Steps of the process of hearing given in their order.

the process of hearing, as it must occur in the case of every sound that produces that sensation. The vibrating air enters the tube of the ear, and, reaching the drum, produces a vibration there. This vibration is communicated to the chain of bones, which, as Dr. Paley very aptly says, like a repeating line of frigates pass it on. It is transmitted from the last of this chain of bones, the stirrup-bone, to the membrane covering the fenestra ovalis, and from this to the fluid contained in all the passages of the labyrinth. The vibration goes through all the semi-circular canals in one direction, and in another up one gallery of the cochlea, and down the other. In all these cavities, are spread out in various ways, the filaments of the nerve which receive the impression of the vibration. This impression is transmitted from the extremities of the nerve, through its trunk, to the brain, where the mind receives it. All this together constitutes hearing; and all of it occurs in the case of any sound which we hear, however closely it may follow any other sound.

426. Most of our hearing is done precisely in the way described, but not all. We sometimes hear directly through the bone surrounding the labyrinth. If you place a watch between the teeth, you hear the ticking; and it gives a very different sound from what it does when held to the ear, because the sonorous vibration is transmitted directly through the solid bones of the skull from the teeth. In the same way was the sound transmitted in the case of the deaf old gentleman, (§ 409) who heard his daughter's music through the stem of his pipe, as he rested the bowl of it on the piano. The fact thus illustrated is often made use of by physicians, in detecting the nature of the difficulty in cases of deafness. Thus, if a watch held between the teeth communicate a very distinct and loud sound to the ear, we infer that the internal ear is in a good condition, and that the difficulty is in some of the other parts connected with it, the drum, or the cavity of the tympanum, or the Eustachian tube..

427. I have described the apparatus of hearing as we find it in man. But it varies in different animals, according to the circumstances in which they are placed, and their necessities. Animals that live in water of course have a different apparatus of hearing from those that live in air. In most fishes the semi-circular canals exist, but there is nothing like a cochlea. As sounds are transmitted so easily through water, (§ 410,) fishes have no need of so complicated and perfect an apparatus

Hearing in other animals. Only a part of the process of hearing understood.

as animals that live in air. They are fitted to hear in their own element, and probably the moment that a fish is taken out of the water he becomes quite deaf, because his hearing apparatus is so poorly fitted to receive and transmit vibrations from the air. But in many animals that live in air the ear differs from that of man in its arrangements. The cochlea in birds is nearly straight instead of being spiral. Such facts lead to the inference, that the peculiar arrangements in the hearing apparatus of man have regard, not merely to the medium in which he is placed, but to peculiar uses which are necessary in his case, as the determination of the direction of sound, the appreciation of its pitch and its character, the power of hearing very slight sounds, &c. The simplest form of apparatus found in animals, is a cavity excavated in bone, with a fluid shut in it by a membrane, and nervous filaments distributed so as to be impressed by the vibrations of the fluid. And this is all that is absolutely essential to hearing.

428. Many speculations have been broached in regard to the special offices of particular parts of the labyrinth. Thus, it has been supposed that the semi-circular canals have an agency in informing us of the direction of sounds; for it is observed that they are always arranged in the same relative angle to each other. It has been supposed also, that the cochlea gives us the idea of the note of sounds, because it is noticed that the development of this part in different animals is in proportion to the variety of note which they produce. These suppositions, though quite probable, require farther investigation in comparative anatomy to test their truth.

429. In the process that makes up the sensation of hearing, there is one part which we can in some measure understand, and to which we can apply the known principles which govern the transmission of sonorous vibrations. But there is another part, that which links the process to the immaterial mind, that we cannot understand. We can trace the vibration received from the air through the several parts to the fluid in the labyrinth, but here we come to a stand in our knowledge. The vibration stops here, and what is transmitted through the nerve to the mind we know not. We call it an impression; but this is only an indefinite word, implying simply that something is transmitted, without defining what it is. Neither do we know how the transmission is made. All that we do know is, that the nerve is essential to the completion of the sensation of hearing, and that it spreads out its minute fibrils or tubuli in the

Ear equal to the eye in delicacy, beauty, and complication of structure. halls of audience, in order to receive impressions from the vibrations that come there, and transmit them to the brain where the mind takes cognizance of them. Every part of the apparatus may be mechanically perfect, so that the vibrations may be transmitted to the fluid which bathes the nervous fibrils, but if the nerve be paralyzed, or if the communication between its extreme fibrils and the brain be in any way interrupted, the mind knows nothing of the vibration, and there is no hearing.

