1462 



HUMAN ANATOMY. 



a disease due to increased intraocular tension from retention results. It is neces- 

 sary, therefore, in iritis to keep the pupil dilated, so as to prevent such adhesions 

 as far as possible. 



THE NERVOUS TUNIC. 



The Retina. The retina, the light perceiving portion of the eye, with its con- 

 tinuation, the optic nerve, in contrast to the other sense organs represents a portion 

 of the brain itself, and develops in close connection with it. It is a delicate mem- 

 brane, which extends from the optic nerve entrance to the pupillary border. The 

 functionating portion, or pars optica retina;, reaches as far forward as the ora serrata, 

 where it terminates as an irregular, wavy line ; anterior to this the retina is repre- 

 sented by an atrophic portion, consisting of the double layer of cells covering the 



FIG. 1218. 



Vitreous 



Cerebral layer 



Sustentacular cell 



Neuroepithelial layer 



Pigmented layer 



Fibre layer 

 Ganglion cells 



Inner plexiform layer 



Inner nuclear layer 

 (bipolar nerve-cells) 



fy A d~\ Outer plexiform 



plexiform layer 



Outer nuclear layer 

 (bodies of visual cells) 



Rods and cones 



Pigmented layer 



Choroid 

 Diagram illustrating structure of retina and relations of three fundamental layers. (Greeff.) 



ciliary body and the iris, already referred to in the description of these structures, 

 and known respectively as the pars c iliari s retina:, and^arf iridica retina. 



The pars optica retinae is closely applied to the inner surface of the choroid 

 and is in contact with the hyaloid membrane investing the vitreous body. It grad- 

 ually diminishes in thickness from .4 mm. at the posterior pole to .1 mm. near the 

 ora serrata. During life the membrane is transparent and possesses a purplish red 

 color, owing to the presence in its outer layers of the so-called visual purple ; after 

 death the retina rapidly becomes opaque and has the appearance of a grayish veil. 

 The inner surface is smooth and presents at the posterior pole of the eye, a small 

 circular or transversely oval yellow spot, the macula lutea, from 1-2 mm. in diam- 

 eter. At the centre of the macula is a small depression, the fo-rea centra/is, from 

 .2-.4 mm. in diameter, in which position the retina is reduced in thickness to .1 mm. 



The entrance of the optic nerrc forms a conspicuous spot of light color, situated 

 3 mm. to the nasal side of the macula lutea. This area, called the optic papilla or 

 porus optic/is, is in form of a vertical oval, about 1.5 mm. in its horizontal and 



