18 A TYPICAL VERTEBRATE EYE: THE HUMAN 



their contraction opens up the pupillary aperture, throwing the body of 

 the iris into concentric folds or contraction furrows. The dilatator, being 

 only a part of an epithelial layer, contains no nuclei and no blood vessels, 

 nor any connective tissue forming septa within it or a sheath outside it. 

 The sphincter shows all of these features, however. 



Both sphincter and dilatator are derived embryologically from the 

 anterior layer of the double epithelial pars iridica retinae which is, em- 

 bryologically, the zone of the optic cup nearest its lip. The cells which 

 become sphincter muscle fibers separate completely from the epithelium 

 late in fetal life, and the epithelium exhibits a gap underneath them (Fig. 

 7a, g) ; but the dilatator cells remain permanently, so to say, in a half- 

 way stage of conversion from epithelium into muscle. As a rare anomaly 

 in man, even this development may fail and there may be no trace of a 

 dilatator, the pupil then remaining strongly contracted throughout life 

 ('microcoria'). Wherever among the lower animals the dilatator is lack- 

 ing (the pupil then being opened by the elasticity of the iris tissue alone, 

 upon relaxation of the sphincter) the spindle cells of the anterior layer 

 of the pars iridica retinae remain wholly epithelial, like the cells of the 

 posterior layer, and fail to lose their pigmentation during development as 

 do the elements which produce dilatator fibers in other animals and man. 



The sphincter and dilatator have very different nerve supplies from 

 the autonomic system, and respond very differently to pharmacological 

 agents and to substances which duplicate or imitate the natural chemical 

 intermediators between nerve and muscle. They are involved in a num- 

 ber of reflexes. The fundamental one is the contraction of the pupil to 

 protect the retina from dazzlement when the external illumination is 

 suddenly increased. As the retina adapts to the new illumination (by 

 reducing its sensitivity) the pupil slowly reopens. Other reflexes include 

 the 'consensual' contraction of the pupil of a covered eye when the other 

 eye is illuminated, its dilatation in emotional states or when the skin of 

 the neck is pinched, its contraction when the eyes converge and accommo- 

 date for nearby objects, etc. The last-mentioned of these reactions is not 

 a true reflex, but the result of co-innervation of the sphincter pupillae 

 and the muscles of accommodation, the common nerve also running 

 with that which supplies the convergence-muscle, the internal rectus. The 

 complexity of some of the pupil reflexes is only realized when an attempt 

 is made to analyze their neurological basis in cases where the reflexes 

 have been lost or altered, due to traumatic or pathological lesions in the 

 central nervous system. 



