894 



THE ORGANS OF SENSE. 



number (Kdlliker), forty to forty-five (Waldeyer and Sumisch) ; they are derived 

 from the ciliary nerves and enter the laminated tissue of the cornea. They 

 ramify throughout its substance in a delicate network, and their terminal fila- 

 ments form a firm and closer plexus on the surface of the cornea proper beneath 

 the epithelium. This is termed the subepithelial plexus, and from it fibrils are 

 given off which ramify between the epithelial cells, forming a network which is 

 termed the intra-epithelial plexus. 



Dissection. In order to separate the sclerotic and cornea, so as to expose the second tunic, 

 the eyeball should be immersed in a small vessel of water and held between the finger and 

 thumb. The sclerotic is then carefully incised, in the equator of the globe, till the choroid is 

 exposed. One blade of a pair of probe-pointed scissors is now introduced through the opening 

 thus made, and the sclerotic divided around its entire circumference, and removed in separate 

 portions. The front segment being then drawn forward, the handle of the scalpel should be 

 pressed gently against it at its connection with the iris, and, these being separated, a quantity 

 of perfectly transparent fluid will escape ; this is the aqueous humor. In the course of the 

 dissection the ciliary nerves may be seen lying in the loose cellular tissue between the choroid 

 and sclerotic or contained in delicate grooves on the inner surface of the latter membrane. 



Second Tunic. This is formed by the choroid behind, the iris and ciliary 

 processes in front, and by the Ciliary muscle, opposite the junction of the scle- 

 rotic and cornea. 



FIG. 528. The choroid and iris. (Enlarged.) 



The choroid is the vascular and pigmentary tunic of the eyeball investing the 

 posterior five-sixths of the globe, and extending as far forward as the cornea, 

 the ciliary processes being appendages of the choroid developed from its inner 

 surface in front. The iris is the circular muscular septum which hangs vertical- 

 ly behind the cornea, presenting in its centre a large circular aperture, the pupil. 

 The Ciliary muscle forms the white ring observed at the point where the choroid 

 and iris join with each other and with the sclerotic and cornea. 



The Choroid is a thin, highly vascular membrane, of a dark brown or chocolate 

 color, which invests the posterior five-sixths of the central part of the globe. It is 

 pierced behind by the optic nerve, and extends in front as far forward as the 

 ciliary ligament, where it is connected with the iris, and bends inward, forming 

 on its inner surface a series of folds or plaitings, the ciliary processes. It is thicker 



