214 THE ORGANS OF SPECIAL SENSE 



as antagonists of the meridional fibres. The ciliary processes 

 project inwards behind the iris but in front of the crystalline 

 lens. 



The Iris forms a contractile, perforated diaphragm, which 

 is separated from the posterior aspect of the cornea by the 

 anterior chamber of the eye. Its peripheral border is con- 

 tinuous with the ciliary body and its free, central border 

 bounds the pupil. The iris contains a number of unstriped 

 muscle fibres. Some of these are arranged around the peri- 

 phery and constitute the sphincter pupillcz. They are supplied, 

 through the ciliary ganglion, by the oculo-motor nerve (p. 58) 

 and their action serves to diminish the size of the pupil. 

 Other fibres extend from the periphery towards the free, 

 central margin, constituting the dilatator pupillce. They are 

 supplied, through the ciliary ganglion, by the sympathetic 

 fibres which enter the skull along the internal carotid artery 

 (p. 186). The ultimate origin of these nerves is said to be in 

 the oculo-motor nucleus. 



Inflammation of the iris is usually accompanied by severe 

 pain, experienced, for the most part, over the area of distribu- 

 tion of the ophthalmic nerve (Fig. 41). The inflammation 

 may spread to the adjoining ciliary body and chorioid, but the 

 most important complication arises from the formation of 

 adhesions between the posterior surface of the iris and the 

 anterior aspect of the crystalline lens. Such adhesions pre- 

 vent the fluid in the posterior chamber of the eye from passing 

 forwards into the anterior chamber (p. 216) and an increased 

 intra-ocular tension results, constituting acute glaucoma. In 

 order to prevent the occurrence of this complication, mydri- 

 atics should be employed at an early stage and, provided that 

 the measure is successful, should be continued until the con- 

 dition is cured. 



The Eetina forms the innermost coat of the eyeball. 

 Its outer, pigmented layer is continued forwards over the 

 ciliary processes on to the posterior aspect of the iris, but the 

 nervous elements of the retina only extend as far forwards as 



