IRIS. RETINA. 155 



contracts on the contrary in a bright light, only allowing 

 that quantity of luminous rays to enter the eye which is neces- 

 sary to vision. Certain substances also when taken into the 

 system act similarly on the iris; such are opium and the 

 Calabar bean, which cause the pupil to contract; belladonna, 

 on the contrary, dilates it. Changes in the diameter of the 

 pupillary opening also result from certain affections of the 

 eye and brain. Physiologists consider the dilatation and 

 contraction of the pupil as belonging to muscular move- 

 ments; in fact the microscope demonstrates the existence of 

 muscular fibres in the iris; it contracts also under the influ- 

 ence of electricity. 



It has been remarked that the posterior surface of the iris, 

 the ciliary processes, and the choroid were covered with a layer 

 of pigment. This name is also given to a dark brown substance, 

 appearing black in a mass, which colours certain portions of 

 the skin in the white man, and the whole tegument of the negro. 

 In the eye this pigment plays the same part as the lamp-black 

 in the interior of certain optical instruments, as the telescope 

 and magic-lantern; it absorbs the luminous rays, and prevents 

 them from being reflected, which would confuse the vision. 



Retina. The internal surface of the choroid, or rather the 

 pigmentary layer which covers it, is lined by the retina, a 

 nervous membrane, upon which the objects are depicted 

 that we see. It appears to be formed by the expansion of 

 the optic nerve, which enters the eye at its posterior part, 

 and forms at the bottom of the globe an enlargement, which 

 is called the papilla of the optic nerve. The retina develops 

 itself from the papilla, around which it forms a fold, and ex- 

 tends over the cavity of the eye to the circumference of the 

 ciliary processes of the vitreous body, where, according to 

 Cruveilhier, it abruptly terminates. It is of an opaline white 

 colour, semi-transparent, and easily torn. Its centre, which 

 corresponds to the antero-posterior axis of the eye, is to the 

 outside of the papilla of the optic nerve, where there is a 

 yellow spot (macula luted) and a depression (fovea centralis). 

 The yellow spot seems to be the point in the eye where 

 vision is most distinct. Microscopists describe the retina as 

 being composed of five, or even eight layers, of which the 



