As no fewer than six of the nine nerves which arise from the brain are connected with the organ of vision, he finds it necessary to announce, not only the distinct sensibilities possessed by the organ of vision, but the motions to which it is subjected. Having shewn the connection of the voluntary motions of the eye-ball with the sensation on the retina, and the motions involuntary and for the protection of an organ so delicate and so exposed, he proceeds to shew the necessity of combinations among the muscles of the eye. As there are no direct communications between the muscles, necessarily united in action, he shews the necessity of distinct nerves, and also the necessity of their relations being established at their roots, and hence deduces the reason of the fourth and sixth nerves deviating from the others in their place and mode of origin.