1102 



THE ORGANS OF SPECIAL SENSE 



ward in the perichoroidal lymph space, in which they form a plexus, from which plexus filaments 

 pass to supply the bloodvessels of the choroid. After reaching the iris they form a plexus around 

 its attached margin; from this are derived amyelinic fibres which terminate in the circular and 

 radial muscular fibres. Their exact mode of termination has not been ascertained. Other 

 fibres from the plexus terminate in a network on the anterior surface of the iris. The fibres 

 derived from the motor root of the lenticular ganglion (oculomotor nerve) supply the circular 



fibres, while those derived from the sympathetic supply the radial fibres. 



\ 



MenUana Pupillaris. In the fetus the pupil is closed by a delicate transparent 

 vascular membrane, the membrana pupillaris, which divides the space into which 

 the iris is suspended into two distinct chambers. This membrane contains numer- 

 ous minute vessels, continued from the margin of the iris to those on the front part 

 of the capsule of the lens. These vessels have a looped arrangement, and converge 

 toward each other without anastomosing. Between the seventh and eighth months 

 the membrane begingsto disappear, by gradual absorption from the centre toward 

 the circumference, andVtbirth only a few fragments remain. It is said sometimes 

 to remain permanent ana produce blindness. 



III. The Retina, or Tunica Interna (Figs. 800, 801). 



The retina is a delicate nerve membrane, in which the fibres of the optic nerve 

 are spread out and upon the surface of which the images of external objects are 

 received. Its outer surface is in contact with the choroid ; its inner with the vitre- 

 ous body. Behind, it is continuous 

 with the optic nerve; it gradually 

 diminishes in thickness from behind 

 forward; and, in front, extends nearly 

 as far as the ciliary body, where it 

 appears to terminate in a jagged 

 margin, the ora serrata (Figs. 802 

 and 820). Hejre__the_nerve^ tissues 

 of the retina end, but a thin pro- 

 longation of the membrane extends 

 forward over the back of the ciliary 

 processes and iris, forming the pars 

 ciliaris retinae and pars iridicae reti- 

 nae, already referred to. This for- 

 ward prolongation consists of the 

 pigmentary, layer, of L the retina, to- 

 gether with a stratum of columnar 

 epithelium. The retina is soft, semi- 

 transparent, and of a purple tint in 

 the fresh state, owing to the presence 



of a coloring material named rhodopsin or visual purple; but it soon becomes 

 clouded, opaque, and bleached when exposed to sunlight. Exactly in the centre 

 of the front surface of the posterior part of the retina, corresponding to the visual 

 axis, and at a point in which the sense of vision is most perfect, is an oval yellowish 

 spot, called the yellow spot (macula lutea) (Figs. 799 and 810), having a central 

 depression, the fovea centralis. The retina in the situation of the fovea centralis 

 is exceedingly thin, and the dark color of the choroid is distinctly seen through it. 

 It exists only in man, the quadrumana, and some saurian reptiles. About 3 mm. 

 (one eighth of an inch) to the nasal side of the yellow spot, and about 1 mm. 

 below it, is the point of attachment of the optic nerve, the optic disk (porus options) 

 (Figs. 810 and 811), the circumference of which is slightly raised so as to form 



FIG. 810. The arteria centralis retinae, yellow spot, 

 etc., the anterior half of the eyeball being removed 

 (Enlarged.) 



