242 SENSE OF SIGHT. 



same exaggerated opinion would not be produced whatever were the 

 object that impressed the organ. 



The iris has been compared, more than once, to the diaphragm of a 

 lens or telescope. Its function consequently must be, to correct the 

 aberration of sphericity ', which would otherwise take place. This it 

 does by diminishing the surface of the lens on which the rays impinge, 

 so that they meet at the same focus on the retina. M. Biot has re- 

 marked, that this diaphragm is situate in the eye precisely at the place 

 where it can best fulfil the office, and yet admit the greatest possible 

 quantity of light. 



The iris is capable of contracting or dilating, so as to contract or 

 dilate the pupil. It has been already observed, that the views of anato- 

 mists regarding the muscular structure of the iris have been discrepant ; 

 and that some esteem it to be essentially vascular and nervous, the ves- 

 sels and nerves being distributed on an erectile tissue. The partisans 

 of each opinion explain the motions of the iris differently. They who 

 admit it to consist of muscular fibres affirm, that the pupil is contracted 

 by the action of the circular fibres, and dilated by that of the radiated. 

 Those, again, that deny the muscularity of the organ say, that contrac- 

 tion of the pupil is caused by the afflux of blood into the vessels, or by 

 a sort of turgescence similar to what occurs in erectile parts in general ; 

 and dilatation, by the withdrawal of the surplus fluid. 



Admitting and we think this must be conceded that the iris is 

 really muscular, we meet with the singular anomaly in its physiology 

 that no ordinary stimulus, applied directly to it, has any effect in excit- 

 ing it to contraction. It may be pricked with the point of a cataract 

 needle without the slightest motion being excited ; and, from the expe- 

 riments of Fontana 1 and Caldani, 2 it seems equally insensible when 

 luminous rays are made to impinge upon it ; yet MM. Fowler, Rinhold, 

 and Nysten 3 have proved, that it contracts like other muscular parts on 

 the application of the galvanic stimulus. Like them, too, it is under 

 the nervous influence, its movements being generally involuntary ; 

 but, there is some reason to believe, occasionally voluntary. Dr. Roget 

 asserts, that this is the case with his own eye. 4 In the parrot, and 

 certain nocturnal birds, its motions are manifestly influenced by voli- 

 tion ; 5 and when the cat is roused to attention, the pupil dilates, so as 

 to allow a greater quantity of light to reach the retina. M. Magendie 6 

 affirms, that the attention and effort required to see minute objects' dis- 

 tinctly occasion contraction of the human pupil. He selected an indi- 

 vidual whose pupil was very movable ; and placing a sheet of paper in 

 a fixed direction as regarded the eye and light he marked the state of 

 the pupil. He then directed the person to endeavour, without moving 

 the head or eyes, to read very minute characters traced on the paper. 

 The pupil immediately contracted, and continued so, as long as the 

 effort was maintained. 



' Dei Moti dell' Iride, cap. i. p. 7, Lucca, 1765. 



2 Institutiones Physiologicse, &c., Lips., 1785. 3 Magendie, Ibid., i. 75. 



4 Outlines of Physiology, Amer. edit., by the Author, p. 286, Philad., 1839. 



5 Mayo, Outlines of Physiology, 4th edit., p. 286, Lond., 1837. 

 e Precis Elementaire, 2de edit., i. 74. 



