STKUCTURE OF THE RETINA. 363 



retina as composed of layers, however, modern inves- 

 tigators, following KoUiker and Heinrich Miiller, are 

 generally agreed in considering that the fibres of the 

 optic nerve pass radially through the retina : thus 

 from the fibres a thread passes downwards till it meets 

 a cell of the vesicular layer, which in turn is in con- 

 nection with a granule of the granular layer, which 

 terminates in a cone and rod ; these latter forming the 

 real termination of the optic fibre in the pigment layer 

 of the choroid coat. — (See Plate III, fig. 4.) It is now 

 universally held that the rods and cones are the per- 

 cipients of light, which they communicate to the ceEs 

 of the vesicular layer, thence to the optic fibres, and 

 thence to the optic ganglion. The point to be borne 

 in mind in this description is that the sensitive part of 

 the retina is not the surface on luhich the light imincr 

 diately falls, hut the surface which is in contact tuith 

 the black pigment. 



In a parenthesis I may add, that one of the Dorpat 

 school* has considerably disturbed the harmony which 

 existed on the subject of the retina, by the pubhcation 

 of a series of researches, wliicli led him to the con- 

 viction that only the optic fibres of the retina are of 

 nervous structure, the rest being formed of "con- 

 nective'' tissue. Whatever may be the issue of the 

 quarrel thus raised, it will not affect the points to 

 which our argument will be directed ; indeed, Funke-f* 

 already suggests that, inasmuch as the function of 



* Blessig : De Retince Strudura : 1855. See an abstracl; in Can- 

 statt's J ahresbericht: 1855. 

 ■i* Lehrbuch cler Physiologie, 711. 



