Wherever impressions _first_ act on the body, nature has placed a most vigilant guard. This is variously managed in different cases; the result is the same, and, as it would appear, the final cause also. In the eye, there is the most beautiful contrivance for moderating the ingress of light, as also any abrupt increase of intensity. Fringed curtains are provided which can close with electrical celerity. Again, the aperture by which light is finally admitted into the eye is vividly contractile or expansive, as the occasion may require; then again there are various media of different densities, through the influence of which even the velocity of light undergoes practical retardation by repeated refractions; and lastly, there are powers of sensual adaptation in the nerve with which the light is ultimately brought in contact, more wonderful than all.
The ear, being likewise a portal for external impressions, is guarded with equal care. Not a single vibration of air can ever reach the nerve of the ear with the crude intensity (if I may use the expression) with which it is generated. Passing over preliminary apparatus, by which the vibrations of air are first collected, the impressions of sound are first received on the parchment of a little drum, which parchment can be relaxed or tightened with the quickness of thought, so as to modify the force of the impression. This impression is then, by means of a little chain of bones, conveyed across the drum, which is filled with air. It then reaches a portion of the ear in which are found very curious cavities and canals, of various forms, and taking different directions, and which, from the curious and complex arrangement of the whole, is not inappropriately called the labyrinth. This is the mysterious seat of those nerves which convey impressions to the brain. There is, however, here, an arrangement more exquisite than any we have yet mentioned.
In these cavities and canals, which are themselves so small as to be not unfit objects for magnifying glasses, there are corresponding delicate sacs and tubes, and these are filled with a limpid fluid. On this delicate apparatus, so exquisitely calculated to modify any undue force of impression, the sensitive extremities of the auditory nerves are spread out, which convey impressions to the brain. We see, therefore, how carefully these portals of the body are guarded; arrangements equally conservative prevail throughout. We might show a similarly exquisite arrangement in the laws governing the mind; but that is not our present object. We have seen hitherto that, beautiful as the arrangement is for securing us against painful impressions, it has been in a great degree mechanical.