revealed, with the utmost fidelity, in the expression of his eye, and it has been rightly called "the window of the soul."

82. Many of the invertebrate animals have the eye constructed upon the same plan as that of the vertebrate animals, but with this essential difference, that the optic nerve which forms the retina is not derived from a nervous centre, analogous to the brain, but arises from one of the ganglions. Thus, the eye of the cuttle-fish contains all the essential parts of the eye of the superior animals, and, what is no less important, they are only two in number, placed upon the sides of the head.

83. The snail and kindred animals have, in like manner, only two eyes, mounted on the tip of a long stalk, (the tentacle,) or situated at its base, or on a short pedestal by its side. Their structure is less perfect than in the cuttlefish, but still there is a crystalline lens, and more or less distinct traces of the vitreous body. Some bivalve mollusks, the scollops for example, have likewise a crystalline lens, but instead of two eyes, they are furnished with numerous eye-spots, which are arranged like a border around the lower margin of the animal.

Fig. 15.

84. In spiders, the eyes are likewise simple, and usually

the corner, the crystalline lens, the vitreous body, are found in

hem, and even the choroid, which presents itself in the form of a black ring around the crystalline lens. Many in sects, in their caterpillar state, also have simple eyes.

85. Rudiments of eyes have been observed in very many of the worms. They generally appear as small black spots on the head; such as are seen on the head of the Leech, the Planaria and the Nereis. In these latter animals there are four spots. According to Müller, they are small bodies, rounded behind, and flattened in front, composed of a black, cup-shaped membrane, containing a small white, opaque body, which seems to be a continuation of the optic nerve. It cannot be doubted, therefore, that these are eyes; but as they lack the optical apparatus which produces images, we must suppose that they can only receive a general impression of light, without the power of discerning objects.

86. Eye-spots, very similar to those of the Nereis, are found at the extremity of the rays of some of the star-fishes, in the sea-urchins, at the margin of many Medusæ, and in some Polypi. Ehrenberg has shown that similar spots also exist in a large number of the Infusoria.

*

Fig. 17.

87. In all the above-mentioned animals, the eyes, what ever their number, are apart from each other. But there is still another type of simple eyes, known as aggregate eyes. In some of the millipedes, the pill-bugs, for instance, the eyes are collected into groups, like those of spiders; each eye inclosing a crystalline lens and a vitreous body, surrounded by a retina and chorov. Such eyes consequently form a

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 facettes 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 per-u fectly 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 nu

merous.

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 facettes 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 a very 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 spelaus,) 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.

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