THE SENSES. 



of elastic and tendinous tissue, and gives 



muscle on the one hand, and on the other to the outer 



iris. The canal is regarded by most anatomists as occupied by a 



venous plexus, receiving veins from the ciliary muscle and from the 



anterior part of the sclerotic. 



Choroid. The choroid coat is a vascular and pigmentary mem- 

 brane, lining the inner surface of the sclerotic, and presenting ante- 

 riorly a thickened portion, the " ciliary body." The inner part of the 

 ciliary body is thrown into radiating folds, the " ciliary processes," 

 which surround the borders of the crystalline lens. The inner surface 

 of the choroid is occupied by a layer of hexagonal nucleated cells, filled 

 with blackish-brown pigment. Similar pigment is also deposited, 

 though less abundantly, in the substance and near the external sur- 

 face of the choroid. At its anterior part, the choroid is separated from 

 the sclerotic by the ciliary muscle (Fig. 130, 6 ). This muscle is com- 

 posed of unstriped fibres, which arise from the posterior wall of the 

 canal of Schlemm, at the junction of the sclerotic and cornea, and 

 thence diverge in a radiating direction, outward and backward, to be 

 inserted into the external surface of the choroid, where it passes into 

 the folds of the ciliary processes. At the anterior and inner part of the 

 muscle there are also bundles of circular fibres, parallel with the margin 

 of the cornea. The muscle is thus composed of two parts, namely, an 

 internal circular, and an external radiating portion, the fibres of which 

 are more or less interwoven with each other at its inner edge. 



Iris. The iris is a variously colored membrane, extending across 

 the cavity of the eyeball, attached by its external border to the pos- 

 terior wall of the canal of Schlemm, and presenting at its centre the 

 nearly circular orifice of the pupil. It consists of connective and mus- 

 cular tissue, with an abundant supply of blood-vessels, and is covered 

 on its posterior surface by a layer of blackish-brown pigment cells, 

 continuous with those of the choroid. The color of the iris, which 

 appears, in different individuals, blue, gray, brown, or black, depends 

 on the abundance and disposition of its pigmentary elements. In gray 

 and blue eyes, the visible hue of the iris comes from the diffused light 

 of its semi-transparent tissues, seen against the dark background 

 of the pigment layer on its posterior surface. In brown and black 

 eyes, the pigment is more abundant, and is deposited, according to 

 Kolliker and Cruveilhier, not only on the posterior aspect of the iris, 

 but also in its stroma, between its fibres, and to some extent even on 

 its anterior surface. It thus predominates, and extinguishes more or 

 less completely the diffused light of the remaining elements of the tissue. 



The position of the iris is such that while its outer border is attached 

 to the junction of the cornea and sclerotic, its central portion is in con- 

 tact with the anterior surface of the crystalline lens. According to 

 Helmholtz,* the iris in myopic eyes is sometimes so nearly flat that 

 it throws no perceptible shadow under an extreme lateral illumination ; 

 but in normal eyes, as a rule, the portion immediately surrounding the 



