THE EYEBALL. 



741 



The iris is a fibre-muscular curtain which is suspended between the 

 cornea and the crystalline lens. It is attached by its circumference to the 

 internal wall of the sinus c. iridis. In its centre is a round perforation called 

 the pupil, which is susceptible of considerable variations in size. This mem- 

 brane is composed of a fibre-connective tissue having a general radiating 

 direction from the pupillary border. Within this tissue are found pigment- 

 cells and unstriated muscular tissue. The muscular tissue element consists 

 of radiating and circular fasciculi. (Fig. 160.) The circular fasciculi form 



FIG. 160. 





Muscular Structure of the Iris of a White Rabbit, a, sphincter of the pupil; b, b, radiating 

 fasciculi of dilator muscle; c, c, connecting tissue with its corpuscles. 



a sphincter at the pupillary margin ; the radiating fasciculi radiate from 

 the sphincter to the circumference. At the circumference of the iris the 

 membrane lining the anterior chamber forms fibrous processes, which are 

 termed the ligamentum iridis pectinatwn. The posterior surface is covered 

 with a pigmentary layer, which is a continuation of the pigment layer of 

 the retina. 



The retina or third coat consists of two portions : the pigmentary mem- 

 brane and terminal elements of the optic nerve. The pigmentary membrane 

 or external layer, which has been called the system of the uvea, covers the 

 whole of the internal surface of the ciliary processes, the iris, and the 

 choroid. It consists of a single layer of hexagonal nucleated pigment-cells 

 of a dark-brown color. From the internal surface of this membrane delicate 

 fibres are continued between the cellular elements of the nervous layer. It 

 is frequently dissected with the choroid coat, and spoken of as one of its 

 laminae. The color of the iris in different individuals is dependent upon the 

 density of the fibro-connective tissue anterior to the uvea, and to the amount 

 of pigment-granules in it. In persons with dark eyes the pigment in this 

 tissue is relatively more abundant. 



The internal or nervous layer of the retina is composed essentially of the 

 terminal nerve elements of the optic nerv r e. Externally it is covered with 

 the pigmentary layer ; internally it is lined by a homogeneous transparent 

 structure called the hyaloid membrane. The structure of the retina is one 

 of great complexity. It consists of nine distinct layers, seven of which are 

 layers of nerve elements. All of these layers are bound together and sup- 

 ported by a connective tissue which contains bloodvessels. This layer ex- 



