Asymmetry

Asymmetry in biology denotes the absence or disturbance of symmetry in the structure of an organism or its parts. A. can be both a normal phenomenon characteristic of a given genus or family of animals, developed in the process of the struggle for existence, and a pathological phenomenon peculiar only to individual individuals and arising in connection either with irregularities of embryonic development or with pathological disturbance of nutrition or innervation of a given part of the body in the post-embryonic period. Examples of normal A. are found among various groups of invertebrates; for instance, in hermit crabs the right limbs are developed much more strongly than the left; among vertebrates an example of normal A. is the flounder, in which, in connection with a lying-down mode of life, there is a displacement of organs, as a result of which both eyes in them are on the side turned upward. Examples of pathological A. in man can serve the cases of so-called hemiatrophy of the face and A. of half of the spinal cord. Mild degrees of A. of the face occur quite often, combining with other symptoms of degeneration, which applies even more to cases of more sharply expressed A. But A. of the face can also result from paralysis of the facial nerve. Along with hemiatrophies, hemihypertrophies are also encountered, including some cases of so-called partial gigantism.