Note 45 — Chapter XI · The Special Senses.

This change in the convexity of the lens is only a slight one, as the difference in the focal point between rays from an object twenty feet distant and one four inches distant is only one-tenth of an inch. While this muscular action is taking place, the pupil contracts and the eyeballs converge by the action of the internal rectus muscles. These three acts are due to the third nerve (the motor oculi). This is necessary in order that each part should he imprinted on the same portion of the retina, otherwise there would be double vision.