HISTOLOGY.

CHAPTER I.

HISTOLOGY is the microscopic science of normal tissues. No matter how carefully we dissect the various tissues or organs of the body, we can with the unaided eye obtain but little knowledge of their ultimate composition. An examination of the smallest portion of muscle, cartilage or bone, for instance, will reveal nothing which could not be seen in a gross specimen. The moment, however, we bring the microscope to bear upon any organ, an astonishing complexity of structure is revealed. The whole field of view is seen to be composed of minute bodies, differing greatly in appearance from the tissues they compose, and arranged in more or less definite order. These small bodies have received the name of cells, and are the histologic units.

CELLS.

A cell is a microscopic, protoplasmic mass, which, when typical or complete, contains cell-wall, cell-body, nucleus, nucleolus, and centrosome.

Some cells lack one or more of the above, and are said to be atypical or not complete―e. g., the red corpuscle contains usually neither nucleus, nucleolus, nor centrosome. The loss of these constituents can usually be traced to some stage of development.

Cell-body. The cell-body, protoplasm, or cytoplasm, as its name implies, constitutes the largest part of the cell. It is surrounded by the cell-wall, and incloses the nucleus, nucleolus, and centrosome. The relative size and the internal structure of the cell-body vary greatly according to the locality from which the cell is taken. When examined under a high magnification, the cell-body is seen to consist of a network or reticulum of exquisitely delicate fibers (the spongioplasm), the interstices of which are filled with a transparent,

Fig. 1.-Diagram of a typical cell: A, Cell-wall; B, cell-body; C, nucleus; E, nucleolus; F, centrosome; a, spongioplasm; b, hyaloplasm; c, metaplasm or microsomes; d, exoplasm; e, nuclear membrane; f, nuclear network or chromatin filaments; g, nuclear matrix; h, nodal enlargements or net knots.

homogeneous substance (the hyaloplasm). Besides these two substances, we usually find some small granules embedded in the hyaloplasm. These are usually believed to be not a part of the cell, but rather the result of some vital change in the cell-substance, or in some cases to be partially assimilated substances from without. These granules have received the special name of metaplasm or microsomes.

At the outer edge of the cell-body there is usually

NUCLEUS.-CENTROSOME.

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a narrow zone which is free from microsomes, and which has received the distinguishing name of exoplasm.

Cell-wall. The cell-wall is a delicate, structureless membrane enveloping the cell, and is derived from the cell-body.

Nucleus. The nucleus is an easily discernible, highly refracting body lying within the cell. It is quite complex in structure. Under a high magnification we are able to distinguish

(a) Nuclear Membrane.-A very thin membrane surrounding the nucleus (sometimes absent).

(b) Nuclear Network.-A network of fine, strongly staining threads or filaments called chromatic filaments. These filaments exhibit along their course, and also at their points of intersection, certain thickenings called nodal enlargements or net knots. (c) Nuclear Matrix.-A transparent, gelatinous, semifluid substance which occupies the space between the fibrils and the nuclear network.

Nucleolus. The nucleolus (plural, nucleoli) is a small spheric body usually lying in the nuclear network. There may be several of these bodies in a single cell.

Centrosome. The centrosome is a very minute body usually situated in the nucleus and surrounded by a small clear area. During cell-division, however, it is found in the cell-body. Radiating from the centrosome to the chromatin filaments are found a number of straight lines, which are extremely fine and very difficult to

see.

The nucleus seems to be the part of the cell which contains and transmits the hereditary characteristics. The centrosome apparently controls the reproduction of the cell.

Properties of the Cell.-As the body is but a collection of cells and intercellular substance, it is natural

for us to expect to find the cell possessing certain attributes which characterize a living organism. Among these properties may be named

Metabolism-absorbing nutrition and excreting waste.

Fig. 2.-Cells from various localities: a, Squamous epithelial cell from tongue; b, ciliated epithelial cell from trachea; c, smooth muscle-cell from uterus; d, pigment cell from choroid coat of eye; e, eosinophile cell from blood; f, cell from liver.

Motion-in some cells from place to place, in others simply the extension and extraction of the cell-body. Function.-All normal cells have a certain life-work to perform, which is called their function.

Origin of Cell.-All cells originate from a preexisting cell, and can not be generated spontaneously.