THE PROTECTIVE ORGANS OF THE EYE. 469 



With regard to the posterior hyaloid vessels, the generally ac- 

 cepted theory is that they later enter into the formation of the 

 retinal vessels. Little is known as to the details of this process ; 

 but the fact remains that, in the rabbit, for instance, the larger 

 branches of the retinal vessels are internal to the inner limiting 

 membrane, and, therefore, within the vitreous body, and that they 

 send smaller branches into the retina (His, 80). 



L INTERCHANGE OF FLUIDS IN THE EYEBALL. 



The anterior lymph- channels of the eye comprise (i) the 

 lymph-canaliculi of the cornea, which communicate with similar 

 structures in the sclera ; (2) the system of the anterior chamber, 

 which is indirectly connected, on the one hand, with the canal of 

 Schlemm by means of the spaces of Fontana, and with the stroma 

 iridis, into which the ligamentum pectinatum extends ; while, on the 

 other hand, it communicates with the posterior chamber and its 

 recesses, and with the canal of Petit. 



In the posterior region of the eyeball are the lymph-channels 

 of the retina (the perivascular spaces), those of the optic nerve, the 

 space between the pigment layer and the remaining portion of the 

 retina (interlaminar space, Rauber), and the lymph-spaces of the 

 choroid and sclera. The influx and efflux of intraocular fluid 

 occur principally by means of filtration. The influx takes place 

 through the ciliary processes ; that the choroid has to do with this 

 process is very improbable. The efflux takes place through the 

 veins of the canal of Schlemm, into which the fluid filters through 

 the cement lines of the endothelial lining of the canal of Schlemm, 

 finally emptying into the anterior ciliary veins. A posterior efflux 

 from the vitreous body probably does not exist, or at least occurs 

 to a veiy limited extent. The anterior chamber possesses no efferent 

 lymph-vessels (Leber, 95). 



J. THE PROTECTIVE ORGANS OF THE EYE. 



J. THE LIDS AND THE CONJUNCTIVA. 



At the end of the second month of embryonic life the eyelids 

 begin to develop in the shape of two folds of skin. At the end 

 of the third month these folds come in contact in the region of 

 what is later the palpebral fissure, and grow together at their outer 

 epithelial margins. Shortly before birth the two lids again separate 

 and the definitive palpebral fissure is formed. 



The eyelids show three distinct layers: (i) the external cutis, 

 which presents special structures at its free margin and continues 

 about I mm. inward from the inner border of the free margin ; (2) 

 the mucous membrane, or palpebral conjunctiva, beginning from 



