. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. Fig. 156.—Diagram to illustrate the theory of accommodation of the human eye to changes in distance. In the first figure the eye is at rest and is said to be focused at infinity. When vision is accommodated to observe a near object the ciliary muscles contract. This lessens the tension on the suspensory ligament of the lens. Then the lens, being elastic, tends to become spherical. It will be noted that in this state the ciliary muscles are expending energy; hence prolonged close vision causes
. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. Fig. 156.—Diagram to illustrate the theory of accommodation of the human eye to changes in distance. In the first figure the eye is at rest and is said to be focused at infinity. When vision is accommodated to observe a near object the ciliary muscles contract. This lessens the tension on the suspensory ligament of the lens. Then the lens, being elastic, tends to become spherical. It will be noted that in this state the ciliary muscles are expending energy; hence prolonged close vision causes fatigue. since the crest of the ciliary body is thereby raised. The relief of tension on the ligament and lens capsule allows the elastic lens to assume a more spherical shape by taking up the slack in the liga- ment. Thus the curvature of the lens is increased. At rest the ciliary muscles are relaxed but by reason of their elasticity and that of associated tissues there is a draw exerted on the lens capsule which flattens the lens. Thus the resting eye is set for far vision, that is, focused at infinity. The mechanism of accommodation is very sen- sitive and constantly in action during waking hours. Defects in Vision. The most common defects in human vision are near-sightedness, myopia, far-sightedness, presbyopia, and astig-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Buchanan, James William, 1888-. New York, London, Harper & brothers
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