Relationship between limbal incisions and the structures of the anterior chamber angle.

(2) The angle made by the blade with the surface of the globe, i.e., the degree of obliquity of the incision. From the same external site an incision perpendicular to the scleral surface may penetrate the ciliary body, whereas a slanting incision parallel to the iris surface would enter the anterior chamber (Swan, 1959). Much controversy still exists concerning some of the external landmarks of the limbus, perhaps because of confusion between the conjunctival and the scleral overlap over the cornea. According to Wolff (I968), the limbus is the transitional zone between the conjunctiva and the sclera on the one hand and the cornea on the other. The transparent corneal tissue ends just behind a line which joins the ends of Bowman's membrane and Descemet's membrane. Mlainly because of the overlap of the conjunctiva and the subconjunctival tissue, the former is I mm. shorter than the latter, and Descemet's membrane also approaches nearer to the angle of the anterior chamber, although still separated from it by the whole of the corneo-scleral trabecular system, i.e. by about I *5 mm. The oblique fibres and star-shaped cells characteristic of the sclera commence a little sooner anteriorly than posteriorly, but the main overlap of the cornea is by the conjunctiva and subconjunctival tissue rather than by the sclera as usually stated. According to Stallard (i 965), this scleral overlap is most extensive in the upper part of the limbus, less in the lower, and least in the horizontal meridian. Its extent shows individual variations, but accurate measurement of it is of prime importance in making an accurate incision into the apex of the filtration angle. The limits of the "surgical limbus", the apex of the filtration angle, may be judged with fair accuracy by using the well-known illumination phenomenon of "scleral scatter". To mark this site in the upper part of the globe, where most glaucoma operations are performed, a fine pencil of light is applied to the limbus at 6 o'clock. This illuminates a crescentic band about 2 mm. wide, the scleral scatter, in the corneo-scleral zone. The apex of the filtration angle is then marked on the sclera at operation after reflecting the conjunctival flap. Swan (I 959) stated that the limbus is not a sharp linear junction but a zone of transition which may be more than i mm. wide. He commented that many authors, whein referring to the limbus as a landmark, mean the junction of the limbal and corneal epithelium, i.e. the anterior border of the limbus. Swan defined the following landmarks: (i ) The anterior border of the limbus is the junction of the corneal and limbal epithelium. An incision made perpendicular to the surface of the globe at this border would be entirely