CHAPTER SECOND. GENERAL PROPERTIES OF ORGANIZED BODIES SECTION I. ORGANIZED AND UNORGANIZED BODIES. 30. NATURAL HISTORY, in its broadest sense, embraces the study of all the bodies which compose the crust of the earth, or which are dispersed over its surface. 31. These bodies may be divided into two great groups; inorganic bodies, (minerals and rocks,) and living or organized bodies, (vegetables and animals.) These two groups have nothing in common, save the universal properties of matter, such as weight, extension, &c. They differ at the same time as to their form, their structure, their chemical composition, and their mode of existence. 32. The distinctive characteristic of inorganic bodies is est; the distinctive trait of organized bodies is independent motion, LIFE. The rock or the crystal, once formed, never changes from internal causes; its constituent parts or molecules invariably preserve the position which they have once. taken in respect to each other. Organized bodies, on the contrary, are continually in action. The sap circulates in the tree, he blood flows through the animal, and in both there is, besides, the incessant movement of growth, decom position, and renovation. 33. Their mode of formation is also entirely different. Unorganized bodies are either simple or made up of elements unlike themselves; and when a mineral is enlarged, it is simply by the outward addition of particles. constituted like itself. Organized bodies are not formed in this manner. They always, and necessarily, are derived from beings similar to themselves; and once formed, they always increase interstitially, by the successive assimilation of new particles, derived from various sources. 34. Finally, organized bodies are limited in their duration. Animals and plants are constantly losing some of their parts by decomposition during life, which at length cease to be supplied, and they die, after having lived for a longer or shorter period. Inorganic bodies, on the contrary, contain within themselves no principle of destruction; and unless subjected to some foreign influence, a crystal or a rock would. never change. The limestone and granite of our mountains remain just as they were formed in ancient geological epochs; while numberless generations of plants and animals have lived and perished upon their surface. SECTION II. ELEMENTARY STRUCTURE OF ORGANIZED BODIES. 35. The exercise of the functions of life, which is the essential characteristic of organized bodies, (32,) requires a degree of flexibility of the organs. This is secured by means of a certain quantity of watery fluid, which pene trates all parts of the body, and forms one of us princ.pal constituents. 36. All living bodies, without exception, are made up of tissues so constructed as to be permeable to liquids. There is no part of the body, no organ, however hard and compact it may appear, which has not this peculiar structure. It exists in the bones of animals, as well as in their flesh and fat; in the wood, however solid, as well as in the bark and flowers of plants. It is to this general structure that the term r ganism is now applied. Hence the collective name of organized beings, which includes both the animal and the vegetable kingdoms.. 37. The vegetable tissues and most of the organic structures, when examined by the microscope in their early states of growth, are found to be composed of hollow vesicles or cells. The natural form of the cells is that of a sphere or of an ellipsoid, as may be easily seen in many plants; for example, in the tissue of the house-leek, (Fig. 1.) The intervals which sometimes separate them Fig. 1. from each other are called intercellular passages or spaces (m.) When the cellules are very numerous, and crowd each other, their outlines become angular, and the intercellular spaces disappear, as seen in figure 2, which represents m * Formerly, animals and plants were said to be organized, because they are furnished with definite parts, called organs, which execute particular functions. Thus. animals nave a stomach, a heart, lungs, &c; plants have leaves, petals, stamens. pistils, roots, &c., which are indispensable to the maintenance of life and the perpetuation of the species. Since the discovery of the fundamental identity of structure of animal and vegetable tissues, a common denomination for this uniformity of texture has been justly preferred; and the existence of tissues is now regarded as the basis of organization. the pith of the elder. They then have the form of a honey-comb ; whence they have derived their name of cellules. Fig. 2. 38. All the organic tissues, whether animal or vegetable, originate from cells. The cell is to the organized body what the primary form of the crystal is to the secondary, in minerals. As a general fact, it may be stated α b that animal cells are smaller than vegetable cells; but they alike contain a central dot or vesicle, called nucleus. Hence such cells are called nucleated cells, (Fig. 3, a.) Sometimes the nucleus itself contains a still smaller dot, called nucleolus, (b.) Fig. 3. 39. The elementary structure of vegetables may be ob served in every part of a plant, and its cellular character has been long known. But with the animal tissues there is far greater difficulty. Their variations are so great, and their transformations so diverse, that after the embryonic period it is sometimes impossible, even by the closest exam ination, to detect their original cellular structure. 40. Several kinds of tissues have been designated in the animal structure; but their differences are not always well marked, and they pass into each other by insensible shades. Their modifications are still the subject of investigation, and we refer only to the most important distinctions. 41. The areolar tissue consists of a network of delicate fibres, intricately interwoven so as to leave numberless communicating interstices, filled with fluid. It is inter posed in layers of various thickness, between all parts of the body, and frequently accompanied by clusters of fat cells. The fibrous and the serous membranes are mere modifications of this tissue. 12 The cartilaginous tissue is composed of nucleated cells, the intercellular spaces being filled with a more com pact substance, called the hyaline matter. Figure 4 represents a slip of cartilage from the horse, under a magnifying power of one hundred and twen-00000000 ty diameters. 0 0 0 0 0 0 0 0 C 0000 43. The osseous or bony tissue differs from the cartilaginous tissue, in having its meshes. filled with salts of lime, instead of hyaline substance, whence its compact and solid appearance. It contains, besides, minute, rounded, or star-like points, improperly called bonecorpuscles, which are found to be cavities or canals, sometimes radiated and branched, as is seen in figure 5, representing a section of a bone of a horse, magnified four hundred times. 000 Fig. 4. Fig. 5. flesh of ani 44. The muscular tissue, which forms the mals, is composed of bundles of parallel fibres, which possess the peculiar property of contracting or shortening themselves, under the influence of the nerves. In the muscles under the control of the will, the fibres are commonly crossed by very fine lines or wrinkles; but not so in the involuntary muscles. Every one is sufficiently familiar with this tissue, in the form of lean meat. 45. The nervous tissue is of different kinds. Fig. 6. In the Fig. 7. is found in the white portion of the brain. But the gray substance of the brain is composed of very minute granulations, interspersed with clusters of larger cells, as seen in figure 7. |