Experiment 149. The retina is not sensitive where the optic nerve enters the eyeball. This is called the “blind spot.” Put two ink-bottles about two feet apart, on a table covered with white paper. Close the left eye, and fix the right steadily on the left-hand inkstand, gradually varying the distance from the eye to the ink-bottle. At a certain distance the right-hand bottle will disappear; but nearer or farther than that, it will be plainly seen. The vitreous humor fills about four-fifths of the eyeball and prevents it from falling into a shapeless mass. It also serves to hold the choroid and the retina in position, and to maintain the proper relations of the inner structures of the eye. The iris consists of a framework of connective tissue, the surface of which is lined by cells containing pigment, which gives color to the eye. Bundles of involuntary muscular fibers are found in the substance of the iris. Some are arranged in a ring round the margin of the pupil; others radiate from it like the spokes of a wheel. When the circular fibers contract, the pupil is made smaller, but if these fibers relax, the radiating fibers cause the pupil to dilate more or less widely. 329. The Crystalline Lens. Just behind the pupil and close to the iris is a semi-solid, double-convex body, called the crystalline lens. It is shaped like a magnifying glass, convex Illustration: Fig. 129.—Diagram showing the Change in the Lens during Accommodation. On the right the lens is arranged for distant vision, the ciliary muscle is relaxed and the ligament D is tense, so flattening by its compression the front of the lens C; on the left the muscle A is acting, and this relaxes the ligament and allows the lens B to become more convex, and so fitted for the vision of near objects] on each side, but with the posterior surface more convex than the anterior. In health it is perfectly clear and transparent, and highly elastic. When the lens becomes opaque, from change in old age, or from ulcers or wounds, we have the disease known as _cataract_. The lens is not placed loosely in the eyeball, but is enclosed in a transparent and elastic capsule suspended throughout its circumference by a ligament called the suspensory ligament. This ligament not only retains the lens in place, but is capable of altering its shape. In ordinary conditions of the eye, this ligament is kept tense so that the front part of the lens is flattened somewhat by the pressure on it. All around the edge, where the cornea, sclerotic, and choroid meet, is a ring of involuntary muscular fibers, forming the ciliary muscle. When these fibers contract, they draw forwards the attachment of the suspensory ligament of the lens, the pressure of which on the lens is consequently diminished. The elasticity of the lens causes it at once to bulge forwards, and it becomes more convex.