Regional differences within the external 'duct' of the rat exorbital lacrimal gland

The lacrimal cord is the fibrous structure that extends between the rat exorbital lacrimal gland and the eyelid. Disagreement exists about the number of ducts it contains and with which lid the orifice is associated. Therefore, 18 lacrimal cords from adult rats were studied by light microscopy. Additionally, in five specimens the continuations of the major ducts were observed immediately intraglandularly. Adjacent to the exorbital lacrimal gland, the lacrimal cord may contain six or more ducts. Farther toward the eye, it has five, four, and then three ducts; the latter typically at about its midpoint. Close to the infraorbital gland two or one are present. Approaching the temporal aspect of either lid, the ducts of both the infraorbital and exorbital lacrimal glands course within the lacrimal cord and usually merge. In some adult rats, when the eyelids are retracted laterally, it is evident that a crescentic fold of the palpebral conjunctiva contains a vertical pair of whitish, convex structures, the 'temporal canthal domes'. When present, one can serve as a marker for the duct orifice located close to the conjunctival fornix. Pseudostratified columnar epithelium lines the ducts from the orifice to the region of the infraorbital lacrimal gland. From the latter toward the exorbital lacrimal gland the epithelium may become pseudostratified cuboidal. That type exists close to and immediately within the exorbital gland. The variation in duct number within the lacrimal cord would have physiological consequences. By analogy with the circulatory system, it is expected that flow would occur more slowly within the multiple ducts in and near the exorbital lacrimal gland than in the one or two near the canthus. Where flow is slower, contact time of the secretion with the duct epithelium would be greater. Thus, modification of the lacrimal secretion is likely not to occur uniformly in the duct system but would take place primarily in and near the exorbital lacrimal gland rather than toward the duct orifice.