THE EYE. 251 



anterior chamber of the eye. This is the ligamentum pcctina- 

 tum, which has just been mentioned. Its trabecular are 

 covered with epithelial cells. The anterior surface of the iris 

 also has such a layer. An incompletely closed, ring-shaped 

 canal, which is bounded by the trabecular of this ligamentum, 

 has been called the canalis Fontanae. 



Small ganglia of the ciliary nerves occur in the choroid. 

 The ciliary muscle and the iris are more plentifully supplied 

 with nerve fibres, but their manner of termination we do not 

 yet know. 



Concerning the crystalline lens and the vitreous body in 

 general, we refer to pages 78 and 45. There is one circum- 

 stance which requires more special mention here. According 

 to a widely disseminated acceptation, the hyaloid membrane 

 (Fig. 199 in the vicinity of k), separates into two leaves, a 

 posterior and an anterior, the so-called zonula Zinnii, which 

 is impressed in a ruffle-like manner by the ciliary processes. 

 Both continue on to the crystalline lens at its equatorial zone. 

 The zonula Zinnii presents a peculiar pale and resistant sys- 

 tem of fibres. A three-cornered annular sinus, bounded by 

 both lamellae, bears the name of the canalis Petiti. Much is 

 still obscure here, and the space is, after all, only an artefact 

 (Merkel, Mihalcovics). 



Let us now turn to the expansion of the optic nerve into the 

 retina. Our membrane has its greatest thickness (0.38 to 

 0.23 mm.) at the place of the entrance of the optic nerve. It 

 becomes thinner (to about the half) towards the periphery. 

 Passing beyond the equator (thinned to 0.09 mm.) it termi- 

 nates as the so-called ora serrata (Fig. 199, k). Externally 

 from the place of entrance of the optic nerve (z 1 ), about 3 to 4 

 mm. removed from it, is the macula lutea, the seat of the most 

 distinct vision (g). In its centre there is an excavation, the 

 so-called fovea centralis. 



The retina, provided with numerous other elements, appears 

 to be an extraordinarily complicated structure, and, at the 

 same time, of extreme delicacy and variability. It has been 

 the object of infinite research in older and more recent times ; 



