and above the known forces of the laboratory.68 If the vital machine is given, it works by physical forces; but to produce it and keep it in order needs, so far as we now know, more than mere physical force. To this controlling power we may apply the name vitality. Life is exhibited only under certain conditions. One condition is the presence of a physical basis called protoplasm. This substance is found in all living bodies, and, so far as we know, is similar in all — a viscid, transparent, homogeneous, or minutely granular, albu minoid matter. Life is inseparable from this protoplasm; but it is dormant unless excited by some external stimulants, such as heat, light, electricity, food, water, and oxygen. Thus, a certain temperature is essential to growth and motion; taste is induced by chemical action, and sight by luminous vibrations. The essential manifestations of animal life may be reduced to four: contractility; irritability, or the power of receiving and transmitting impressions; the power of assimilating food; the power of reproduction. All these powers are possessed by protoplasm, and so by all animals all move, feel, grow, and multiply. But some of the lowest forms are without any other trace of organs than is found in a simple cell; they seem to be almost as homogeneous and structureless as a drop of jelly. They could not be more simple. They are devoid of muscles, nerves, and stomach; yet they have all the fundamental attributes of life, moving, feeling, eating, and propagating their kind. The animal series, therefore, begins with forms that feel without nerves, move without muscles, and digest without a stomach, protoplasm itself having all these properties: in other words, life is the cause of organization, not the result of it. Animals do not live because they are organized, but are organized because they are alive. CHAPTER VIII* ORGANIZATION We have seen that the simplest living thing is a formless speck of protoplasm, without distinctions of structure, and therefore without distinctions of function, all parts serving all purposes-mouth, stomach, limb, and lung - indiscriminately. There is no separate digestive cavity, no separate respiratory, muscular, or nervous systems. Every part will successively feed, feel, move, and breathe. Just as in the earliest state of society all do everything, each does all. Every man is his own tailor, architect, and lawyer. But in the progress of social development the principle of the division of labor emerges. First comes a distinction between the governing and governed classes; then follow and multiply the various civil, military, ecclesiastical, and industrial occupations. In like manner, as we advance in the animal series, we find the body more and more heterogeneous and complex by a process of differentiation, i.e., setting apart certain portions of the body for special duty. In the lowest forms, the work of life is carried on by very simple apparatus. But in the higher organisms every function is performed by a special organ. For example, contractility, at first the property of the entire animal, becomes centered in muscular tissue; respiration, which in simple beings is effected by the whole surface, is specialized in lungs or gills; sensibility, from being * See Appendix. common to the whole organism, is handed over to the nerves. An animal, then, whose body, instead of being uniform throughout, is made up of different parts for the performance of particular functions, is said to be organized. And the term is as applicable to the slightly differentiated cell as to complex man. Organization is expressed by single cells, or by their combination into tissues and organs. 1. Cells. A cell is the simplest form of organized life. In general, it is a microscopic globule, consisting a 12 D FIG. 198. - A, diagram of a cell; w, cell wall, with inclosed cytoplasm; », nucleus, consisting of nuclear membrane inclosing granular substance, in which are seen a spherical nucleolus and several irregular masses of chromatin; a, attraction sphere containing a centrosome. B-F, changes which take place in the cell during fission. of a delicate membrane inclosing a minute portion of protoplasm. The very simplest kinds are without granules or signs of circulation; but usually the protoplasm is granular, and contains a defined separate mass called the nucleus, within which are sometimes seen one or two, rarely more, dark, round specks, named nucleoli. The enveloping membrane is extremely thin, transparent, and structureless; it is only an excretion of dead matter acting as a boundary to the cell contents.69 The nucleus generally lies near the center of the protoplasm, and is the center of activity. Cells vary greatly in size, but are generally invisible to the naked eye, ranging from 0 to 10000 of an inch in diameter. About 4000 of the smallest would be necessary to cover the dot of this letter i. The natural form of isolated cells is spherical; but when they crowd each other, as seen in bone, cartilage, and muscle, their outlines become angular, either hexagonal or irregular. Within the narrow boundary of a simple sphere, the cell membrane, are exhibited all the essential phenomena of life, nutrition, sensation, development, and reproduction. The physiology of these minute organisms is of peculiar interest, since all animal structure is but the multiplication of the cell as a unit, and the whole life of an animal is that of the cells which compose it in them and by them all its vital processes are carried on.70 The structure of an animal cell can be seen in blood corpuscles, by diluting with a weak (.6 per cent) solution of salt a drop of blood from a frog, and placing it under the microscope. (See Fig. 260.) With this may be compared vegetable cells as seen in a drop of fluid yeast or a drop of water into which pollen grains from some flower have been dusted. 2. Tissues. There are organisms of the lowest grade (as Paramecium, Fig. 9) which consist of a single cell, living for and by itself. In this case, the animal and cell are identical: the Paramecium is as truly an individual as the elephant. But all animals, save these unicellular beings, are mainly aggregations of cells; for the various parts of a body are not only separable by the knife into bones, muscles, nerves, etc., but these are susceptible of a finer analysis by the microscope, which shows that they arise from the development and union. of cells. These cellular fabrics, called tissues, differ from one another both chemically and structurally, but agree in being permeable to liquids a property which secures the flexibility of the organs so essential to animal life. Every part of the human body, for example, is moist; even the hairs, nails, and cuticle contain water. The contents as well as the shape of the cells are usually modified according to the tissue which they form: thus, we find cells containing earthy matter, iron, fat, mucus, etc. In plants, the cell generally retains its characters well defined; but in animals (after the embryonic period) the cell usually undergoes such modifications that its structural features become altered. The cells are connected together or enveloped by an intercellular substance (matrix), which may be watery, soft, and gelatinous, firmer and tenacious, still more solid and hyaline, or hard and opaque. In the fluids of the body, as the blood, the cells are separate; i.e., the matrix is fluid. But in the solid tissues they are held together by intercellular substance. In the lowest forms of life, and in all the higher animals in their earliest embryonic state, the cells of which they are composed are not transformed into differentiated tissues: definite tissues make their first appearance in the sponges, and they differ from one another more and more widely as we ascend the scale of being. In other words, the bodies of the lower and the immature animals are more uniform in composition than the higher or adult forms. In the vertebrates only are all the following tissues found represented: (1) Epithelial Tissue.—This is the simplest form of cellular structure. It covers all the free surfaces of the body, internal and external, so that an animal may be said to be contained between the walls of a double bag. That which is internal, lining the mouth, windpipe, lungs, blood vessels, gullet, stomach, intestines - in fact, every cavity and canal—is called epithelium. It is a |