Taste and smell are senses which afford us no information with regard to time or space. They give rise to massive sensations. Such sensations, devoid of detail, produce a frame of mind rather than thought. The smell of tobacco does not distract attention. On the contrary, the steady flow of impulses to which it gives rise helps to inhibit, to subdue, the yapping of more exigent sensations. And since sensations of smell have no features of their own, they form a background to sensations of other kinds, entering with them into memory. No two scenes are exactly alike. One cannot recall another. But the scent of syringa is always the same. Wherever smelled, it opens the pathways in the brain in which were first associated a June evening and syringa, with a scene and a situation upon which memory loves to dwell.
CHAPTER XIII
VISION
The eye is enclosed in a globe of fibrous tissue, of which the front part, or cornea, being transparent, admits light. The epithelial layer which covers the cornea, conjunctiva, is also transparent. No bloodvessels enter these colourless tissues, unless as the result of inflammation due to infection or to exposure to sunshine or dust. For nutrition they are dependent upon the plasma which, exuding from, and returning to, the vessels which surround them, circulates in their tissue-spaces. In advancing years, when the circulation is less brisk, a ring of opaque tissue, arcus senilis, encroaches on the cornea. In the interior of the globe, just behind the cornea, is a projecting shelf, formed of a ring of tissue supported by buttresses, ciliary processes. It is continued inwards as the iris, a muscular curtain. The “hyaloid membrane” lines the back portion of the globe. Continued on the inner side of the ciliary processes, it splits into several layers, which pass, one in front of the lens, others to its edge, to which they are attached, and still another, very thin, behind it. Since it holds the lens in place, the anterior portion of the hyaloid membrane is known as its “suspensory ligament.” Thus the eyeball is divided into three chambers. The anterior is filled with watery lymph, aqueous humour. In it, resting on the anterior surface of the suspensory ligament of the lens, is the iris. The middle chamber contains the lens. The posterior chamber is filled with a liquid jelly, vitreous humour.
By the contraction of the circular fibres of the iris, the aperture of the pupil is diminished, limiting the light which enters the globe. This adjustment occurs when the illumination is bright. It is also brought into action for the purpose of cutting out divergent rays, which would not be clearly focussed when objects near at hand are looked at. The posterior surface of the iris and the inner surfaces of the ciliary processes are covered with dense black pigment. It is this pigment, showing through the uncoloured connective tissue and plain muscle-fibres of which the iris is composed, that gives their colour to grey and blue eyes. In many eyes the iris contains a brown pigment in its substance.
Fig. 27.— Horizontal Section through the Right Eye.
The slight depression in the retina in the axis of the globe is the fovea centralis, or yellow spot; the optic nerve pierces the ball to its inner or nasal side. The lens, with its suspensory ligament, separates the aqueous from the vitreous humour. On the front of the lens rests the iris, covered on its posterior surface with black pigment. On either side of the lens is seen a ciliary process, with the circular fibres of the ciliary muscle cut transversely, and its radiating fibres disposed as a fan.