NERVOUS PORTION OF THE RETINA. 



799 



rith the optic nerve, arid a pigmentary layer outside, which adheres 

 o the choroid coat. 



Dissection. The retina can be satisfactorily examined only on 



L eye which is obtained within forty-eight hours after death. To 

 )riug it into view on the eyeball in which the middle coat was 

 lissected, the choroid must be torn away carefully with two pairs 

 )f forceps, while the eye is immersed in fluid. In this dissection 

 he pigmentary layer separates from the nervous portion of the 

 etina, and is removed with the choroid coat. 



The pigmentary portion of the retina is a very thin, dark layer, 

 vhich lines closely the inner surface of the choroid coat, and is 

 :ontinued over the ciliary processes into the uvea on the posterior 

 urface of the iris (fig. 291). 



The nervous portion of the retina is a soft membrane of a pinkish 



ey tint and semitransparent when 

 fresh ; but it soon loses this trans- 

 ucency, and is moreover rendered 

 )paque by the action of water and 

 other substances. In the living state, 

 lowever, the retina is characterised by 

 he existence of a purplish red colour, 

 vhich is discharged under the influence 

 )f sunlight. This part of the retina 

 extends over about the posterior two- 

 hirds of the eyeball, reaching from 

 he entrance of the optic nerve to the 

 outer extremities of the ciliary pro- 

 cesses, where it ends in an irregular 

 wavy border the ora serrata. Its 

 thickness diminishes from behind for- 

 wards. 



The outer surface of the dissected 

 retina is slightly flocculent, owing to 

 the tearing away of the pigmentary 

 layer. The inner surface is smooth : 

 it is covered with folds in a prepara- 

 tion of the eye cut in two, but these are accidental, in conse- 

 quence of the membrane having lost its proper support. At 

 the spot where the optic nerve expands (porus opticus, optic due; 

 fig. 293) the surface is slightly elevated (papilla optica) ; but in 

 the centre of this is a slight excavation where the central vessels 

 appear. 



In the interior of the human eye, in the axis of the ball, is a 

 slightly elliptical yellow area (fig. 293), one-twelfth of an inch in 

 diameter, which is named the yellow spot (macula lutea). Almost 

 in the centre of this spot is a minute hollow, the fovea centralis, 

 which appears black in consequence of the thinness of the wall 

 allowing the dark pigment outside to be seen. 



From the ora serrata a very thin layer is continued on as far as 

 the tips of the ciliary processes ; it is called the ciliary part of tJie 



Dissection 

 to see the 

 retina. 



Pigmentary 

 membrane. 



Nervous 

 retina : 



extent : 



FIG. 293. OBJECTS ON THE 

 INNER SURFACE OF THE 

 RETINA (SCEMMERRING). 



In the centre of the ball is 

 the yellow spot, here repre- 

 sented by shading : and in its 

 middle the fovea cen trails. To 

 the inner side is the optic disc 

 with the branching of the 

 artery. 



thickness ; 



outer 

 surface ; 



inner sur- 

 face presents 



optic disc, 



central 

 vessels, 



yellow spot, 



and central 

 fovea. 



Ciliary 

 part of 

 retina. 



