The Encyclopedia Americana (1920)/Organ (part)
ORGAN, a functional part of a living being. It is an essential quality of a living being (see Life) that it shall exhibit activities, the expression or product of organic constitution. Hence, any living being either vegetable or animal is termed an organism — the broadest possible name for the animate world as opposed to the inanimate.
Nature of Organs.— Organs may be spoken of in two senses. In common speech they are large functional parts of gross anatomy, such as roots, leaves or blossoms of plants, the limbs, the heart and blood-vessels, the lungs, the stomach, the liver and excretory glands, the generative parts, etc., of animals; these are termed, properly enough, the respiratory, digestive, locomotive or reproductive organs, and so on. But each is made up of many parts, which unite to effect the great functions that characterize the group as a whole, and all are more or less interdependent. The number and diversity of these structures, large and small, visible and invisible, vary enormously in different plants and animals, and are the result of an organic evolution, that is, a struggle for existence among organs, or intra-evolution, as it has been termed by some German naturalists.
Degrees of Organization.— Where the organs are few and simple, in some cases more than one distinct function being performed apparently by the same part, the animal or plant is said to be of simple or synthetic structure or organization; where the subdivision of labor in the organism is extensive, separate structures doing each a more particular work for the benefit of the whole, the organization is said to be complex or specialized. Increase of specialization is regarded as an advance, hence we speak of “lower” and “higher” animals, referring to the less or greater degree of complexity and specialization of their organs. Comparison of existing organisms, considered with reference to their activities, and a study of the phylogeny of animals and plants as revealed by palæontology and embryology, establishes the truth of this standard of comparison, and throws light upon the evolution of organs. Tracing backward any particular group, as, for example, that of the digestive system, we find it less and less complex as we descend the scale of organization until we arrive at the simplest forms — the one-celled moners, which have no structure that our power is able to detect, the drop of undifferentiated protoplasm which constitutes their whole being accomplishing all the work of nutrition, locomotion and reproduction. A little higher stands the amœba (q.v.), and other protozoans and protophytans, in which a beginning of differentiation and structure appears in the presence of a nucleus, a vacuole, microsomes, etc. Still higher stand the sponges, cœlenterates, and so on, whose comparative superiority is manifested by the assignment of certain parts with gradually increasing definiteness to perform distinct offices. But the higher organisms primarily differ from the lower ones only in the fact that they are composed of many cells in more and more complicated arrangement, while the lowest consist of only a single cell; hence in the last analysis, an organ can be defined only as a cell or group of cells devoted to doing a special part of the labor required for the continued life, activities, and prosperity of the organism. This is only a corollary of the general doctrine of the cell as the seat of life. (See Cell). Of the origin of function or organic activity we know nothing.
Form and Symmetry.— Returning now to the ordinary sense of the word “organ” as a functional part, certain general facts may be considered. Among these are the form and symmetry of organs. The form is determined by the work to be done, the organ as an instrument assuming that shape and relative position in the body, and acquiring that structure, which will be most effective for the purpose. Many of the problems here which seemed so difficult to early investigators have been solved by the application of mechanical laws. Mechanical principles, controlling, to begin with, the original cell-division of the egg, and regulated in growth by external conditions, as gravity, air or water pressure, strains, muscle-leverage, etc., are no doubt at the basis of the symmetry which so remarkably characterizes the arrangement of organs throughout the living world. Two general types may be recognized — radial symmetry and bilateral symmetry. The former is most characteristic of plants, where the majority of organs occur in circles around an axis (see Plants, Structure of; Flower; Phyllotaxis, etc.); and of such animals as the echinoderms, cœlenterates and many worms, whose organs are radially disposed about an oral-aboral axis, as in polyps and starfish. The superior orders generally, and the arthropods and vertebrates particularly, display bilateral symmetry in a marked degree in all their outward parts, the external organs being in pairs (with one important exception in the higher forms) and these approximately, though not absolutely, alike. The internal organs of the vertebrates are by no means symmetrical at present, although they seem to have reached their asymmetry by evolution from a primitive paired condition. In insects and arthropods generally the internal parts show much bilateralism. Among mollusks the bilateral arrangement of the organs is normal, but one side is often developed at the expense of the other, even to the complete loss of the latter.
Correlations.— As might be expected under such circumstances certain definite correlations exist between organs as to size and relative proportions. Thus in the human frame, the limbs are equal in length, the head is one-seventh of the total height, the internal organs must be of a relative size in respect to one another and to the frame, and so on. The symmetry and correlation of organs is necessary to the continuance of healthful interacting functions.
At the same time homologous parts — that is, those structurally alike — may vary greatly among animals of the same class, according to the varying requirements of habits and environment, as for instance among vertebrates, the locomotive organs are in the form of fins or paddles for aquatic species, wings for aerial creatures, and legs and feet for those of terrestrial habit; and these may be vastly modified among the different groups, adapted to diverse habits, giving all the difference, for example, between the massive, shovel-like, digging-paws of the mole and the long slender legs of the antelope, or even the total disappearance of homologous parts, as in the snakes and limbless lizards. Such alterations of form adaptive to the requirements of environment and habit come about gradually, and are likely to require a corresponding change in other organs, directly or indirectly; since all or many organs in the frame are dependent upon the coaction of others, and conditions of space, gravity, etc., require mechanical as well as physiological conformity. The stomach, lungs, etc., of a long slim animal will be elongated and narrow, while those of a related but more compactly built species will be more globular. Sometimes, however, the changes render some secondary organ unnecessary, and its continual disuse results in its gradual reduction and perhaps extinction, as has happened in the case of the loss of eyes by many burrowing or cave-dwelling animals. When such a loss has become permanent in a type the remains may occur only in the embryonic stage, or may exist in adulthood, quite useless to the animal in its present condition. Such obsolete structures, of which the false hoofs of many ungulates, the “balancers” of flies and the appendix vermiformis in man are examples, are called vestigial organs, and were most difficult of explanation until this relationship was recognized.
Function Change.— Organs may not only change their form, but may also change their function. Many perform more than one function, even in the higher animals, and often, especially in the case of the limbs, this has produced a change of form as between the fore and hind limbs, relegating to one pair, say the hind limbs, the main work of progression, while the other pair assist in this respect only a little, but serve mainly as the means of seizing and holding food; most of the rodents afford examples of this case, of which the kangaroos and the extinct iguanodons are extreme instances. But function-change has often been more radical, the original or primary service by an organ having been completely superseded by some other service, to accomplish which the organ underwent gradual transformation in accordance with change of habit and the arising of a novel bodily need to which existing machinery could be adapted. The origin of lungs in air-breathing amphibians, reptiles and warm-blooded mammals and birds is directly traceable to the transformation for this new purpose of the swimming-bladder of fishes; teeth came into existence as a relic of the hard scales of ancient fishes whose function was purely as an armature, the structure and office surviving only in a few such forms as the gar (see Gar); the three pairs of “jaws” of crabs are structures which, in the early history of the crustacean type were primarily swimming and breathing organs, but long ago lost those functions altogether and became fitted wholly for mastication of food. Similar examples in great number might be drawn from both animals and plants. Indeed, it might be said that most if not all of the successful and persistent variations have involved change of function on the part of organs, the result of which in many cases was greater specialization.