62 W. J. DAKTN. 



that the nerve in the eye-stalk divided into two branches, 

 one of which ran up to the optic vesicle, where he lost it, 

 whilst the other passed up the side and entered the vesicle, 

 lying- on the septum. 



Will (6) noticed the cellular structure of the lens, and 

 Keferstein (12) recognised the retina in the hinder trans- 

 parent body of Krohn. This brings us to Hensen's paper (13) 

 published in 1865, which is the first account of the histology 

 of the retina. Hensen divided this part of the eye into five 

 layers : 



1. First cell layer. 



2. Second cell layer. 



3. Rods. 



4. Tapetum. 



5. Pigment layer. 



The cells of the first layer, which may be arranged in a 

 single or double row, are spindle-shaped. The second layer 

 is made up of cylindrical cells (the rod-cells), the third layer 

 is that of the rods, and then follow two others — the tapetum 

 (first demonstrated by Krohn), and the pigment layer. 



The innervation is described as follows : The proximal 

 branch of the optic nerve does not bore through the optic 

 vesicle below, as Keferstein had assumed, but splits into a 

 number of small branches which enclose the lower part of the 

 optic vesicle, and these branches of the nerve form a plexus 

 in the peripheral region of the retina. Apparently Hensen 

 assumed that they were connected, with his second cell-layer 

 (the rod-cells) — " Der Zellenauslaufer geht so continuirlich in 

 den Nerven iiber, dass man nicbt sagen kann, wo der eine 

 anfangt und der andere aufhort." 



The other nerve-branch penetrates the septum, and the 

 fibres become connected to the cells of the first layer. 

 Hensen, it will be seen, discovered the different groups of 

 cells in the retina, described the nerve innervation correctly 

 (though since he did not recognise two types of cells in the 

 outer layer and in the rod-cell layer, this was probably more 

 accidental than otherwise), and saw the axial fibre in the 



