In many of these instances the motions so nearly resemble those of some of the lower animals, that it is not easy to say wherein they differ in principle, although in such cases there is no danger of supposing the plant to be an animal. Probably the principle of distinction is this. In the plant, the phenomena are purely local, and do not imply any connection with, and consciousness of, the individual considered as a whole. In the animal, on the contrary, we have reason to believe that the whole individual experiences a degree of consciousness. Touch a stamen of the barberry and a leaf of the sensitive plant, and that stamen and that leaf contract. But touch the petals of the barberry, and the stamen will not contract; touch the stem of the sensitive plant, and the leaf will not. On the contrary, touch any part of an oyster whose shell is open, and the muscle which closes the shell, though at a distance, contracts and closes it. So when the arms or feelers of a snail are extended, touch any one, or touch any part of the body, and they will all be contracted.

The inference is, that in an animal there is some central power or principle giving to it a unity of being and purpose, which is wanting in a vegetable. This is called the sensorial power. It is that directing and controlling principle which receives all impressions from without, and from which proceed all the voluntary motions. Its residence is in the brain and nervous system. Its powers become less complete and extensive as we descend from man to the lower animals, in proportion as the brain and nervous system become less perfect; but, go as low as we will, traces of it are still to be found.

CHAPTER III.

GENERAL STRUCTURE OF ANIMALS.

HAVING thus endeavored to illustrate the relation which animals bear to other bodies, and the peculiarities by which their

mode of existence is characterized, it is our next object to inquire how their existence is maintained.

This will be done most clearly by an examination of the subject in connection with man, the animal in whom all the purposes of life are carried out on the largest scale and in the most complete manner. We call him the most perfect of animals. Not that the structure of the lowest is less perfect in itself than that of the highest. Each species is adapted equally well to its own end. But the end of his being is higher in man; its purposes are more numerous, varied, and elevated; we place him, therefore, at the head of the creation, and yet in the lowest animals the purposes of their existence, such as they are, are carried out with as much skill and by as perfect an apparatus as in him.

The life of animals is maintained, as has been already stated, by two sets of functions; one set common to all living things, by which the life of the individual is maintained; and another set by which the individual maintains a connection with the external world. The former are called the organic functions, or functions of organic life; the latter, the animal functions, or functions of animal life. The former are in man wholly subsidiary to the latter. It is the animal functions which make him what he is. He digests no better, he breathes no better, he circulates no better than the lowest of his class. . It is in the combined perfection of sensation, motion, intelligence, and speech that he excels them all.

As the distinctive character of man depends upon the mode in which these functions are performed, so his external form is the expression of that character. The organs of intellect, of sense, of motion, of voice, constitute this form. As the basis or support of it, there is a bony structure or skeleton, which will be first described.

The bones of the human body are divided into those of the head, trunk, and extremities.

The head includes the cranium, or skull, and the face. The skull is a large bony cavity, composed of several wide, thin, and arched bones, united together by what are called sutures. It contains the brain, and gives passage to the spinal nerve through hole situated in its lower part, where it proceeds from the

brain, and goes to the backbone. The face is formed of the upper and lower jaws, and of the organs of seeing, smelling, and tasting. The bones which form the basis of these organs, are very numerous and difficult to describe. When taken together they give the general shape and configuration of the counte

nance.

The head is placed on the top of the backbone, and is capable of a variety of motions upon it. The backbone is the main support of the trunk of the body, and is composed of twenty-four distinct bones called vertebræ, placed one above another, so as to form a kind of pillar or column. The body of each vertebra consists of a solid cylindrical piece of bone, and this is united firmly to those contiguous to it, above and below, by strong and elastic cartilages. The body of the vertebra is solid; but behind it, and on each side, are projections of bone, called processes, which are arched over and connected together in such a manner as to form a canal from one end of the spine to the other. This canal contains the spinal nerve or marrow, and between the vertebræ are holes through which branches are sent out to the different parts of the body. (Fig. 1 and Fig. 2.)

Fig. 1.

Vertebra.

Seven of the vertebræ belong to the neck, twelve to the back, and five to the loins. They are called respectively the cervical, dorsal, and lumbar vertebræ. They increase in size from above downwards, so that the lumbar vertebræ are much larger, thicker, and stronger than those of the back and neck. To the dorsal vertebræ are affixed twelve ribs on each side, which arch over forwards, and are joined to the sternum or breastbone by means of cartilage or gristle. In this way they form the cavity of the thorax or chest, which contains the heart and lungs. This cavity is terminated below by a muscular membrane, called the diaphragm or midriff, which extends from the edges of the lower ribs, and stretches across to the backbone, so as to form a complete curtain or division between the chest, and the abdomen which lies below it. This is another important cavity, usually called the belly, containing the stomach, liver, spleen, intestines, and other important organs. It is formed below by four bones attached to the lower end of the back, which spread out

and constitute a sort of basin, called the pelvis. This serves as a solid basis to support all the heavy organs contained in the abdomen, which is protected before and at its sides only by skin, fat, and muscles, and has no bones, except below and behind.

The limbs of man and other animals are called their extremities. The arm, or upper extremity, is composed of the shoulder, which has two bones, the collar-bone and shoulder-blade, by which it is connected with the trunk; the arm, which has only one bone, long and firm, extending to the elbow; the forearm, which has two long bones, parallel to each other, extending from the elbow to the wrist; the wrist, having eight small and irregular bones; and the hand, on which there are four fingers, each with four bones, and the thumb with three. These bones are united together, so as to form movable joints of various degrees of flexibility and power, by means of firm substances called ligaThe surfaces which move upon one another are covered by a smooth, polished substance, cartilage, always lubricated by a fluid like the white of eggs, that renders all their motions easy and free from impediment.

ments.

The lower extremities are constructed in a similar manner. The thigh-bone, the largest and strongest bone in the body, is connected above with one of the bones of the pelvis, by means of a large, round head, which is received into a socket of corresponding size, and thus forms the hip-joint. Its lower end, together with the kneepan and one of the two bones of the leg, .contributes to form the knee-joint. These last are parallel to each other, and extend from the knee to the ankle. The ankle is composed, like the wrist, of a number of small bones, of which there are seven, one of them projecting behind to form the heel. The toes have the same number of bones as the fingers and thumbs, but are shorter and less capable of free and extensive

motions.

These different bones are covered by muscles, fat, and skin, which constitute the principal soft parts of the body. The muscles are fibrous organs, which, attached to the bones generally by tendons, by their contractions put the bones in motion, and thus originate all the movements of which we are capable. They act,