106 



HEALTH AND DISEASE 



Numerous dark-looking pigment cells, of irregular forms, are distributed 

 between the blood-vessels, giving to the membrane a soft, velvety-black 

 colour, except at one part, which forms a broad band just above the en- 

 trance of the optic nerve. Here the choroid presents a metallic yellowish- 

 green colour, and reflects the light strongly. 



This appearance is attributed to the jieculiar arrangement of some thin 

 (ell -plates found in this part, producing 

 the phenomena of interference of light, 

 and is believed to assist the animal in 

 peiceiving objects in twilight. 



The Ciliary Processes (fig. 242, o), 



which are thickly covered with black pig- 

 ment, are about one hundred and twenty 

 ui number, and are arranged in a circle, 

 pioiecting into the interior of the globe 

 to l)ecome connected with the vitreous 

 humour. They probably play an important 

 pait in the secretion of the aqueous humour. 

 Externally they present a whitish band, 

 which is the ciliary body or muscle. Some 

 of the fibres of this muscle radiate backwards from the margin of the 

 cornea over the choroid, others are circular; both, when acting, modify the 

 curvation of the lens, and thus influence the accommodation of the eye. 

 The Iris (hg. 241, 5) is a thin membrane composed of blood-vessels and 



of muscular fibres united together by connective tissue. Some of these 

 fibres are arranged in the form of a ring around the inner margin of the 

 pupil, while a second set, outwardly placed to these, and connected with 

 them, are disposed in a radiating manner like the spokes of a wheel. 

 In the centre of the iris is an ovoid opening, the " pupil " (fig. 241, 8), 



