REPTILES. 



The bat, fwallow, frug, and falamander, exemplify the 

 former. 



Organs of the Senfes. 



Organ of V'ifion. — Reptiles, like all vertebral animals, have 

 two inoveable eyes occupying the cavities of the head called 

 orbits, and poilefling in their llnidlnre the fame elTential 

 parts as in man. Their ftrufture has not been much iii- 

 veftigated : there is fomc information in a memoir of Petit, 

 in the Acad, dcs Sciences, 1737; and an account of the 

 tortoifes in CalJefi. 



They occupy the fides of the head, except in the cro- 

 codile, where they are placed above, between the cranium 

 and jaw. 



In the proteus they are covered by the integuments, and 

 thus concealed until the latter are removed : this animal is, 

 therefore, like the zemni or mus typhlus among mammalia, 

 and conlequently blind. All other reptiles have the fcnfe 

 of fight, and fome poflels even large and prominent eyes. 



The tortoife has, at the anterior part of the fclerotica, 

 fimilar ofleous lamina to thofe of birds. They are inclofed 

 in that membrane, without being continued into its fub- 

 flance, and may be eafily feparated from it. There are 

 fimilar laminae in the camcleon and fevcral other lizards, 

 but they do not form the anterior difk, they merely fur- 

 round the lateral part. Daudin has found this ftrufture in 

 the common iguana and the great tupinambis of America. 



" In the tortoife," fays Cuvier, " the ciliary procefles 

 projeft fo very little, that we could fcarcely recognize them, 

 were it not for the elegant impreffion they leave on the 

 vitreous body ; but in the crocodile thefe procefles are very 

 beautiful, and very confpicuous : they are each terminated 

 by a nearly right angle. I have obfervcd thefe proceiles 

 in the form of elongated threads in a large foreign tree- 

 frog ; there are alfo fuch, though not dilfini!:!;, in the toad. 

 I have not ebferved them in the common lizards, nor in 

 the ferpents. 



" The iris of reptiles refemblcs that of fifhes in its golden 

 colours ; fometimes it is red or brownifh. The vellels are 

 more vifible than in fUb ; they form a beautiful net-work 

 on the iris of the crocodile. 



" The ligure of the pupil varies ; in the crocodile it is ver- 

 tically oblong, as in the cat : in frogs rhomboidal. The 

 tortoife, cameleon, and common lizards, have it round ; the 

 gecko rhomboidal. 



•' The optic nerve in all reptiles pafTes through the mem- 

 branes of tlie eye direitly, and by a round hole, as in quad- 

 rupeds ; it forms internally a fmall tubercle, from the 

 edges of which the retina proceeds. 



" The cryftalline is more fpherical in the turtle afid frog, 

 than in the mammalia : the ratio of its axis to its diameter 

 J9 in the former 7 to 9, in the latter 7 to 8 : thefe propor- 

 tions are nearly thofe which it has in tifhcs." 



Mufcles of the Ghlc. — There are iix in the tortoife, dif- 

 pofed like thofe of fifties : and befides, four fmall ones, 

 which clofely embrace the optic nerve, and fpread over the 

 convex portion of the fclerotic, after being interrupted by 

 the mufcle of the third eye-lid. The fame ftrufture is found 

 in the crocodile. 



In frogs and toads there is a great funnel-like mufcle, 

 which embraces the optic nerve, and is divided into three 

 portions ; its inferior fibres advance more towards the edge 

 cif tlie eye than the fuperior. There is only a fingle ftraight 

 mufcle on the inferior part, and coiifequently only a fingle 

 depreflbr. There is one very (hort oblique mufcle attached 

 to the anterior part of the orbit, and inferted direftly into 

 the adjoining part of the globe. The mufcle of the third 



eye-lid is fo clofc to the inferior jpart of the fiifpenfory 

 mufcle, that it becomes ftrctclied when the latter fwells ; 

 which accounts for the elevation of the third eye-lid when 

 tlie eye is lowered. 



Blumenbach remarks, that no inftance of the leucsethiopic 

 formation, or deficiency of the colouring matter of the 

 choroid coat and iris, has ever been fcen in reptiles ; al- 

 though it is fo common in the two warm-blooded claffcs, 

 and in the human fiibjeft. Specimen, p. 27. 



Many reptiles fhun the light, lying hid by day, and 

 coming out to feek their food at night. Others appear very 

 fond of light, whether tiiat of the fun, as the green lizard 

 and green frog ; cu" of artificial flame, like that of a candle, 

 as the tree-frog. 



It feems peculiar to the cameleon to have the power of 

 moving the two eyes at the fame time in different dii eftions ; 

 fo that he is not neceffarily obliged, like other animals, to 

 bring tiie two optic axes into the fame dircftion. 



Eye-lids — Reptiles vary fingularly with rcfpeft to the 

 number and difpofition of tlieir eye-lids : ferpents have none ; 

 crocodiles and tortoifes have three, and the third is verti- 

 cal, as in birds : tliere are alfo three in frogs, but the 

 third is horizontal like the other two. The third eye-lid is 

 the part called membrana niflitans. 



Tlie crocodiles arc remarkable for having bone in the 

 upper eye -lid : in general there is only a fingle bit near the 

 anterior angle ; but in the crocodilus palpebrofus the whole 

 eye-lid is occupied by a plate divided into three pieces. See 

 Cuvier in Annales du Muleum, v. 10. 



The horizontal eye-lids of crocodiles and tortoifes clofc 

 cxaftly ; they have each an enlargement at their edge, but 

 no cilia ; the third eye-lid is femi-tranfparent ; it moves 

 from behind forwards, and may cover the whole eye ; it has 

 only one mufcle, which is analogous to the pyramidalis of 

 birds : it is fixed in the fame manner to the poftcrior part 

 of the globe inferiorly. After having turned round the 

 optic nerve, it rcpaffes under the eye to fend its tendon to 

 that eye -lid ; but there are neither the mufculus quadratus, 

 nor its flieath, as in birds. 



In the other lizards there are alfo very remarkable va- 

 rieties. The common lizards have, for eye-lids, a kind of 

 circular veil extended before the orbit, and perforated by 

 a horizontal fiiiuie, which is capable of being clofed by a 

 fphinClcr mufcle, and opened by a levator and dcpreflor j 

 its inferior part has a fmooth round cartilaginous difl<, as 

 in birds ; there is, befides, a fmall internal eye-lid, but it 

 has no proper mufcle ; it is entirely wanting in the came- 

 leon, in which animal alfo the flit of the eye-lids is fo fmall, 

 that the pupil can fcarcely be obferved through it. The 

 gecko has no moveable eye-lid ; its eye is protefted by a 

 (light margin of the flcin, as in ferpents. A fimilar difpo- 

 iition feems to prevail in the fcink. 



Blumenbach lias examined more particularly the ftrufture 

 of the part which covers the front of the eye in ferpents. 

 " Thefe animals," he obferves, " are commonly faid to throw 

 off the external layer of the cornea Tvith the reil of their 

 epidermis when they change their fkin. On examining this 

 matter more miiuitcly in the coluber natrix, I find that that 

 part of the epidermis, which is perfeftly tranfparent and 

 Itretchcd before the eye, is not actually connefted to the 

 cornea, but feparated from it by a imall quantity of water. 

 It is immove^.ble, fo that the globe moves behind it, as 

 behind a window." Specimen, Sec. p. 26. 



In frogs and toads the fuperior eye-lid is only a projec- 

 tion of the ftin, and almolt immoveable ; the inferior is 

 more moveable, and has a fwoln edge ; but the third, which 

 moves from below upward, is moft employed by thefe ani- 

 mals: 



