more or less dilated. Now, the pupil is enlarged or diminished, by the 

 contraction or expansion of the iris. The motions of this membrane 

 depend entirely on the manner in which light affects the retina. The iris 

 itself is insensible to the impressions of the rays of light, as Fontana has 

 proved, who always found it immovable, when he directed on it alone the 

 tlje luminous rays. When the retina is disagreeably affected by the lus- 

 tre of too strong a light, the pupil contracts, to give passage only to a 

 smaller number of rays: it dilates, on the contrary, in gloom, to admit 

 enough to make the requisite impression on the retina. 



To explain the motions of the iris, it is not necessary to admit that 

 muscular fibres enter into its structure*; it is enough to know its vascu- 

 lar, spongy, and nervous texture; the irritation of the retina sympatheti- 

 cally transmitted to the iris, determines a more copious afflux of humours; 

 its tissue dilates and stretches, the circumference of the pupil is pushed 

 towards the axis of this opening, which becomes contracted by this vital 

 expansion of the membranous tissue. When the irritating cause ceases 

 to act, by our parsing from light into darkness, the humours flow back 

 into the neighbouring vessels, the membrane of the iris returns upon itself, 

 and the pupil enlarges the more, as the darkness is greater. ' 



The rays, admitted by the pupil, pass through the aqueous humour of 

 the posterior chamber, and soon meets the crystalline, which powerfully 

 refracts them, both from its density and its lenticular form. Brought 

 towards the perpendicular by this body, they pass on towards the retina 



* Some anatomists contend, and we think justly, that two sets of muscular fibres enter 

 into the structure of the iris, the one radiating 1 , and the other orbicular. Amongst those 

 who have lately argued in support of this position, we may mention M MAU^OIB, of 

 Geneva. It appears to us, that those who deny muscularity to this part, do so in conse- 

 quence of a mistaken idea, which seems to be too generally adopted, viz. that no part 

 is really muscular, but that which possesses fibres of a similar appearance to those which 

 perform the function of voluntary motion. It shouldjrshowever, be kept in recollection 

 that involuntary muscles those fibrous textures who'll receive only nerves proceeding, 

 from the ganglia, which are not supplied with voluntary nerves, and which are, conse- 

 quently, not directly influenced by the will differ very essentially from voluntary 

 muscles in their structure. Indeed, muscular textures vary not only in their functions, 

 but even in their external characters, according as they are more or less plentifully sup- 

 plied with either class of nerves the cerebral or ganglial. The structure also of mus- 

 cular parts, especially those which are removed from the influence of volition, has some 

 relation to the kinds of irritants by which they are designed by nature to be influenced. 

 Thus, th-e eye being formed with an intimate relation to the functions which it has to 

 perform, and to the external influences which act on it, possesses, in the structure of 

 its iris, a muscular texture of peculiar delicacy, and hence it is more sensible to the 

 irritations which accompany, and are subservient to, the right performance of this im- 

 portant animal function. 



If the structure of the iris, the number of the soft and delicate nervous fibrils wliich 

 proceed to it from the lenticular ganglion, and the connexion which they form with the 

 retina in their course, be kept in view, we shall readily be able to comprehend the 

 procession of phenomena which lead to the motions of the iris. It is not improbable 

 that the nervous fibi-ils proceeding from the ganglia to the iris, form, with the capillary 

 arteries wliich supply its cellular texture, that particular organization which may be 

 considered as muscular; or, in other words, that these ganglia! nerves terminating 

 conjunctly with capillary ramifications in the delicate cellular substance of the iris 

 constitute, by such a disposition, its particular structure, and enable it to perform its 

 peculiar functions. Hence it will be seen that impressions made upon the retina, (in the 

 sensible state of that nervous expansion, and in proportion to the extent of its sensibility) 

 and transmitted to the iris, by means of the connexion which exists between the retina 

 and the nerves supplying the iris, cause a contraction of its circular fibres: as soon as 

 such impressions cease, these fibres relax, and the comparative action, in the circular and 



2 H 



