THE EETIXA. 



605 



tliese nerves are seen lying on the surface of the choroid, into -which 

 they send branches, and in which they form between and amongst the 

 blood-vessels a fine plexus of j^ale fibres rich in gangiion-cells, the 



Fiff. 415. 



Fi- 416. 



Fig. 415. — Lateral View of tiik Ciliakt Xehves (Arnold). Diagrammatic. 



a, optic nerve ; b, back j^art of the sclerotic ; c, ciliary muscle ; d, iris ; e, outer 

 surface of the choi'oid coat ; 1, five of the ciliary nei-ves jiassing along the sheath of 

 the optic nerve, piercing the sclerotic fiosteriorh', and thence passing forward on the' 

 choroid membrane to the ciliary mnscle and iris. The nerves are represented too large. 



Fig. 416.— DiSTRiBuxioN of Nerves in the Iris (Kolliker). 50 Diameters. 

 The preparation was from the eye of an albino rabbit; a, smaller branches of the 

 ciliary nerves advancing from the choroid ; b, loops of nniou between tliem at the 

 margin of the iris ; c, arches of nuion in the iris ; c', finer network in the inner part ; e, 

 sphincter pupillse muscle. 



groups of cells being often applied to the walls of the vessels. Within 

 the ciliary muscle the nerves also subdivide minutely, forming here 

 another plexus, which contains a number of mednllated fibres and the 

 cells of which are smaller. A few recurrent branches appear to pass 

 back from it into the choroid coat, but the greater number pass on to 

 the iris (fig. 416, a, n). In the iris the nerves follow the course of the 

 blood-vessels, dividing into branches, which communicate with one 

 another as far as the pupil, forming a close plexus of fine non-medul- 

 lated fibres. Their ultimate termination is not ascertained. 



RETIlSrA OR ]^^:EIlVOUS tujstic. 



The retina is a delicate almost pulpy membrane, which contains the 

 terminal part of the optic nerve. It lies within the choroid coat, and 

 rests on the hyaloid membrane of the vitreous humour. It extends 

 forwards nearly to the outer edge of tlie ciliary processes of the choroid, 

 where it ends in a finely indented border — ora serrata. From this border 

 there is continued onwards a thin layer of transparent, nucleated cells of 

 an elongated or columnar form, constituting the ixirs cUiarh rcliiut, 

 which reaches as far as the tips of the ciliary processes, and there dis- 

 appears. The thickness of the retina diminishes from beliind forwards, 

 varying from Joth of an inch to aihjth of an inch. In the fresh eye 

 it is translucent and of a light pink colour; but after death it soon 

 becomes opaque, and this change is most marked under the action of 

 water, alcohol, and other fluids. Its outer surface is covered with a 

 layer of hexagonal pigment-cells which, as the study of the develop- 

 ment of the parts shows, must be regarded as belonging to the retina 

 and not to the choroid, to which it has usually been ascribed. More- 



