Expanding forces in aqueous outflow pathways of a nonaccommodating mammal: an approach via comparative dynamic morphology.

Six dog eyes were fixed by intracameral perfusion of fixative at pressures of 0, 5, 10, 15, 20, and 25 cm of water. Eight dog eyes were fixed after the injection in both ocular chambers of a number of cholinergic agents, either singly or in combination. Under the effect of miotics and under increased ocular pressure, the aqueous pathways expand. An analysis of the forces involved in expansion of the exit pathways reveals the primary role of the detached ciliary body in nonaccommodating mammals. Two mechanisms appear to have been conserved in dogs and humans throughout evolution. The first is an active mechanism: the opening of the trabecular meshwork as a consequence of the combined action of the ciliary muscle and the iris and its insertion ligaments-the uveoscleral trabeculate-in dogs, and the longitudinal portion of the ciliary muscle and scleral spur in humans. The second is a passive mechanism: the infundibular arrangement of the drainage structures assisted by the traction on the zonular ligament of the lens, which responds to an increase in pressure in the anterior chamber by widening the pathways, thus favoring outflow.