440 COMPLEMENTARY COLOURS. -DIRECTION OF MOVEMENTS.

570. Upon these properties of the eye are founded the laws of harmonious colouring; a full knowledge of which should be possessed by artists of every kind who are concerned with contrasts of colour, whether in pictures, architectural decorations, or even in dress. All complementary colours have an agreeable effect when judiciously disposed in combination; and all bright colours which are not complementary have a disagreeable effect, if they are predominant: this is especially the case in regard to the simple colours (red, blue, and yellow), strong combinations of any two of which, without any colour that is complementary to either of them, are extremely offensive. Painters who are ignorant of these laws, introduce a large quantity of dull grey into their pictures, in order to diminish the glaring effects which they would otherwise produce; but this benefit is obtained by a sacrifice of the vividness and force which may be secured in combination with the richest harmony, by proper attention to physiological principles.

571. The Eye is endowed with common sensibility (§ 487) by the fifth pair of nerves; and when this is paralysed, all parts of it are completely insensible to the touch, although the power of vision may remain unimpaired. It seldom preserves its healthy condition in this state, however; for the lachrymal and mucous secretions which protect its surface, are no longer formed as they should be; and inflammation, often terminating in the destruction of the eye, is the result.

572. The visual sensations obtained through the Eye have numerous and varied purposes among the lower animals. That they chiefly serve to direct their movements, is evident from observation of these movements; and from the fact, that, when the eyes are covered or destroyed, most animals make little attempt at determinate motions, though they frequently exhibit a great deal of restlessness. There are a few Vertebrata, however, which do not possess perfectly-formed eyes, and which are consequently guided in their movements by other senses. This is the case with the Mole, which spends its whole life in burrowing beneath the ground; and also with the Proteus, and some others of the lower Amphibia, which inhabit the dark recesses of subterranean lakes and channels.

573. In the Articulated series of animals, we meet with

COMPOUND EYES OF ARTICULATA.

441

eyes of a kind entirely different from those which have been previously described. In most Insects we notice a large black or dark-brown hemispherical body, situated on either side of the head (fig. 212); and in Crabs, Lobsters, &c., we find spherical bodies of similar appearance mounted on short footstalks, which are capable of some degree of motion. When these are examined with the microscope, their surface

Fig. 212.-HEAD AND EYES OF THE BEE, SHOWING THE DIVISION INTO FACETS. a, a, antennæ; A, facets enlarged; B, the same with hairs growing between them. is seen to be divided into a vast multitude of hexagonal (sixsided) facets. In a species of Beetle (Mordella) upwards of 25,000 of these have been counted; in a Butterfly, above 17,000; in a Dragon-fly, more than 12,500; and in the common House-fly, 4,000. Every one of these facets may be regarded as the front of a distinct eye, which, however, instead of being globular, is conical in its form; the front being the base of the cone, and the apex or point being directed towards the optic nerve, which swells-out into a bulbous expansion that fills a large part of the interior of the hemisphere. Each individual eye consists, therefore, of its facet, which (being convex on both surfaces) acts as a lens; of the transparent cone behind this, which may be compared to the vitreous humour; and of the fibre which passes from the bulbous expansion of the optic nerve to the point of this cone. The several fibres are separated from one another by a considerable quantity of black pigment, which also fills up the spaces between the cones; and it is to this that the black appearance of the whole compound eye is due.

574. We must thus regard each of the cones, which, united together, constitute the hemispherical or globular mass, in the light of a distinct eye; but the entire aggregate seems to

442

COMPOUND EYES OF ARTICULATA.

correspond in function with the single eye of the Vertebrate animal. For no rays except those which correspond in direction with the axis of each cone, can reach the fibre of the optic nerve at its apex; all others being stopped by the layer of black pigment which surrounds it. Hence it is evident that each separate eye must have an extremely limited range of vision, being adapted to receive but a very small collection of rays proceeding from a single point in any object; and as these eyes are usually immoveable, animals with but a small number of them would be very insufficiently informed of the position of external things. But by the vast multiplication in the number of the eyes, and the direction of their axes to every point in the hemisphere, their defects are compensated; a separate eye being provided, as it were, for every point to be viewed. And it is quite certain, from observation of the movements of Insects, that their vision must be very perfect and acute.1

