A practical treatise on the diseases of the eye / by William Mackenzie ; to which is prefixed an anatomical introduction explanatory of a horizontal section of the human eyeball by Thomas Wharton Jones.

Date:
1840
Catalogue details

Licence: Public Domain Mark

Credit: A practical treatise on the diseases of the eye / by William Mackenzie ; to which is prefixed an anatomical introduction explanatory of a horizontal section of the human eyeball by Thomas Wharton Jones. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    :if the resistance of its omi weight being thus in a great measure remov- ■jd. The notion of the structure of the iris being erectile is disproved by iJl the phenomena attending its motions. It is certain that when there is idilatatiou of the pupil, the iris is as much in an active state, as it is w hen ixhere is contraction of the pupil. In the former case, the larger ring of tihe iris is contracted in the direction of its radius, and in the latter case iche smaller ring is circularly contracted. It is also certain that the state Dt relaxation of the iris is that in which the pupil is neither much con- rtracted nor much dilated, a state in which the pupU always is some time ! liafter death, and to which, in consequence of an elasticity which the tissue tof the iris possesses, it has a constant tendency to return after the con- rtracting or dilating force has ceased to act.® § XX. There are two arteries peculiarly devoted to the iris, called the iridal or long ciliaries; but it also derives blood from the anterior ciliaries .'(§ xi). It receives very few twigs from the branches of the short or posterior ciliaries ramifying in the ciliary processes (§ xvii). The long ciliary • arteries perforate the sclerotica a little farther from the optic nerve than the posterior ciliaries do. Accompanied by their corresponding veins and nerves, they run on the external surface of the choroid, towards the iris, in the direction of the equator of the eyeball, sometimes, however, a little '.higher, sometimes a little lower, the one on the nasal, and the other on the temporal side. At about a quarter of an inch from the iris, they divide [•at an acute angle, each into two branches, an upper and a lower. The ■ two upper branches inosculate above, and the two lower below, in the ' substance of the annulus albidus. There they send off branches, which, together with the anterior ciliary arteries, form a ring at the ciliary edge of the iris. From this, branches radiate in the substance of the iris to its pupillary edge, near which another vascular circle is formed. From the sinus circidaris iridis (§ vi), and principally in the annulus albidus, the anterior ciliary and the long ciliary veins arise. The former make their way through the sclerotica immediately; the latter accompany their cor- responding arteries. The iris is supplied by the ciliary nerves, Avhich are derived from the ophthalmic ganglion, and nasal branch of the first divi- - sion of the fifth nerve. They perforate the sclerotica at the back part of the eye, lie flat on the outer surface of the choroid (§ xiv), and, in the substance of the annulus albidus (§ xv), divide into branches, which pass into the iris. § xxi. By the dilatation and contraction of the pupil, the quantity of light admitted into the eye is regulated, and hence the iris may be consi- dered in some sense as a photometer. It is also to the eye what a dia- ] phragra is to a telescope, serving to correct the aberration of sphericity. § xxii. Pigmentum nigi'um and its membrane. The internal sui-face of the choroid and corpus ciliare, and the posterior sm-face of the iris, are covered with a layer of dark-bi’own mattei*, commonly called pignientuvi