THE ORGANS OF SPECIAL SENSE 2$$ 



deep-black color within. It is connected externally with 

 the sclerotic, by an extremely fine cellular tissue and by 

 the passage of the nerves and vessels ; internally, it is in 

 contact with the retina. The choroid membrane is com- 

 posed of three layers. It secretes upon its internal sur- 

 face a dark substance, called pigmentum nigrum, which is 

 of great importance in the function of vision. 



The iris, so called from the variety of its colors in 

 different persons, forms a partition between the anterior 



FIG. 134 Choroid membrane and iris, exposed by the removal of the 

 sclerotic and cornea: a, One of the segments of the sclerotic thrown back; 

 b, ciliary muscle ; c, iris ; e, one of the ciliary nerves ; /, one of the vasa 

 vorticosa or choroidal veins (Zinn). 



and posterior chambers of the eye, and is pierced by a cir- 

 cular opening called the pupil. It is composed of two 

 layers. The radiating fibers of the anterior layer converge 

 from the circumference to the center. Through the ac- 

 tion of these radiating fibers the pupil is dilated. The 

 circular fibers surround the pupil, and their action pro- 

 duces contraction of the area. The posterior layer is of 

 deep-purple tint, and is called uvea, from its resemblance 

 in color to a ripe grape. 



The ciliary processes consist of a number of tri- 



