CHAP. XVII.] OF THE VITREOUS BODY. 31 



mound, or projection of the retina towards the p^.m. 



vitreous humor, with a minute aperture in the 

 summit. On removing the sclerotic and choroid 

 with the utmost care, the interior of the globe 

 can be seen from the outside, through this hole ; 

 and yet the membrane of Jacob appears to be 

 continued over it. On examining the structure 

 of the retina about the yellow spot, from with- 

 in, the fibrous expansion of the optic nerve 

 (though stretching in every other direction to 

 a much greater distance) cannot be traced 

 quite up to the spot itself. Nucleated cells 

 occupy the elongated meshes of the fibrous plexus already de- 

 scribed, until at length the fibres disappear, and the closely 

 set cells seem to cover the whole surface of the spot. The 

 gradual subsidence of the fibres in the interstices of the cells 

 we have distinctly seen. As for the colouring matter, it is not in 

 grains of pigment, but stains the several tissues, and soon disappears 

 in water. The use of the yellow spot is unknown. It is interesting 

 to observe in connexion with the perfection of vision over the spot, 

 that the principal branches of the artery and vein of the retina, 

 above and below, curve round it at a distance, going, as it were, 

 out of their course to avoid it, so that only capillary vessels are 

 found in its immediate vicinity. 



It now remains to describe the transparent media which occupy 

 the interior of the ball of the eye. 



The vitreous body, lying in the concavity of the retina, and filling 

 all but about the anterior fifth of the globe, has, when entire, the con- 

 sistence of soft jelly. It consists of an exceedingly fine and close, 

 but perfectly transparent web of fibrous tissue, the meshes of which 

 are exceedingly small, and contain an aqueous fluid. If the tissue 

 be cut into, the water will slowly drain off, showing the continuity 

 of the cells with one another; and the manner in which they are 

 constructed by interlacing fibres may be very plainly seen with a 

 high power near the ciliary processes, in the vicinity of which these 

 fibres are particularly strong. The whole vitreous body is bounded 

 by or contained in an envelope of extremely thin homogeneous 

 membrane, having corpuscles or cell-nuclei on its inner surface, 

 where the fibrous tissue is attached (fig. 117, h, Ti] . It would perhaps 

 be convenient to restrict the term hyaloid membrane to this envelope. 

 Where the retina extends, that is, as far as the ciliary body, the 

 hyaloid membrane is in contact with its inner surface, and united 



