Passage of Tear Duct Fluids through the Nasal Cavity of Frogs

Abstract

The tear duct or nasolacrimal duct is present in most tetrapods, yet its function remains poorly understood. The duct extends from the orbit to the nasal cavity. Traditionally its main function is assumed to lie at its proximal end: It is usually described as carrying away from the eye (e.g., Romer and Parsons, 1986; Walker and Liem, 1994; Kent and Miller, 1997). However, this explanation fails to address why there should be tear fluids in the first place or, more significantly, why these surplus fluids should be delivered specifically to the nasal cavity. The fluids conveyed by the nasolacrimal duct are, in large part, produced by the glandular tissues of the orbit and typically enter the nasolacrimal duct only after passing over the cornea; a primary function of these fluids is to serve in orbital lubrication. However, a growing body of literature suggests that the secretions of a specific orbital gland, the Harderian gland, may have significant functions at the distal end of the nasolacrimal duct as well (e.g., Olcese and Wesche, 1989; Thiessen, 1992; Payne, 1994; Buzzell, 1996). In lepidosaurs, and in caecilian and urodele amphibians, the nasolacrimal duct opens rostrally in or near the entrance of the vomeronasal organ (VNO; Hoppe, 1934; Pratt, 1948; Bellairs and Boyd, 1950; Jurgens, 1971; Billo and Wake, 1987), and its fluids have been observed within the VNO in snakes (Rehorek et al., 2000b,c) and caecilians (Schmidt and Wake, 1990). A functional role for the Harderian gland secretions in the vomeronasal system is suspected for these taxa (Bellairs and Boyd, 1950; Bellairs, 1970; Jurgens, 1971; Heller, 1982; Wake, 1985; Saint-Girons, 1989; Schmidt and Wake, 1990; Halpem, 1992; Rehorek et al., 2000ac). In these cases, the nasolacrimal duct serves as a conduit to deliver the Harderian gland secretions to their putative functional targets. The extraocular functions, if any, of the nasolacrimal fluids of frogs are unknown. The Harderian gland, the only orbital gland in anurans, has received only limited attention (Di Matteo et al., 1989; Minucci et al., 1989, 1990; Chieffi et al., 1992). No empirical data are available about its functions. Neither is it clear what happens to the fluids once they are released from the distal end of the nasolacrimal duct. The morphology of the anuran nasal cavity is comparatively