EYE. 



[ 254 ] 



EYE. 



broken or deformed. The cones(fig.217,l,4i) 

 are bacilli with a conical or pyriform body ; 

 and are also very easily injured. A slight 

 constriction divides each cone into two parts, 

 the innermost of which (fig. 217, 4 c) contains 

 a nucleus. .The cones are from 1-6000 to 

 1-4000" in diameter. In most parts of the 

 retina the cones are surrounded by several 

 bacilli ; opposite the punctum aureum, they 

 alone form a continuous layer ; whilst at its 

 margins, single bacilli intervene between the 



Fig. 218. 



End view of the rows of bacilli and cones from the out- 

 side. 1, opposite the punctum aureum (cones only) : 2, 

 at its margins ; 3, at the middle of the retina, a, cones 

 or spaces corresponding to them ; b, bacilli of the cones, 

 the ends of which are often situated somewhat beneath 

 the level of those of the true bacilli, c. 



Magnified 350 diameters. 



cones (fig. 21 8). Opposite the entrance of the 

 optic nerve, both bacilli and cones are absent. 

 These curious bodies are more distinctly seen 

 in many animals than in man (PL 41. fig. 5). 



The radial system of fibres pass between 

 the nerve-cells of the retina and the meshes 

 of the optic nerve to reach the limiting 

 membrane. The inner ends of the fibres 

 next the latter membrane, when overlapping 

 each other, and especially when swollen by 

 the action of water, present the appearance 

 of a number of rounded or angular cells 

 (fig. 216, c); for which they were once mis- 

 taken. 



It is thus evident, that, excepting the layer 

 of nerve-cells and that of the fibres of the 

 optic nerves, the retina cannot truly be con- 

 sidered as composed of layers. The series 

 of bacilli and cones when torn from their 

 connexion with the radial fibres, form the 

 so-called Jacob's membrane. 



We cannot enter into the physiology of 

 these radial fibres, which have been shown 

 to be the percipients of light. 



Crystalline lens, or, simply, crystalline. The 

 crystalline lens is contained in a capsule (fig. 

 213, q. 3), consisting of a perfectly structure- 

 less, very elastic membrane, the anterior half 

 of which is lined with a single layer of very 



transparent, polygonal, epithelial cells (fig. 



213, s), from 1-2000 to 1-1200" in diameter. 



The lens itself consists of long, transparent, 



six-sided, flattened fibres (fig. 219), from 



Fibres or tubes of the lens of the ox. 

 Magnified 350 diameters. 



1-4800 to 1-2400" in breadth and 1-8500 to 

 1-1300" in thickness ; these are tubular, at 

 least in the outer portions of the lens, and 

 contain a tenacious sarcodic substance, which 

 escapes from the ends of the broken fibres 

 in irregular globules, The form of the fibres 

 is best seen in a transverse section (fig. 220). 



Fig. 220. 



Transverse section of the fibres or tubes of the human 

 lens. 



Magnified 350 diameters. 



The fibres are firmer, narrower, and more 

 highly refractive towards the centre of the 

 lens. Their general arrangement is such, 



