THE importance of membranes in vital processes has long been recognized. From the earliest times anatomists have been impressed with the frequency with which thin sheets of solid material occur as elements of structure in organisms. Even elementary methods of analysis show that the materials composing the most various organs often tend to dispose themselves in thin, continuous layers. Thus the entire body is enclosed in an extremely resistant and impermeable layer, the skin. Each of the internal organs has its own characteristic enclosing membrane; the peritoneum lines the body-cavity and invests the intestine and its associated glands, the heart is enclosed in the pericardium, the lungs in the pleura, the central nervous system in the pia mater; the muscles are closely surrounded by thin connective tissue sheaths, or perimysia; the walls of the blood-vessels and of the intestine and other hollow viscera consist of several distinct concentric layers. Various products of animals, like the eggs of birds and reptiles, often show this tendency. Plants also deposit a great part of their structural materials in layers; the wood forms concentric circles; leaves and fruits have thin and often waterproof membranous coverings; the orange is partitioned by a system of membranes and each smaller portion of pulp has a membrane of its own. The instances, in fact, are innumerable. Evidently the tendency to deposit material in thin continuous sheaths is highly characteristic of organisms.
This much was clear at a time when anatomists were limited to direct and unaided vision. When the microscope came into use the existence of a similar tendency soon became evident in the minutest tissue elements. The living substance exhibited itself everywhere as minutely subdivided by innumerable thin partitions, or membranes, giving it a characteristic honeycomb-like or cellular structure. These partitions isolate the enclosed portions of living substance and render them at least mechanically separable. Hence the conception that each of these minute membrane-enclosed masses of gelatinous or viscid "protoplasmic" material is an independently living entity, or elementary physiological unit, gained ground, and, as all know, has been universally adopted in biology. The name "cell," originally applied to the minute spaces themselves, has been transferred to the protoplasmic mass within, by whose activity the enclosing membrane is itself formed.
Thus it was early recognized that cells tend to separate materials