CELLS, in Physiology.—I. ANIMAL examining, under a high magnifying power, any of the constituents of the animal body, we perceive that the smallest parts which appear to the naked eye as fibers, tubes, etc., are not ultimate elements in respect to form (morphotic elements), but that they contain and are built up of certain extremely minute particles, which differ in different organs, but always have a similar appearance in the same organs. By far the most important of these microscopic forms, which are known by histplogists as "simple elementary parts," are the C., which not only form the starting-point of every animal and vegetable organism (the ovum in either kingdom of nature being simply a cell), but also—either as C. or after having undergone certain modifications which will be presently described—make up the tissues and organs of the perfect animal. Indeed, some of the lowest plants (red snow, gory dew), and of the simplest forms of animal life (Gregorinw, etc., q.v.), appear to consist of a single cell.
While in plants the elementary parts generally unite directly with one another, in animals they are usually combined by an interstitial substance, which may be either solid or fluid, and is always derived from the blood or general nutrient fluid. If this interstitial substance take a part in the formation of the C., it is called a cytoblastema or a blastema, from kutos, a cell or vesicle, and blastema, germ substance; if it has noth ing to do with their maintenance, it is called the matrix. The cytoblastema is usually fluid, as in the blood, chyle, etc.; while the matrix is solid, as in cartilage, bone, etc.
In every cell, we can distinguish, if we use sufficiently high magnifying powers, a membranous envelope, known as the cell-wall or membrane, and certain contents. The latter are fluid or gelatinous, and besides containing particles or granules, usually exhibit a peculiar rounded body, the nucleus; which, again, contains in its interior a fluid and a still smaller corpuscle, the nucleolus.
The fundamental form of the cell is spherical or lenticular; it is such in all young C., and is persistent in those which occur in fluids, as, for example, the blood-corpuscles Amongst other well-known forms may be mentioned: the polygonal, as in pavement epithelium, or the pigment of the eye; the conical or pyramidal, as in ciliated epi thelium; the cylindrical, as in cylinder epithelium; the fusiform, or spindle-shaped, as in contractile fiber-cells; the squamous, as epidermic scales; and the caudate, polar, or stellate, as the C. in the gray nervous tissue.
With regard to size, the largest animal C.—excepting the unicellular organisms— are the yolk-cells of the ova of birds and amphihia, while the blood-cells of certain ani mals may be taken as representing the smallest cells. Average C. range from 0.005 to 0.01 of a line in diameter.
The cell-membrane is usually transparent and colorless, mostly smooth, and so thin as to exhibit only a single contour, rarely of any measurable thickness. No traces of structure can be detected in it. The granular appearance which the membrane occa sionally presents, is due to projections depending on granules lying on the inside; and it vanishes on the addition of water, which causes the cell to be distended by endos mosis. See ()swine ACTION.
C. which contain only fluid are rare (fat-cell, blood-cell); generally, besides fluid, they contain elementary granules and vesicles, and sometimes crystals. As a general rule, the number of these morphotic elements increases with the age of the cell; sometimes, however, this is not apparent, in consequence of their being grouped in a single mass around the nucleus.
The nucleus is usually spherical or lenticular, transparent and either colorless or yellowish, and ranges from 0.002 to 0.004 of a line in diameter. All nuclei are vesicles, as was originally maintained, in 1841, by Schwanu (Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants, Sydenham society's translation, 1847, p. 173), who must be regarded as the founder of the cell-theory in its relation to animal tissues, and as has since been confirmed by K011iker and other later observers. The contents of the nucleus usually consist, with the exception of the nucleolus, of a limpid or slightly yellowish fluid, from which water and acetic acid pre cipitate granular matter. In general, only one nucleus exists in each cell, except when it is multiplying (a process which we shall presently explain); occasionally, however, we meet with several nuclei—four, ten, or even twenty.