575. Although these Compound Eyes exist in all Insects and in most Crustaceans, Spiders and Centipedes, they are in general not the only organs of vision which these animals possess. Most of them are also furnished with several simple eyes, analogous in their structure to those of higher animals, but less complex and perfect in their organization; these, which are for the most part disposed on the back of the head, are largest in Spiders. The larvæ of some Insects possess the simple eyes without the compound; the latter being only developed at the time of the last metamorphosis. The simple eyes of Insects do not appear to be nearly so efficient as instruments of vision, as are their compound ones; for when the latter are covered, the animals seem almost as perplexed as if they were perfectly blinded. Simple eyes, closely resembling those of Insects in structure, are found in most of

1 It is commonly believed that each of these compound eyes produces its own image of the same external object, as do our two eyes; but from the description here given of their separate directions when united, it is evident that in no two of them can an image of the same object be formed at the same time. The membrane formed of all the lens-like corneæ united together, when separated from the other parts of the eye, and flattened-out, has the properties of a multiplying-glass, each lens forming a distinct image of the same object; but this is not the case when they are arranged in their natural position, because no two of them have the same direction.

EYES OF MOLLUSKS, ETC.-ANIMAL MOTION.

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the Mollusca which possess a head-namely, in the Gasteropods, Pteropods, and Cephalopods; those of the last class present an evident approach to the eyes of Fishes, in the greater completeness of their structure, and in their adaptation for distinct vision. In many of the lower Mollusca, as in the Rotifera and several Annelida, and also at the end of the arms of the Star-fish, red spots may be seen, which appear to be rudiments of eyes; but no distinct organs of vision can be seen in the Zoophytes and lowest Mollusca; although many of them appear very sensible to the action of light.

CHAPTER XII.

ANIMAL MOTION, AND ITS INSTRUMENTS.

576. THE different modifications of the faculty of Sensation which have been described in the preceding chapter, enable Man and other Animals to become acquainted with what is going-on around them. But their connexion with the external world is not confined to this faculty; for if they possessed it alone, they would be nearly as passive as are Plants,-experiencing, it is true, pain and pleasure from their sensations, but not having the power of avoiding the one or of procuring the other. They are endowed, however, with another faculty, that of spontaneous movement; which serves the double purpose of enabling them to act upon the inanimate world around them, and of communicating to each other their feelings and ideas. Thus, if we find ourselves scorched by a flame, we either withdraw our bodies from it, under the direction of the instinct which leads us to avoid suffering, or we set about to extinguish the fire by an act of the will, founded upon our rational knowledge of its injurious tendency. The Plant, even if it had sensation (which some naturalists have supposed), could do neither of these things. Again, it is entirely by the movements concerned in speech, by those giving expression to the countenance, and by the gestures of the body, that we convey to beings like ourselves a knowledge of what is passing in our own minds; of this power we know that plants are entirely destitute, and it is possessed in a very limited degree by the lower Animals.

444 MOVEMENTS NOT DEPENDENT ON MENTAL DIRECTION.

Contractile Tissues:-Muscular Contractility.

577. When we examine into the nature of the movements of the lower tribes of Animals, however, we find that they bear a much closer analogy to those of Plants, than they do to those which are the expressions of the self-determining power of higher Animals. Among the simplest Protozoa, it seems as if the change of form of the single cell of which each individual is composed, were the sole means of movement which it possesses (§ 129); and this change of form often appears rather to be due to the nutrient actions taking place within the cell, than to occur in respondence to any impression made upon its exterior. In such movements it is impossible to suppose with any probability that consciousness can participate. So, again, among Infusory Animalcules, all the movements of the body are effected by the action of cilia (§ 133), which we know in our own experience to be entirely removed from any mental direction, and which we see to be exhibited under a no less remarkable aspect by the 66 ZOOspores" or motile buds of the Alga (BOTANY, § 767).

578. Although the movements of the Hydra (§ 121) and other Zoophytes may appear to indicate the existence of a selfdetermining power, yet it is very doubtful whether such an endowment is really possessed by these animals. For their contractile tissue is of the simplest possible character, resembling that which is found in the early state of newly-forming parts of higher Animals; and when the movements executed by it are carefully compared with our own, it becomes obvious that they are analogous, not to those of the Human body and limbs generally, but to those of the muscular coat of the alimentary canal and of the muscles concerned in deglutition, which not only take place without any voluntary determination on our parts, but may even be performed without our consciousness. In like manner, the rhythmical movements of the umbrella-like discs of the Medusa (§ 120), by which many species of them are propelled through the water, bear a much closer analogy to the rhythmical movements of the heart of higher Animals, than they do to any other of their actions; and are probably performed, like these, without any exercise of will, and even without the guidance of consciousness.

579. In proportion, however, as we ascend the scale, we