natural transition to the compound eyes of insects, to which we now give our attention.

88. Compouna eyes have the same general form as simple eyes; they are placed either on the sides of the head, as in insects, or supported on pedestals, as in the crabs. But if we examine an eye of this kind by a magnifying lens, we find its surface to be composed of an infinite number of angular, usually six-sided faces. If these façettes are removed, we find beneath a corresponding number of cones, (c,) side by side, five or six times as long as they are broad, and arranged like rays around the optic nerve, from which each one receives a little filament, so as to present, according to Müller, the following disposition.

Fig. 18.

(Fig. 18.) The cones are perfectly transparent, but separated from each other by walls of pigment, in such a manner that only those rays which are parallel to the axes can reach the retina A; all those which enter obliquely are lost; so that of all the rays which proceed from the points a and b, only the central ones in each pencil will act upon the optic nerve, (d;) the others will strike against the walls of the cones. To compensate for the disadvantage of such an arrangement, and for the want of motion, the number of façettes is greatly multiplied, so that no less than 25,000 have been counted in a single eye. The image on the retina, in this case, may be compared to a mosaic, composed of a great number of small images, each of them representing a portion of the figure. The entire picture is of course, more perfect,

in proportion as the pieces are smaller and more

merous.

nu

89. Compound eyes are destitute of the optical apparatus necessary to concentrate the rays of light, and cannot adapt themselves to the distance of objects; they see at a certain distance, but cannot look at pleasure. The perfection of their sight depends on the number of façettes or cones, and the manner in which they are placed. Their field of vision is wide, when the eye is prominent; it is very limited, on the contrary, when the eye is flat. Thus the dragon-flies, on account of the great prominency of their eyes, see equally well in all directions, before, behind, or laterally; whilst the water-bugs, which have the eyes nearly on a level with the head, can see to only a very short distance before them. 90. If there be animals destitute of eyes, they are either of inferior rank, such as most of the polypi, or else they are animals which live under unusual circumstances, such as the intestinal worms. Even among the vertebrates, there are some that lack the faculty of sight, as the Myxine glutinosa, which has merely a rudimentary eye concealed under the skin, and destitute of a crystalline lens. Others, which live in darkness, have not even rudimentary eyes, as, for example, that curious fish (Amblyopsis spelœus,) which lives in the Mammoth Cave, and which appears to want even the orbital cavity. The craw-fishes, (Astacus pellucidus,) of this same cave, are also blind; having merely the pedicle for the eyes, without any traces of façettes.

a very

2. Hearing.

91. To hear, is to perceive sounds. The faculty of perceiving sounds is seated in a peculiar apparatus, the EAR, which is constructed with a view to collect and augment the sonorous vibrations of the atmosphere, and convey them to

the acoustic or auditory nerve, which arises from the poste. rior part of the brain. (Fig. 21, c.)

92. The ears never exceed two in number, and are placed, in all the vertebrates, at the hinder part of the head. In a large proportion of animals, as the dog, horse, rabbit, and most of the mammals, the external parts of the ear are generally quite conspicuous; and as they are, at the same time, quite movable, they become one of the promi nent features of physiognomy.

93. These external appendages, however, do not constitute the organ of hearing, properly speaking. The true seat of hearing is deeper, quite in the interior of the head. It is usually a very complicated apparatus, especially in the superior animals. In mammals it is composed of three parts, the external ear, the middle ear, and the internal ear; and its structure is as follows: (Fig. 19.)

94. The external ear, which is popularly regarded as the ear, consists of the conch, (a,) and the canal which leads from it the external auditory passage, (b.) The first is a

gristly expansion, in the form of a horn or a funnel, the object of which is to collect the waves of sound; for this reason, animals prick up their ears when they listen. The ear of man is remarkable for being nearly. immovable. Therefore, persons, whose hearing is deficient, employ an artificial trumpet, by which the vibrations from a much more extended surface may be collected. The external ear is peculiar to mammals, and is wanting even in some aquatic species of these, such as the seals and the Ornithorhyncus.

95. The middle ear has received the name of the tympanic cavity, (k.) It is separated from the auditory passage by a membranous partition, the tympanum or drum, (c;) though it still communicates with the open air by means ɔf a narrow canal, called the Eustachian tube, (i,) which opens at the back part of the mouth.

In the interior of the chamber are four little bones, of singular forms, which anatomists have distinguished by the names of malleus, (Fig. 20, c,) incus, (n,) stapes, (s,) and os orbiculare, (o;) which are articulated together, so as to form a continuous chain, as here represented, magnified.

Fig. 20.

96. The internal ear, which is also denominated the labyrinth, is an irregular cavity formed in the most solid part of the temporal bone, beyond the chamber of the middle ear, from which it is separated by a bony partition, which is perforated by two small holes, called, from their form, the round and the oval apertures, the fora men rotundum, (Fig. 19, g,) and the foramen ovale, (h.) The first is closed by a membrane, similar to that of the tympanum, while the latter is closed by the stapes, one of the little bones n the chamber.

97. Three parts are to be distinguished in the labyrinth, namely, the vestibule, which is the part at the entrance of the cavity; the semicircular canals, (d,) which occupy its upper part, in the form of three arched tubes; and the cochlea, which is a narrow canal placed beneath, at the lower part of the vestibule, having exactly the form of a snail-shell, (e.) The entire labyrinth is filled with a watery fluid, in which membranous sacs or pouches float. Within these sacs, the auditory nerve (ƒ) terminates. These pouches, therefore, are the actual seat of hearing, and the most essential parts of the ear. The auditory nerve is admitted to them by a long passage, the internal auditory canal.

98. By this mechanism, the vibrations of the air are first collected by the external ear, whence they are conveyed along the auditory passage, at the bottom of which is the tympanum. The tympanum, by its delicate elasticity, augments the vibrations, and transmits them to the internal ear, partly by means of the little bones in the chamber, which are disposed in such a manner that the stapes exactly fits the oval aperture, (foramen ovale ;) and partly by means of the air which strikes the membrane covering the round aperture, (g,) and produces vibrations there, corresponding to those of the tympanum. After all these modifications, the sonorous vibrations at last arrive at the labyrinth and the auditory nerve, which transmits the impression to the brain.

99. But the mechanism of hearing is not so complicated in all classes of animals, and is found to be more and more simplified as we descend the series. In birds, the middle and interior ears are constructed on the same plans as in the mammals; but the outer ear no longer exists, and the auditory passage, opening on a level with the surface of the head behind the eyes, is merely surrounded by a circle of peculiarly formed feathers. The bones of the middle ear are also 'ess n 'merous, there being generally but one.