Angioarchitecture of rabbit iris.

We made a scanning electron-microscopic study of the angioarchitecture of the rabbit iris using vascular resin casts, and compared the vascular structure in miosis to that in mydriasis. There were three vascular layers in the iris: the anterior capillary layer, arteriolo-venular layer and posterior capillary layer. The anterior capillary layer was a network which covered the anterior surface of the iris. The posterior capillary layer was a peculiar network composed of many capillary folds, which were arranged radially. The arteriolo-venular layer was sandwiched between the two capillary layers. In this layer, arterioles and venules ran radially toward the pupil. The peripupillary region lacked the posterior capillary layer. In miosis, the vessels of the peripheral iris were straightened radially, while those in the peripupillary region were folded. In mydriasis, the vessels were very tortuous in the peripheral region, while those in the peripupillary region were stretched laterally. The change in the angioarchitecture of the iris was suited to pupillomotoric activity.