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lodget. in these cavities, is a peculiarity belonging to animals. only. In plants, the organs designed for special purposes are never embodied into one mass, but are distributed over various parts of the individual. Thus, the leaves, which answer to the lungs, instead of being condensed into one organ, are scattered independently in countless numbers over the branches. Nor is there any organ corresponding to the brain, the heart, the liver, or the stomach.

54. Moreover, the presence of a proper digestive cavity involves marked differences between the two kingdoms, in respect to alimentation or the use of food. In plants, the fluids absorbed by the roots are carried, through the trunk and all the branches, to the whole plant, before they arrive at the leaves, where they are to be digested. In animals on the contrary, the food is at once received into the diges. tive cavity, where it is elaborated; and it is only after it has been thus dissolved and prepared, that it is introduced into the other parts of the body. The food of animals consists of organized substances, while that of vegetables is derived from inorganic substances; and they produce albumen, sugar, starch, &c., while animals consume them.

55. Plants commence their development from a single point, the seed, and, in like manner, all animals are developed from the egg. But the animal germ is the result of successive transformations of the yolk, while nothing similar takes place in the plant. The subsequent development of individuals is for the most part different in the two kingdoms. No limit is usually placed to the increase of plants; trees put out new branches and new roots as long as they live. Animals, on the contrary, generally have a limited size and figure; and these once attained, the subsequent changes are accomplished without any increase of volume, or essential alteration of form; while the appearance of most vegetables is repeatedly modified, in a notable manner, by the develop

ment of new branches. Some of the lowest animals, however, the polyps for instance, increase in a somewhat analogous manner, (§ 329, 330.)

56. In the effects they produce upon the air by respira tion, there is an important difference. Animals consume the oxygen, and give out carbonic acid gas, which is destructive to animal life; while plants, by respiration, which they in most instances perform by means of the leaves, reverse the process, and thus furnish oxygen, which is so essential to animals. If an animal be confined in a small portion of air, or water containing air, this soon becomes so vitiated by respiration, as to be unfit to sustain life; but if living plants are enclosed with the animal at the same time, the air is maintained pure, and no difficulty is experienced. The practical effect of this compensation, in the economy of Nature, is obviously most important; vegetation restoring to the atmosphere what is consumed by animal respiration, combustion, &c., and vice versa.

57. But there are two things which, more than all others, distinguish the animal from the plant, namely, the power of moving itself or its parts at will, and the power of perceiving other objects or their influences; in other words, voluntary motion and sensation.

58. All animals are susceptible of undergoing pleasure and pain. Plants have also a certain sensibility. They wither and fade under a burning sun, or when deprived of moisture; and they die when subjected to too great a degree of cold, or to the action of poisons. But they have no consciousness of these influences, and suffer no pain; while animals under similar circumstances suffer. Hence they have been called animate beings, in opposition to plants, which are inanimate beings.

CHAPTER THIRD.

FUNCTIONS AND ORGANS OF ANIMAL LIFE.

SECTION I.

OF THE NERVOUS SYSTEM AND GENERAL SENSAT ON.

59. LIFE, in animals, is manifested by two sorts of func tions, viz. First, the peculiar functions of animal life, or those of relation, which include the functions of sensation and voluntary motion; those which enable us to approach, and perceive our fellow beings and the objects about us, and to bring us into relation with them: Second, the functions of vegetative life, which are nutrition in its widest sense, and reproduction; * those indeed which are essential to the maintenance and perpetuation of life.

60. The two distinguishing characteristics of animals, namely, sensation and motion, (57,) depend upon special systems of organs, which are wanting in plants, the nervous system and the muscular system under its influence. The nervous system, therefore, is the grand characteristic of the animal body. It is the centre from which all the commands of the will issue, and to which all sensations tend.

This distinction is the more important, inasmuch as the organs of animal life, and those of vegetative life, spring from very distinct layers of the embryonic membrane. The first are developed from the upper layer, and the second from the lower layer of the germ of the animal See Chapter on Embryology, p. 112.

61. Greatly as the form, the arrangement, and the vol

ame of the nervous system vary in different animals, they may all be reduced to four principal types, which correspond, moreover, to the four great departments of the Animal Kingdom. In the vertebrate animals, namely, the fishes, reptiles, birds, and mammals, the nervous system is composed of two principal masses, the spinal marrow, (Fig. 9, c,) which runs along the back, and the brain, contained within the skull.* The volume of the brain is proportionally larger as the animal occupies a more elevated rank in the scale of being. Man, who stands at the head of Creation, is in this respect also the most highly endowed being.

[graphic]

Fig. 9.

62. With the brain and spinal marrow are connected the nerves, which are distributed, in the form of branching threads, through every part of the body. The branches which unite with the brain are twelve pairs, called the cere

*The brain is composed of several distinct parts which vary greatly, in their relative proportions, in different animals, as will appear hereafter They are 1. The medulla oblongata; 2. Cerebellum; 3. Optic lobes; 4. Cerebral hemispheres; 5. Olfactory lobes; 6. the pituitary body; 7. the pineal body. (See figures 9 and 21.) The spinal marrow is made up by the union of four nervous columns.

bral nerves, and are designed chiefly for the organs of sense located in the head. Those which join the spinal marrow are also in pairs, one pair for each vertebra or joint of the back. The number of pairs varies, therefore, in different classes and families, according to the number of vertebræ. Each nerve is double, in fact, being composed of two threads, which at their junction with the spinal marrow are separate, and afterwards accompany each other throughout their whole course. The anterior thread trans mits the commands of the will which induce motion; the other receives and conveys impressions to the brain, to produce sensations.

63. In the Articulated animals, comprising the crabs,

Fig. 10.

barnacles, worms, spiders, insects, and other animals formed of rings, the nervous system consists of a se

ries of small centres or swellings, called ganglions, (Fig. 10,) placed beneath the alimentary canal, on the floor of the general cavity of the body, and connected by threads; and of a more considerable mass placed above the œsophagus or throat, connected with the lower ganglions by threads which form a collar around the alimentary canal. The number of ganglions generally corresponds to the number of rings. 64. In the Mollusks, (Fig. 11,) the nervous system con

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