430. The eye has generally been spoken of as being more wonderful than any other organ in the body, in view alike of the delicacy, the beauty, and the complication of its structure. But the apparatus of hearing presents a combination of these qualities quite as wonderful. There is nothing more delicate, and beautiful, and complicated than the arrangement of the nervous fibrils in the winding labyrinthic passages of the halls of audience. And as we trace the steps of the process of hearing, from the drum of the ear where the sound strikes, to the gray substance of the brain where the mind receives the impression, and think of each sound as sending a vibration through membranes and a chain of bones to the fluid in which the nervous fibrils are immersed, and of these fibrils as catching from every vibration of the fluid a definite impression and transmitting it to the mind, we see a mingling of the purely mechanical with the spiritual, which greatly enhances our admiration of the mechanism. Though the apparatus is complicated, the mechanical result is a simple one-it is a mere trembling of a fluid inclosed in winding cavities of bone. But simple as the result is, it is made, through the beautiful nervous connections of the ear with the brain, one of the chief inlets of knowledge to the mind, coming to it from nature's multitudinous voices, and is a constant medium of communication for thought and feeling between man and man. Thus intimately in the human body are the simplest mechanical results connected with the complicated and diversified operations of the mind. In the process of hearing the drum of the ear is to be considered one end of the apparatus, and the gray portion of the brain the other. The drum simply vibrates; and instantaneously the mind receives a distinct impression from the vesicles of the gray matter. And thus is the communication established between the immaterial mind, and the vibrations of the material substances with which it is surrounded.

Seeing a compound process. Refraction of light.

CHAPTER XVI.

THE EYE.

431. THE sensation of sight is the result of a compound be divided into two distinct parts, as I reprocess, which may marked in relation to the sensation of hearing, in § 429. The one part is purely mechanical, and the apparatus for it is constructed according to the common principles, which we find illustrated in optical instruments. The object of its arrangements is to form distinct images of objects in the back part of the eye. The other part of the process is executed by the nerve of vision, called the optic nerve. This nerve, expanded upon the membrane where the images are formed, transmits impressions from these images to the brain, just as the nerve of hearing transmits to the brain the impressions which come from the vibration of the fluid of the labyrinth.

Before proceeding to an examination of the eye as an optical instrument, I will call your attention to certain principles, which we shall find illustrated more beautifully and perfectly in the eye than in any optical instrument which man has ever constructed.

FIG. 156.

432. The rays of light coming from any luminous point go in straight lines in all directions, just as the vibrations of sound do, and, like them, become less intense the farther they are diffused. But they move in straight lines only so long as they remain in the same medium. When they pass from one medium into another they are bent out of their straight course, or refracted, as it is termed, unless they pass from one to the other in lines perpendicular to the surface of the medium which they enter. This may be illustrated by the following experiment. Place a coin, a, in the bottom of a basin, as represented in Fig. 156, and then withdraw from it so far that the coin may be hidden from your eye by the edge of the basin, as represented in the figure. Keeping your eye fixed in that position, pour some

a

d

Refraction as light passes from a rarer into a denser medium, and vice versa.

water into the basin up to the level, c. The coin will again become visible to your eye. The reason is, that the rays of light, as they come from the water into the rarer medium, the air, are refracted or bent downwards, that is from the perpendicular. The effect of this may be seen in the figure. A ray of light, coming from the coin in the direction a, d, does not pass to d, but is bent downward, and so passes to the eye at e. And so of other rays coming from the object. The coin, therefore, is seen by the eye at e, but it is not seen in its true direction from the eye which is in the line e, c, a. The only point in which the eye can see the coin in its true position is when the eye is at b, in a perpendicular line directly over it. A ray that passes from one medium to another in a line perpendicular to the surface of the medium into which it passes is not bent out of its course. All other rays are, and the more so the farther they are from the perpendicular.

433. While rays that pass from a dense medium into a rarer, as from water into air, are bent from the perpendicular, those on the other hand, which pass from a rarer medium into a denser, as from air into water, are bent towards the perpendicular. Thus if in Fig. 156 a be the position of the eye of a fish, and where the eye is, at e, there be an insect, the fish can see it, because the ray that strikes the surface of the water, c, is refracted or bent towards the perpendicular line, b, a. And so of other rays. He does not see the insect, however, in its true direction, a, c, e, but it appears to him to be at d. For we always judge of the place of an object by the direction in which the rays from it strike the eye.

434. When light passes from one medium into another which presents a convex or concave surface, instead of a flat one, a very great change is produced in the direction of its rays. Thus suppose, as represented in Fig. 157, three diverging rays coming from a point, a, through the air, enter a convex surface of glass, b, b'. The central ray a, c enters the glass in a direction perpendicular to its surface, and therefore does not bend from its course. But the ray a, d enters very obliquely, and is bent towards the perpendicular at that point, e, and passes on in the direction f. So likewise the ray, a, g, is bent towards the perpendicular h, and passes on in the line i. These rays diverging in the air have become converging in the glass, and the point at which they meet is called the focus. To this point all the other rays entering the convex glass converge also.

435. But if the surface of the glass be concave, as represented