VISION. 



233 



117. POSTERIOR HALF 

 OF EYEBALL, exhibiting the 

 retinal vessels ramifying 

 from the optic pore, and, 

 in the centre, the macula 

 lutea of Sommering with 

 its fovea ceiitralis. 



optic pore, and is there placed on the surface of the retina. 

 Subjacent to the multipolar corpuscles are other layers, marked 

 by the presence of nuclear ele- 

 ments; and on the other side of 

 these, resting on the choroidal 

 epithelium, is what is termed the 

 bacillary layer (fig. 118). 



The bacillary layer, or Jacob's 

 membrane, consists of multitudes 

 of minute structures, called rods 

 and cones, placed vertically to the 

 rest of the retina, like the ele- 

 ments of a columnar epithelium. 

 The rods are the more numerous, Fig. 

 and consist of an outer and inner 

 part of dissimilar nature : the 

 cones have an outer part similar 

 to the rods, while the inner part 

 is swollen to a flask-shape, and 

 they are more distinctly connected, through the medium of 

 structure in the nuclear layers, with the multipolar cor- 

 puscles. In the yellow spot, only cones are present in the 

 bacillary layer, and these are crowded together, and of smaller 

 size than elsewhere; also the multipolar corpuscles are 

 numerous, and there are no fibres of the optic nerve. 



172. When the choroidcoat has been divided, the transparent 

 structures which occupy the cavity of the eyeball can be 

 removed in one mass. They adhere most closely to the 

 ciliary processes, but, when separated from them, they have 

 the appearance of a limpid globe of delicate jelly, in the fore 

 part of which is placed a bead of denser consistence, sur- 

 rounded by a plicated collar, whose plications fit in between 

 the ciliary processes. The main mass is called the vitreous 

 humour; it consists of water entangled in meshes of 

 transparent tissue, and is limited by a structure of firmer 

 consistence, called from its limpidity the hyaloid membrane. 

 The denser bead in front is the crystalline lens; and if a 

 score be made along the face of it with a needle or a point of 

 a knife, the capsule which retains it in its place will be rup- 

 tured, and the lens will start out. It is expelled usually with 



