The Eye, a Treatise on the Art of Preserving This Organ and of Improving the Sight; To Which Is Prefixed, a View of the Anatomy and Physiology of the

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated.1839 Excerpt: ... understand the nature of that contact and intercourse as we are in a condition to investigate the nature of the soul itself. To return after this digression to the sense of sight, let us represent to ourselves an individual with his eyes closed, in whose mind the desire of seeing any external object has arisen. In this case the faculty of the mind which is first called into action is the will; this excites muscular motion, which is followed by a series of functions and actions in rapid succession. At the moment of volition the eyelids first open through the influence of the nerves upon the muscles, the eye moves and directs itself towards the object which the mind wishes to view, and several changes are occasioned in the interior of the eyeball by the stimulus of light. The external world has now become visible to the individual, and he is therefore placed in a condition, as far as regards the objects themselves, to occupy himself with the things around him, and to act upon them according to his subsequent will. Thus the eyelids may be compared to a pair of curtains hung between the mind and the external world, which they alternately unfold to, or veil from the view. But they serve likewise as a defence against external injuries, whose influence they keep off during sleep, as also while we are awake, by closing quickly, for instance, on the approach of any small insect. As these sources of injury, such as particles of dust, &c., enter the eye for the most part from above, the upper lid is the larger, and has moreover a wider compass of motion. The opening and closing of the eyelids, together with the motions of the eyeball, exercise a stimulus upon the lacrymal gland, which augments the secretion of its fluid in the neighbourhood of the outer canthus of the...