So fascinating is everything connected with the eye that the temptation to describe it in detail is great; but in a book of rough outlines, and in consideration of the many important matters yet awaiting their turn, we must confine ourselves to briefly mentioning a few of the more important points concerning it. The light is focussed by the lens upon the nervous curtain at the back, and produces there a picture, as in the photographic camera. Thus we perceive the shape of objects. The different rays of the spectrum affect different elements in this curtain or retina, whereby we get sensations of colour. Finally, the clearness of the picture, its size, the degree of convergence of the two eyes, and the effort of focussing—for the curvature of the surface of the lens can be altered—enable us to estimate the size and distance of an object. And now, though it would take volumes to do justice to the physiology of vision, we must pass on to deal equally briefly with the functions of that no less important organ, the ear. The essential part of the ear is a membranous bag, formed by the pouching in of the outer layer of cells—as shown in Figs. 1, 2, and 3 of Diagram 57—which comes to lie in a bony chamber beneath the skull, and assumes the somewhat complicated shape depicted in Fig. 4. We have not time, nor is it for our purpose necessary, to trace all the steps in the development of the ear, either external or internal, nor need we spend much time upon its structure, beyond indicating its position. But its position, which is shown in Diagram 58, must be grasped in order to understand how it is influenced by sound. [Illustration: DIAGRAM 57.—SHOWING DEVELOPMENT OF THE MEMBRANOUS LABYRINTH OF THE EAR. U, Utricle; C, cochlea; S, saccule; S.C., semicircular canals.] It will be seen that the membranous bag, which is fitly termed the labyrinth, is situated in a bony cavity which fits so closely as to be termed the bony labyrinth (C). The membranous labyrinth is filled with a liquid, called endolymph, and the bony labyrinth (C) is also filled with a liquid, called perilymph, in which the membranous bag swims. All this is called the inner ear. The inner ear communicates with a second cavity—the middle ear (B)—by two apertures in the bony wall, which are closed by membranes. The middle ear is full, not of liquid, but of air, and is separated from the external ear, the cavity marked A, which is open to the external world, by another membrane called the tympanum, or drum, of the ear. The middle ear is connected by a tube with the throat, so that the pressure of the air on both sides of the drum may be the same. [Illustration: DIAGRAM 58.—SHOWING THE POSITION OF THE EAR. A., Outer ear; B., middle ear; C., inner ear.]