S5 2 



THE RETINA. 



[sect. 226. 



Fig. 223. 



to eighteen trunklets, extend forwards in the outer lamella of the 

 choroid, partly in furrows of the sclerotic, and divide several times 

 in a forked manner, even before their entrance into the ciliary- 

 muscle. Here they break up into a rich and dense plexus, from 

 which numerous filaments are distributed to the muscle and the 

 cornea, and the remainder form the proper nerves of the iris. 

 The latter nerves accompanying the blood-vessels, dividing re- 

 peatedly and forming anastomoses, especially in the situation of 

 the annulus minor, until they reach the pupillary border, where 



they terminate, probably 

 by free extremities, after 

 repeated division and the 

 formation of loops. The 

 elements of all these 

 nerves are moderately 

 fine, and fine tubules 

 (croo2'" to o'004'") in the 

 trunks, while in the iris 

 they measure only O' 001'" 

 to o - oo2" in diameter. 



Nerves of one-half of the iris of a white rabbit; treated 

 with caustic soda, and magnified 50 times, a. nervitii 

 ciliares ; b. anastomoses of the same at the border of the 

 iris; c. larger arcuate connections of the same in the iris ; 

 c. finer networks of the same in the inner parts ; d. ter- 

 minations of separate nervous filaments towards the ex- 

 terior of the iris ; e. sphincter pupillae. 



§ 226. The Retina. — 

 The retina is the innermost of the five tunics of the eye-ball, and 

 lies close upon the vascular tunic ; in its proper nervous ele- 

 ments, however, it terminates at the ora serrata by an undulated 

 border, margo undulato-dentatus s. ora serrate? retina?, where it is 

 very intimately connected with the choroid on the one hand, and 

 with the hyaloid membrane on the other. The retina is con- 

 tinued upon the ciliary part of the hyaloid membrane by a 

 peculiar cellular layer, which will be spoken of hereafter. 



The retina is a delicate membrane, almost perfectly transparent 

 and clear when recent, but becoming, soon after death, whitish and 

 opaque. It commences at the place of entrance of the optic 

 nerve, being in part connected continuously with it. It possesses, 

 at first, a thickness of ci'", but soon becomes attenuated anteriorly 

 to o - o6'", till at last, near its anterior border, it measures only 

 o - 04'", terminating eventually by a very well defined edge. Not- 

 withstanding the variations in its thickness, the following lamina? 

 may be everywhere distinctly observed in the retina; they are 

 enumerated in their order from without inwards: i, the layer 

 of the rods and cones; 2, the granular layer; 3, the layer of 

 grey nervous substance ; 4, the expansion of the optic nerve ; and, 



