CHAPTER IV.

EPITHELIUM.

EPITHELIAL tissue is essentially a cellular tissue, the intercellular substance of which is of a cement nature. The cells vary greatly as to size, shape, arrangement, and function, but in a general way may be said to be rich in cell-body (cytoplasm), which is usually granular, and possess a clear, vesicular, smoothly outlined nucleus, containing nucleoli. With the exception of some of the cells of the genito-urinary tract (sexual glands and ducts, kidney, and ureters), all epithelium is either of epiblastic or hypoblastic origin.

For sake of convenience we may classify epithelium in three ways—according to shape of cells, arrangement of cells, and function of cells:

1. Squamous cells are thin and flattened; they may

42

58. What is this fibrous 59. Describe the blood 60. How and where do th 61. Describe the nuclei o 62. What is the shape a.. 63. Where is smooth m 64. Describe the struct: 65. How are the smoot! 66. What is the shape of 67. Describe its structu 68. How are the fibers.. 69. What are the di

muscle? voluntary and I

7

s vary, they may appear pyriform, fusiform, gobletped, or ciliated.

Columnar cells may occur in a single layer, as lining ets, glands, and in the intestine; or they may be atified, as in the trachea. In some localities the free nds of the cells are covered by numbers of very deliate hair-like processes, called cilia. These cilia wave pidly to and fro in a definite direction, with a whipike motion, and serve to produce currents in fluids or to transport foreign particles. Ciliated cells are found in the nasal fossæ, trachea, bronchi, and Fallopian tubes, and in the central canal of the spinal cord.

2. According to the uses to which epithelium is adapted, the cells may vary in arrangement.

In the intestine, lung alveoli, posterior surface of cornea, lining ducts of glands, kidney, urethra, Fallopian tubes, and central spinal canal, the cells occur in a single layer resting on underlying layers of specialized fibrous tissue, called the basement membrane. To this arrangement the name of pavement epithelium may be given.

Again, in other localities, as the skin, the mucous membrane of the mouth and vagina, the cornea, and the trachea, they are arranged in many superposed layers, and are called stratified epithelium.

In a few places, as the pelvis of the kidney, the ureters, and the urinary bladder, an arrangement of cells only a few layers in thickness is found; the outer layers of cells are not flattened to any extent, but are more cuboid or ovoid, while virtually but a variation of the stratified variety; this arrangement has received the special name of transitional epithelium.

3. Epithelium may be further classified according to its function. The protective type would include such localities as the skin and the mucous membrane of the

*

ala imetion is to protect the

und lining duets the chief Cansmit or to retain secretions. is a the bronchi and Fallopian clareas in duids and carries small aybe termed propulsive.

In glands and in the gastromaca fancten of which is either be called secretory

De specide selse—skin, tongue, nose, veel types of epithelium are found macy connected with the nervous pesses some of the characteristics paactum, and have received the neuro-epithelium.

zellig classes, there are some specialreale, was, and enamel of the teeth, rocivadves, and will be considered

CHAPTER V.

NERVES.

Nerves are long, highly specialized fibers originating in nerve-cells, carrying impulses, and usually terminating in special end-organs.

The combination of one nerve-fiber with its cell and terminals is called a neuron.

It may probably safely be said that all true nerve tissue is developed from the epiblast or ectoderm.

A nerve-cell is one of the largest cells in the body. It contains a large spheroid nucleus, rather poor in chromatin, hence staining not very deeply, and usually one or more well-marked nucleoli.

The cell-body, or cytoplasm, contains a greater or less number of granules, which stain in a characteristic manner, and are named chromophilic granules.

The nerve-cells vary greatly in shape, occurring as unipolar (one pole or projection), bipolar, tripolar, and multipolar.

One of the poles in the nerve-cell, called the axiscylinder process, is enormously extended, and becomes the axis-cylinder of the nerve-fiber.

The other processes of the cell may divide and subdivide until a dense feltwork of interlacing fibers is formed about the cell. These processes are called

dendrites.

Nerves are principally of two types-those consis