THE SENSE OF VISION 605 



the bloodvessels of the iris will help to bring about its expansion, 

 and constriction its contraction. 



The iris dilates normally in weak light or in the dark, and when 

 the eyes are at rest. It also dilates under the influence of strong 

 emotion and of pain, particularly if the pain is in the region of the 

 neck. Certain drugs, known as mydriatics, produce dilatation of the 

 pupil. Such are atropine, homatropine, which act locally, paralyzing 

 the terminations of the third nerve; cocaine, which acts locally, 

 stimulating the endings of the sympathetic nerve; curare, which acts 

 centrally. 



FIG. 339. DIAGRAM SHOWING NERVE SUPPLY OF THE IRIS. (I)ixon.) 



///., Third nerve;/, preganglionic fibre to ciliary ganglion; g, postganglionic fibres to 

 circular muscle (m). S.C.G., superior cervical ganglion giving fibres which tr.d 

 in radiating fibres, m, of the iris. Drugs may act iipon the pupil at nerve- 

 endings d and e, or centrally at a. 







In certain nervous diseases, such as locomotor ataxia, the pupil 

 does not react to light, although it still responds when looking at a 

 near object (accommodation). This condition is known as the Argyll- 

 Robertson pupil, and is probably due to degeneration of the cells of 

 the ciliary ganglion. 



The Lens consists of a semi-solid core the nucleus with 

 somewhat more fluid edges. Its main constituent is crystallin, a 

 protein of the globulin type. It is held in position by a fine membrane 

 the capsule to which is attached the suspensory ligament, or 

 zonule of Zinn. Structurally, the lens consists of a number of " lens 

 fibres with serrated edges." These are formed by a process of 

 elongation from the layer of cubical cells from which the lens is 

 originally derived. 



The Suspensory Ligament, or zonule of Zitm, is derived from the 

 hyaloid membrane, which encloses the vitreous humour (see Fig. 337). 

 The canal of Petit is a space enclosed in this ligament in which the 



