Microvasculature of the California sea lion (Zalophus californianus) eye and its functional significance.

Objective To examine the ocular circulation in California sea lions (Zalophus californianus). Animals studied Eyes were obtained postmortem from three sea lions that died while in captivity. Procedures Specimens from sea lions were investigated using scanning electron microscopy (SEM) of vascular corrosion casts. The thermal characteristics of live animal eyes were measured using an infrared imaging system. Results The major orbital artery of the sea lion was the ophthalmic artery. The artery was remarkably thick in diameter, showed a marked convolution and formed an ophthalmic rete around the optic nerve at the posterior pole of the eyeball. The long posterior ciliary artery terminates to form a prominent inner arterial circle at the pupillary margin. The iridial arteries originated from the arterial circle showing either a crimped or somewhat coiled course, extending toward the root of the iris and formed a root supplying a large amount of blood to the iris and ciliary bodies. The venules in the conjunctiva formed a well-developed venous plexus. The vortex veins showed a dilation and constriction at the site passing through the sclera. Thermographic examination revealed that the eye showed a higher degree of thermal emission than adjacent skin areas. Conclusions These characteristics suggest that the ocular vasculature might play roles in thermoregulation as well as in hemodynamics by draining a large amount of blood so that the appropriate operating temperature for the eye can be maintained in a deep and cold aquatic environment.