Scleral spur and ciliary muscle in man and monkey

The development of the human scleral spur and the role of the scleral spur in human and monkey eyes were studied by measuring the height of the scleral spur, the thickness of the uveal meshwork and the thickness of the longitudinal ciliary muscle. In an attempt to study how the tension of the ciliary muscle is transmitted to the inner wall of the Schlemm's canal, interconnections within the trabecular meshwork were also studied. The results indicate that after 40 weeks of gestation there is little or no further change with aging in the height of the scleral spur but there is a change in shape. The scleral spur is much less developed in monkey eyes than in human eyes. The corneoscleral meshwork predominates over the uveal meshwork in most human eyes while in monkey eyes the situation is the opposite. In man the thickness of the meridional part of the ciliary muscle attached to the scleral spur varies greatly in eyes with a well developed scleral spur. Interconnecting trabecular beams composed of elastic-like fiber were observed in the uveal and corneoscleral meshwork, as well as in the juxtacanalicular meshwork, extending to the cells of the inner wall of the Schlemm's canal. These findings and the dense structure of the scleral spur suggest that in monkey eyes, and at least in some human eyes, contraction of the ciliary muscle causes unfolding of the trabecular meshwork, not so much through the movement of the scleral spur as by movement of the interconnecting trabecular beams and fibers. One important role of the scleral spur is probably to keep the corneoscleral meshwork open when the ciliary muscle relaxes, and another is to enable inward-forward movement of the circular part of the ciliary muscle by pulling the posterior tip of the ciliary muscle anteriorly when the ciliary muscle contracts. A rigid scleral spur is an advantage for these two functions.