610 THE SENSES. 



the iris. The canal itself is regarded by most anatomists as occupied 

 by a venous plexus, which receives veins from the ciliary muscle and 

 from the anterior part of the sclerotic. 



Choroid. The choroid coat is a vascular and pigmentary membrane, 



lining the inner surface of the sclerotic, and presenting anteriorly a thick- 

 ened portion, the " ciliary body." The inner part of the ciliary body 

 is thrown into a series of radiating folds, the " ciliary processes," which 

 surround the borders of the crystalline lens. The internal surface of the 

 choroid is occupied by a layer of hexagonal nucleated cells, closely packed 

 side by side, and filled with granules of blackish-brown pigment. Similar 

 pigment is also deposited, though less abundantly, in the substance and 

 near the external surface of the choroid. At its anterior part, the cho- 

 roid is separated from the internal surface of the sclerotic by the ciliary 

 muscle (Fig. 189, e). This muscle is composed of unstriped fibres, which 

 arise from the inner 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, at the point where it begins to pass into the folds of the ciliary 

 processes. At the anterior and inner part of the muscle there are also 

 bundles of circular fibres, running parallel with the margin of the cornea. 

 The whole 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 the inner edge of the muscular layer. 



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

 the antero-posterior axis of the eyeball, attached by its external border 

 to the inner wall of the canal of Schlemm, and presenting at its centre 

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

 muscular tissue, with an abundant supply of bloodvessels, and is covered 

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

 tinuous with that of the choroid. The color of the iris, which appears, 

 in different individuals, blue, gray, brown, or black, depends upon the 

 abundance and disposition of its pigmentary elements. In gray and blue 

 eyes, the visible hue of the iris depends upon the diffused light of its 

 semi-transparent tissues, seen against the dark back-ground of the pig- 

 ment layer upon its posterior surface. In brown and black eyes, the 

 pigment is more abundant, and is deposited, according to Kolliker and 

 Cruveilhier, not only upon the posterior aspect of the iris, but also in 

 its stroma, between its fibres, and to some extent even upon its anterior 

 surface. It thus predominates, and extinguishes more or less com- 

 pletely the reflected and 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 lies in 

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

 the observations of Helmholtz, 1 the iris in myopic eyes is sometimes so 



1 Optique Physiologique, traduit par Javal et Klein. Paris, 1867, p. 20. 



