235 



distilled water, saltish, changing to a green colour, vegetable blues, and 

 containing soda, muriate, and carbonate of soda, and a very small quan 

 tity of phosphate ot soda and of lime*. 



In ophthalmia, the irritation of the conjunctiva, transmitted by sympa- 

 thy, to the lachrymal gland, not only augments the quantity of its secre- 

 tion, but appears, likewise, to alter the qualities of the fluid that is se- 

 creted. The tears which, in those cases, flow in such profusion, bring 

 on a sense of burning heat in the inflamed part; do they not, perhaps, 

 contain a greater quantity of the fixed alkali then in the ordinary state of 

 the parts, and may not the painful sensation depend as much on the in- 

 creased proportion of soda in the tears, as on the greater sensibility of 

 the conj unctiva ? 



This last membrane is merely a fold of the skin, which is exceedingly 

 thin, covers the posterior surface of the eye-lids, and is then reflected over 

 the anterior surface of the eye, which it thus unites to the eye-lids. From 

 the whole extent of this surface, there oozes an albuminous serosity, which 

 mingles with tears, and adds to their quantity!. 



The tears are equably difused over the globe of the eye, by the alter- 

 nate motions of the palpebrae ; they prevent the effects of friction, and 

 save the organs of sight from being dried, at that part which is exposed 

 to the air. The air dissolves and carries off, by evaporation, a part of 

 the lachrymal fluid. This evaporation of the tears is proved by the weep- 

 ing to which those in whom that secretion is very profuse, are subject, 

 whenever the atmospherical air, from being damp, does not carry off a 

 sufficient quantity of the fluid. The unctuous and oily fluid, secreted by 

 the meibonian glands, smears the loose edge of the palpebrae, prevents 

 the tears from falling on the cheek, and answers the same purpose as the 

 greasy substances with which one anoints the edges of a vessel, filled 

 above its level, to prevent the overflowing of the contained fluid. 



The greatest part of the tears, however, flow from without inward and 

 towards the inner canthus of the eye ; they take that direction in conse- 

 quence of the natural slope of the moveable edge of the palpebrae of the 

 triangular groove, which is formed behind the line of union of the edges 

 whose round and convex surfaces touch each other only in a point, and 

 this course of the tears is likewise promoted by the action of the palpebral 

 portions of the orbicularis palpebrarum, whose fibres, having their fixed 

 point at the inner angle of the orbit, where the tendons is inserted, al- 

 ways draw inward their external commissure. 



* " The saline parts amount only to about 01 of the whole. The mucus contained 

 in the tears has the property of absorbing oxygen from the atmosphere, and of becom- 

 ing thick and viscid, and of a yellow colour. This property of acquiring new properties 

 from the absorption of oxygen explains the changes which take place in tears in some 

 diseases of the eye." See the Chapter, at the end of the APPENDIX, on the Chemical 

 Constitution of the Textures and Secretions. Copland. 



f There is no opening in the skin at the part which corresponds to the globe of the 

 eye; it is exceedingly thin, and is continued, under the name of the conjunctiva, over 

 the transparent cornea, to which it adheres so firmly, that it is not easily separated from 

 it. In some animals that have no palpebrx, the skin is continued, of the same thickness, 

 over the tore part of the eye. The conjunctiva (if, however, this portion of skin de- 

 serves that name) when opaque, renders the globe of the eye, in other respects imper- 

 fect, absolutely useless. This is observed in the kind of eel called in books of natural 

 history, mnrena cceciUa: the gastrobranchus ccecus is blind from the same circumstance. 

 M. BIBKS is of opinion that it terminates at the circumference of the cornea 1 



