal glands are two, fig. 222, 

 a ; the smaller (harderian) one is internal and inferior in position ; 

 the larger is external, applied to the eyeball, fig. 224, and sending 

 its ducts to a deep fossa in the outer angle of the eyelids. These, 

 fig. 225, are thick, opake, covered by polygonal epidermic scales ; 

 the lower lid is largest, most moveable, and has fewest scales 



Section of the eye-feaH of the 

 Emi/s Europcea. xaxviii. 



224 



22.") 



Eye-ball of Emi/s Europcea : shewing the external 

 lacrymal gland, xxxvm. 



Eye-lids of Emus 

 Europcea. xxxviir. 



upon it in Chelone : there is also a nictitant membrane situated 

 vertically at the inner canthus, and having a horizontal motion. 

 The duct of the harderian gland opens on its internal surface 

 near the line of reflection of the conjunctive membrane upon it ; 

 and the secretion subserves the movements of the third lid. Be- 

 sides the four recti and two oblicp^ii muscles of the eyeball, there 

 is a choanoid or retractor muscle divided into four fasciculi. 



In the Crocodile, the sclerotic plates are not developed : the 

 membrane, fig. 226, x, u, is of a firm fibro-cartilaginous tissue, 

 allowing the dark hue of the choroid to appear through it : the 

 cornea, t, is large and convex. The choroid is thinner and with 



