ORGANS OF THE SENSES. 26*7 



are generally provided with the ordinary straight and oblique 

 muscles, a distinct lachrymal apparatus, two moveable eye- 

 lids, and a membrana nictitans. In serpents the skin forming 

 the eye-lids passes transparent and continuous over the 

 eyes and their lachrymal organs, and the epidermis is thus 

 shed from the closed eye- lids as from the rest of the skin ; 

 the tears here bathe the concealed conjunctiva, and pass by 

 the lachrymal duct into the nose, as in higher animals. But 

 in the slow-worm, the eye-lids and the membrana nictitans 

 are formed as in saurian reptiles. The sclerotic is sometimes 

 firm and cartilaginous, sometimes it contains adipose sub- 

 stance behind the retina as in fishes, and in many as the 

 crocodilian reptiles, the iguana, the lizard, and the monitor, 

 and the tortoises and turtles, the fore part of the sclerotic 

 supports a circle of osseous plates which surround the 

 transparent cornea, as in birds. These plates around the 

 cornea existed also in the ichthyosaurus. The ciliary pro- 

 cesses are now considerably developed around the margin 

 of the flattened lens, especially in the crocodilian reptiles, 

 and the greater freedom and mobility of the iris from the 

 abundance of aqueous humour, allows of more extensive 

 and quick changes in the diameter of the pupil than in 

 the inferior vertebrata. The pupil is often extended vertically 

 in saurian and ophidian reptiles, as in many carnivorous 

 mammalia, and a dark pecten in many lacertine and cro- 

 codilian reptiles, is prolonged from the choroid coat into 

 the vitreous humour, as in birds. The pulpy and fibrous 

 layers are obvious in the retina, and its central foramen 

 is seen in many of the species. The eyes of the chamseleon 

 do not move simultaneously, the two eye-lids are united 

 over the eye excepting a small vertical slit opposite to 

 the middle of the pupil, and the concealed membrana 

 nictitans is nearly as large as in birds. In the eye of 

 emys europaea (Fig. 104. B.) the cornea () is pretty convex 

 from the abundance of aqueous humour (b) in the anterior 

 chamber, and the margin of the cornea is supported by 

 ten osseous plates (104. C. d.) imbricated like those of birds, 

 and placed in the anterior part of the sclerotic (104. B. d, d,) 

 near to the ciliary processes, (104. B. /.) and to the fixed 

 margin of the iris (104. B. e.) The crystalline lens (104. B. 

 g.) has a compressed eliptical form, and a smaller axis than 



