Lectures on Comparative Anatomy Volume N . 3; In Which Are Explained the Preparations in the Hunterian Collection

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1828 Excerpt: ...these projecting lateral portions cross the pupil and fold over one another, but do not adhere together; although this is the usual state of the iris, it admits of variation, for in some specimens these lateral projections had receded sufficiently to render the pupil of an oval form and single; it is therefore probable that the iris can so enlarge the pupil as to make it of considerable size, forming a pyramidal oval, large above and small below, w'hile at other times it covers the middle space, and leaves only two apertures for light to come to the crystalline lens, the uppermost the largest. That vision can only be carried on while the pupil is thus made double, there can be no doubt, since the crystalline lens instead of being spherical is not even circular, having a small projection at the lower edge, directly behind the smaller aperture in the iris, and therefore must be adapted to make_the vision through the smaller pupil distinct. When the lens is examined in the microscope inclosed in its capsule, there is distinctly seen a small bundle of muscular fibres, coming from the capsules of the vitreous humour at the lower part, and inserted into the capsule of the crystalline lens, just at the disk where the small curve joins the large one, the action of which will necessarily be, to bring the lower mammary process of the lens downwards and backwards, into the centre of the lower aperture in the iris, thus constituting a complete organ for the purpose of distinct vision at near distances, and quite independent of that part of the lens opposed to the larger aperture, which is only calculated to see more distant objects, and from the convexity of the cornea, through a rarer medium. When we consider the breadth of the body of the fish, compared with its thic...