96 



LACRYMAL ORGANS. 



2. The conjunctiva.- Stmilunar fold, mem- 

 bruna nictitans or third eyelid. Lacrymal 

 caruncle and glandule of Harder. 

 The existence of eyelids supposes a con- 

 junctiva, that is, the integument modified 

 into a mucous membrane lining the posterior 

 surface of the eyelids, covering the front of the 

 eyeball, and thus connecting these two parts 

 together. In those animals in which there is 

 no palpebral covering either in the usual form 

 of eyelids, or in that anomalous form in which 

 there is no palpebral opening, there is, properly 

 speaking, no conjunctiva : at the most, the com- 

 mon integument may be softer, thinner, and 

 more transparent where it covers the eye. 



The points deserving particular notice in the 

 comparative anatomy of the conjunctiva are, 



1. The oculo-palpebral space of the con- 

 junctiva. 



2. The membrana nictitans and third eyelid, 

 with the glandule of Harder. 



In most animals the oculo-palpebral space of 

 the conjunctiva is as it has been described in 

 man. Serpents have been generally regarded 

 as being without eyelids and via lacrymales. 

 The eye, indeed, has been commonly described 

 as covered by a transparent lamella of epider- 

 mis, which is cast with the rest of the epider- 

 mis; and this has been often adduced as an ar- 

 gument in favour of the extension of the con- 

 junctiva over the cornea in other animals. Ser- 

 pents have a palpebral covering, a conjunctiva 

 and vise lacrymales; but the conformation of 

 these parts is quite peculiar. For the first true 

 exposition of the point we are indebted to Jules 

 Cloquet.* 



In the article HEARING, ORGAN OF, at the 

 end of the section on the parallel between 

 the ear and the eye, it is said, " A part in 

 the composition of the appendages of the 

 eye analogous to the membrana tympani is 

 only to be conceived by supposing the exist- 

 ence of a mediate anchi/lo-blep/taron, that is, 

 an irregular membrane stretched between the 

 edges of the eyelids, uniting them together, and 

 closing in the space lined by the conjunctiva, 

 which space would now communicate with the 

 exterior only by the lacrymal canalicules and 

 nasal duct, in the same way that the tympanic 

 cavity communicates with the exterior only by 

 the Eustachian tube." The mediate anchi/lo-b/e- 

 pharon here supposed is the actual and regular 

 structure in serpents, and is the sole cause of 

 the apparent anomaly in the conformation of 

 their eye appendages. The structure is this: 



Around the margin of the orbit the skin ap- 

 pears as if it formed a palpebral fold, covered 

 with scales, and representing a sort of frame, in 

 which is set, as it were, a transparent continua- 

 tion of the skin before the front of the eye. But 

 this is not the conjunctiva; it is a natural me- 

 diate anchylo-blepharon, or a palpebral covering 

 without palpebral fissure. It is, however, 

 transparent, and was therefore formerly con- 

 founded with the cornea. A conjunctiva lines 

 the inner surface of this palpebral covering 



* Memoire sur 1'Existence et la Disposition des 

 Voies Lacryraales dans les Serpens. Paris, 1821. 



without palpebral fissure, and invests the greater 

 part of the orbital cavity, from which it is re- 

 flected on the sclerotica, and is from thence 

 continued over the cornea, closely adhering to 

 it. 



The conjunctiva thus forms a sac (called by 

 Jules Cloquet oculo-palpebral sac of the con- 

 junctiva), confining a space (which may in like 

 manner be called oculo-palpebral) into which 

 open the excretory ducts of the lacrymal gland. 

 From it the tears are drawn off by a canal be- 

 tween the jaw and palate bone into the mouth, 

 of which more will be said farther on. 



Fig. 17. 



Vertical section of the eye of the common viper, to show 

 the disposition of the conjunctiva. (From Jule 

 Cloquet.) 



a, the eyeball ; b, the optic nerve ; c, the eyelid ; 

 d, d, the scales surrounding the eyelid, and into 

 which it appears as if enchased ; e, e, folds and 

 culs-de-sac formed bv the oculo-pal| ebral conjunc- 

 tiva on being reflected from the orbital cavity upon 

 the eyeball ; f, cavity of the oculo-palpebral sac of 

 the conjunctiva. 



From the above description it will be readily 

 understood how it is from the palpebral exten- 

 sion of the skin that the epidermis falls when 

 the skin is said to be cast, and not from the con- 

 junctiva. 



Muller* has found the above structure in all 

 true serpents, even in the amphisbffinffi, whose 

 eyes are covered with thick skin. I do not 

 know if it has been observed in coecilise also. 

 He has found a similar structure among the 

 saurian reptiles in the geckoes, even in the 

 genus phyllurus. He has even found it in a 

 mammal, spalax typhlus,f the eyes of which 

 appear to be covered by the thick hairy skin, 

 underneath which, however, there is a small 

 sac of conjunctiva. In the chameleons, which 

 follow the family of the geckoes, in Cuvier's 

 arrangement, there is a near approach to the 

 same structure, the palpebral covering present- 

 ing only a very small palpebral opening oppo- 

 site the pupil. 



The me?nbrana nictitans and third eyelids. 

 In the quadrumana, as in man, the conjunc- 

 tiva forms at the nasal canthus a simple semi- 

 lunar fold, but larger. In the other mammi- 

 fera, including the herbivorous cetacea, and ex- 



* Ueber eine Merkwiirdige Eigenthumlichkeit im 

 Baa der Augen und Thrlinenwerkzeuge bci den 

 Geckonen. In Amman's Zeitschrift, Bd. i. p. 179. 



t Handbuch der Physiologic des Menschen, 

 Zweiten Bandes Zweite Abtheilung, S. 313. Co- 

 blentz, 1838. 



