424 NERVOUS SYSTEM. 



what thickened. Just at the edge, the retina adheres to the 

 vitreous humour, and is supposed, erroneously by some anato- 

 mists, as Bichat and Monro, to be continued on to the circum- 

 ference of the lens. Repeated dissections, and the substantial 

 testimony of Soemmering,* have satisfied me that the retina 

 cannot be fairly traced beyond the greater circumference of 

 the impressions made on the vitreous humour by the ciliary 

 striae of the Choroides. When the eye is slightly macerated, 

 the retina always parts from" the vitreous humour at this line; 

 moreover, when its structure is still more slightly changed by 

 freezing and then thawing, the retina manifests a decided pre- 

 ference to separate there, and, under the most careful dissec- 

 tion, it is very difficult to prevent it. In addition to these con- 

 siderations, there is a well marked change of colour at the line 

 mentioned: in front of this line, the surface is transparent when 

 cleaned from the pigmentum ni^rum; whereas, if it were retina, 

 it should be the colour of ground glass, as is usual in the dead 

 body: also the veios of the retina never trespass beyond this 

 line, but are seen to cruise along it. 



Most anatomists teach that the retina is an expansion of the 

 optic nerve. Bichat believed that the latter terminated at the 

 bulb, and that the retina was another part of the structure, but 

 still consisting of the same sort of nervous matter. The latter 

 opinion is probably the more strictly correct, because there is 

 more pulpy matter in a section of the retina than can be found 

 in the same length of the optic nerve; also, if the retina were 

 simply an expansion of the nerve without any addition of mat- 

 ter to it, it should, from its hollow globular shape, be thinner in 

 the middle, where it is most expanded, than it is where the ex- 

 pansion first begins at the bulb of the optic nerve, but this is 

 not the case. 



The rerina does not adhere to the choroid coat, neither to the 

 vitreous humour which it encloses, except at the line mentioned: 

 when this line of attachment is broken, the retina quickly col- 

 lapses. 



The texture of the retina is extremely soft and pulpy; in the 

 living state, it is probably nearly transparent, but this can 

 only be conjectured from the readiness with which the vessels 



* Icones Oculi Humani. 



