424 JACOB, ON THE MUSCULAR STRUCTURE OF THE IRIS. 



induced by irritation : and by the contraction of the rest of the 

 membrane in consequence of its elasticity ; which always 

 exhibits'its effects when the muscular action ceases. 



The elastic power In this case may have some analogy to 

 the elasticity of the arteries, which certainly produces some of 

 the contraction of those vessels; but there is another principle 

 of contraction superadded to ihis, which produces effects differ- 

 ent from those of mere elasticity.* 



It is a question not yet wholly decided, whether the con- 

 traction and dilatation of the pupil is dependent upon an invo- 

 luntary sphincter muscle, placed at the pupillary margin, and a 

 series of longitudinal or radiated fibres, running from the 

 greater to the lesser circumference of the iris, as asserted by 

 Monro and more recently still by Professor Maunoir and 

 others ; or whether it is simply an erectile tissue, dependent 

 in its contraction upon a rush of blood, under the influence of 

 some stimulation, into its serpentine vessels. Anatomists of 

 nearly equal reputation, have advocated these opposite theo- 

 ries. Small doses of belladonna, stramonium, or opium, dilate 

 the pupil ; larger and poisonous doses, contract it. The 

 motions of the iris, contracting to diminish the quantity of 

 light admitted, when the rays excite too intense an impression 

 on the retina, and dilating to receive more when they are 

 feeble, appear, in many respects, like muscular action, and have 

 been compared to the heart, dilating to receive blood, and con- 

 tracting to expel it. We find the pupil after death in a 

 medium state between dilatation and contraction : if its move- 

 ments were dependent upon muscular action, this is what we 

 should expect necessarily to be the result of the relaxation of 

 the opposing sets of fibres. Professor Jocob, of Dublin, f 

 describes the circular muscle as very visible, in some quadru- 

 peds after the pigment has been removed, on the back surface 

 of the iris, near the pupillary margin, and that it is about the 

 twentieth part of an inch in diameter. The longitudinal or 



* Some persons, like the late Dr. Wollaston, exercise a voluntary control 

 over the movements of the iris, dilating or contracting it at will. P. 

 f Medico-Chirurgical Transactions. 



