CELLS. The ultimate structure of animal and vegetable bodies consists of minute vesicles which are called Cells. In both animal and vegetable structures these organs are not generally visible to the naked eye, as they vary from the 1-500th to the 1-10000th part of an inch in diameter. In all cases they consist of an enveloping membrane or cell-wall, which indorses in n space more or less enlarged certain constituents, called cell-contents. The nature of the substances which enter into the composition of the cell-walls and constitute the cell-conteds, differs in the animal and vegetable kingdoms, but there are certain properties which all cella possess in common. Sometimes these properties are called vital, to distinguish them from the pro perties possessed by inorganic or mineral bodies, which are called physical. It will however be seen that, independent of the formative power by which particles of gelatine, cellulose, kc., arrange them selves in the form of cells, and again these cells arrange themselves into the forms of organs and beings of a specific form, there are few of the functions performed by cells that may not be referred to the action of physical forces. One of the first and most necessary con ditions of the cell is, that it shall allow of the pi stage, through the membrane of which its walls are composed, of those substances by means of which it grows, and which it acts upon for the production of the peculiar secretions which characterise either specific beings or parts of their organisation. This function, which is called Absorption, seems referriblo to the physical relations which exist between liquids and gases and the membrane of which the cell-wall is composed. [AirsoRrriox.] The liquid or gaseous contents which are thus introduced into the interior of cells undergo a variety of changes, according to the position, ago, or other circumstances of the cell. Sometimes the fluid that is absorbed appears to be transmitted in compound structures from cell to cell without undergoing any great amount of change. In other cases the most decided chemical changes take place in the elements introduced. The cells of some parts of vegetable structures are an instance of the latter, in which carbonic acid and ammonia nre absorbed with water, and converted, either during their passage through the cell-wall, or whilst in the interior of the cell, into cellulose, starch, sugar, protein, and other constituents of the cell. in other parts of plants the cells convey solutions of sugar and other substances without producing on them any change.
The constituents absorbed into the interior of the cell are the materials from which the cell-wall and all its contents are derived. The process by which the cell appropriates to itself these matters is called Assimilation. This function is supposed to be carried on by an independent force or power residing In the cell, or congeries of cells, which form an organ or a body, and has been called the 'assimilative force or property,' organising force,' plastic force.' It is necessary however in this process to separate between the changes by which ono substance is converted into another, and which is probably the result of ordinary chemical force under other circumstances, and the power or force by which these substances nre made to assume definite forms in cells and organs. The latter is a special force in the case of each cell, plant, or niumal, and to which alone, of the changes involved in the function of assimilation, the term vita] can be properly applied.
The result of the appropriation of the new matter absorbed from without in all cells is their enlargement or growth. This takes place in two ways : either the new matter is taken up into the interior of the substance of the cell-wall, which is always the case where the cell becomes augmented in size, or it is deposited in the form of layers in the interior of the cell. According as the first mode of growth is regular or irregular will be the form of the cell. The vege table and animal kingdoms present almost all conceivable forms of cells, from the spherical and hexagons] cells observed in the lower forms of plants, and the less organised tissues of animals, as cartilage, up to the elongated vessels of the plant, and the irregular cells of bone or areolar tissue in animals. The animal kingdom presents by far the greatest variety in this respect, and so great are the changes that some of the animal cells undergo, that the terms Metamorphoses or Transformations have been applied to these changes. As examples of these cells we may quote—the horny scalea of the epidermis, of the hair and the nails, and the laminated pavement, epithelium—in which the cells are flattened, polygonal, or fusiform, and the cell-wall is fused into one mass with the cell-contents; the contractile fibre cella of the smooth muscles ; the tubules of the lens; the prisms of the enamel ; the various forms of bone-cells ; and the transversely striated cella of muscular fibre.