Function, the technical term in physiology for the vital activity of organ, tissue, or cell. Thus it is the dominant function of the pancreas to secrete digestive juice, of a muscle to contract, of a sensory cell to receive and pass on external stimulus. The classification of the various functions or vital processes presents considerable difficulty, though it is easy enough roughly to catalogue the most important: (1) contractility (by muscular cells, tissues, and organs); (2) irritability to sensory stimulus, transmission of nervous stimulus, 'automatic' origin of nervous impulse (by sensory organs, nerves, brain, &c.); (3) secretion and excretion (by glandular cells, or complexes of these); (4) respiration (by skin, gills, lungs, &c., or necessarily in every actively living cell); (5) nutrition, digestion, assimilation (in the manifold ways in which the income of energy in the form of food is received and worked up into living matter). Somewhat apart from these, and of more periodic occurrence, are the great processes of growth and reproduction. Or the various vital phenomena may be thus arranged in diagrammatic fashion :
In a single-celled organism, such as an Amœba, all the vital processes take place within narrow limits, and just because of the simplicity of structure there must be great complexity of function compared with what occurs in a single cell of one of the higher organisms. For here division of labour is possible, and in the different cells special functions predominate over the others. Thus, a muscle-cell is contractile but not strictly nervous, and a glandular cell is secretory without being definitely contractile. With the division of labour and resultant complexity of structure in a higher organism, various functions appear which are only foreshadowed in a protozoan. Such, for instance, is the circulatory function, establishing nutritive and respiratory communication between the distant parts. But such a multiple process can readily be seen to be the sum of several more fundamental functions. It must also be noted that, while a cell, tissue, or organ may have one dominant function, it may at the same time retain several sub-functions.
Another fact of general importance is the change of function which may be exhibited by the same organ in the course of its history—that is to say, through an ascending series of animals, or even in the development of an individual. Thus, what is a mere bladder, of little apparent account, near the hind end of a frog's gut, becomes the respiratory and sometimes nutritive Allantois (q.v.) of reptile and bird, and an important part of the Placenta (q.v.) in placental mammals. The importance of this in relation to the general theory of evolution has been emphasised by Dohrn in what he terms the principle of functional change.
Fundamentally, the functions of organs, the properties of tissues, the activities of cells, are reducible to chemical changes in the living matter or protoplasm. To the constant change in the protoplasm the general term 'metabolism' is applied, while this is again subdivided into processes of upbuilding, construction, chemical synthesis, or 'anabolism,' and reverse processes of down-breaking, chemical disruption, or 'katabolism.' See AMÆBA, BIOLOGY, CELL, PHYSIOLOGY, PROTOPLASM, and the various functions, DIGESTION, &c.—In speaking of disease, 'functional' is opposed to 'organic.'