Page:Proceedings of the Royal Society of London Vol 4.djvu/465

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The following papers were read, viz. —

1. " Magnetical Term- Observations made at the Observatory at Prague, for )September, October, November and December, 1842 :" by Professor Kreil. Communicated by S. Hunter Christie, Esq., Sec. R.S.

2. " On the Structure and Mode of Action of the Iris :" by C. R. Hall, Esq. Communicated by P. M. Roget, M.D., Sec. R.S.

After reciting the various discordant opinions entertained at dif- ferent periods by anatomists and physiologists, relative to the struc- ture and actions of the iris, the author proceeds to give an account of his microscopical examination of the texture of this part of the eye, in different animals. He considers the radiated plicae, which are seen on the uvea in Mammalia, as not being muscular ; but he agrees with Dr. Jacob in regarding them as being analogous in structure to the ciliary processes. The white lines and elevations apparent on the anterior surface of the human iris, he supposes to be formed by the ciliary nerves which interlace with one another in the form of a plexus. The iris, he states, is composed of two portions ; the first, consisting of a highly vascular tissue, connected by vessels with the choroid, ciliary processes, sclerotica and cornea, and abun- dantly supplied with nerves, which, in the human iris, appear, in a front view, as thread-like striae ; and which are invested, on both surfaces, by the membrane of the aqueous humour. They are more or less thickly covered with pigment, which, by its varying colour, imparts to the iris on the anterior surface its characteristic hue ; and, by its darkness on the posterior surface, renders an otherwise semi- transparent structure perfectly opake. The second component por- tion of the iris consists of a layer of concentric muscular fibres, which fibres, in Man and Mammalia generally, are situated on the posterior surface of the pupillary portion of the iris ; but which in Birds ex- tend much nearer to the ciliary margin, and consequently form a much broader layer. In Fishes and in some Reptiles they do not exist at all.

The author then proceeds to inquire into the bearings which these conclusions may have on the physiology of the iris. He thinks that the phenomena of its motions can receive no satisfactory expla- nation on the hypothesis of erectility alone, or on that of the anta- gonism of two sets of muscular fibres ; the one for dilating, the other for contracting the pupil. He is convinced that the contrac- tion of the pupil is the eff'ect of muscular action ; but does not con- sider the knowledge we at present possess is sufficient to enable us to determine the nature of the agent by which its dilatation is efifected. He, however, throws it out as a conjecture, that this latter action may be the result of an unusual degree of vital contractility, residing