As a result of the tissue injury in disease, repair is brought about by cell multiplication or reproduction. The extent of this regeneration depends upon the degree of specialization of the tissue.

The Cell

The adult cell consists primarily of a mass of protoplasm or cytoplasm surrounded by a limiting membrane called the cell wall, and containing a nucleus within which there may be a small body called the nucleolus.

The cytoplasm, which is a semifluid substance, is divided into two portions - the spongioplasm, which consists of a very elastic and extensible framework, and the hyaloplasm, which is homogeneous and less active.

Embedded in the cytoplasm are minute granules known as microsomes. These are most numerous toward the center of the cell; the peripheral zone, called exoplasm, not containing them.

Foreign bodies and vacuolations may also be found within the cell.

The arrangement of the constituents of the cytoplasm varies at different times. Frequently the spongioplasm is arranged as a distinct reticulum. This is, however, not permanent, and seems to depend upon the relative proportion of the hyaloplasm.

The nucleus is confined by a distinct wall, the nuclear membrane, within which is the nuclear substance or karyomitome. This is divided into a framework of fibrils, the nuclear fibril, and an interfibrillar substance, the nuclear matrix.

The fibrils consist of a part called chromatin or nuclein that has a marked affinity for nuclear stain. This portion is supported by fine fibrils of linin that do not stain.

There is also present a semifluid substance known as the karyoplasm or nuclear juice.

The nucleolus lies within the nucleus and consists of a substance known as pyrenin. Just what is its function is not known. It probably has a distinct purpose during cell multiplication, as it disappears during the division of the nucleus, but reappears when the new nucleus is formed.

Another body, the centrosome, is also sometimes found. It is a small, highly refracting body, situated within the nucleus.

Fig. 15. - Diagram of a Cell (Huber).

It is surrounded by a clear area called the attraction sphere. This body, although it may be found during the stage of rest, becomes most noticeable during the stage of division of the nucleus. At that time it divides into two and passes to opposite poles of the cells.

Occasionally a small irregularly spheric body, the paranucleus, is present in the cytoplasm near the nucleus. Its function is not known.

The relation between the size of the nucleus and that of the cell varies greatly. In certain cells, as in the lymphocyte, the nucleus may occupy nearly the entire area.

The nuclei of the same kind of cells are usually similar in shape and size. They may be round, oval, or, as in some of the lower animals particularly, irregular. The shape of the cell depends partly on environment, partly on specialization.

A cell may also have one or more nuclei, the latter being known as giant cells.

With the exception of the red blood-corpuscles and the horny layer of the skin, all cells under normal conditions contain nuclei. The absence of a nucleus, therefore, usually denotes the loss of cellular activity.

The functions of cells which distinguish living from inorganic tissues can be divided into:

1. Metabolism, the power of selecting and assimilating food, anabolism; and the power of casting off excrementitious matter, catabolism.

2. Growth, the result of assimilation producing an increase in the size of the cell.

3. Irritability, response of the living cell to external influences.

4. Motion, which may be of three different kinds. There is a constant passage of a "circulating albumin" from one part of the cell to another. It may be ameboid, so called on account of its resemblance to the motion of the ameba. It consists of a streaming of the cytoplasm to one point, giving rise to prolongations or pseudopodia extending from the surface of the cell.