Diagram 54.
Diagram 55.
Diagram 56.
So much for the chemical sense organs; they are comparatively simple. But between a single cell, which always makes towards or always hurries out of a ray of light passing through the water in which it swims, and an animal with eyes capable of recognising the colour, shape, size, and distance of objects in space, there really does seem to be a wide gulf. It is not, however, too wide to be bridged.
After the single-cell stage has been passed, and we have beasts consisting of an inner layer of cells which is digestive in function, and an outer layer which is protective, motor, and sensory, the power of perceiving light is doubtless vested in the outer layer. When we get beasts consisting of three layers progressing along the straight path of development which leads to man, we find the outer layer becoming too opaque for this purpose, and the torch is handed on to the sensory tube derived from it. ([See Diagram 5.]) As more and more protection is required, the skin thickens, and the neural tube comes to lie deeper, as in [Diagram 51]. In order not to lose the light altogether, it has to throw out buds, which concentrate in themselves the peculiar faculty of perceiving it, and at the same time little pits are formed in the skin just over them to help the light to reach them. ([See Diagram 52.]) In [Diagram 53] both the nervous elements and the integumentary are developing their possibilities; and in [Diagram 54] a large surface has been prepared for the reception of light, and a lens formed to focus the rays upon it. [Diagrams 55 and 56] give the concluding stages in the development of the eye: the formation of the cornea and its protecting eyelids. The two cavities are filled with clear liquids, and the whole eyeball supported by connective tissue.
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.
