CHAPTER IV.

EPITHELIUM.

EPITHELIAL tissue is essentially a cellular timur tu intercellular substance of which is of a cement matte The cells vary greatly as to Size, Sipe, arrangemeır” and function, but in a general way may **

rich in cell-body (cyt yliau, wild is teua y grama and possess a clear, vesicular, smoothly outlines meses containing nucleoli. With the exegrin off-d' ta cells of the genito-urinary tract 22:21 HA ducts, kidney, and ureters, epithelium » eilut epiblastic or hypoblastie origit,

For sake of convenience we Tury Pete es de in three ways—according to age of ele qe of cells, and function of bles

1. Sqramons 23 m na mí firmned; meg mag

be found in the outer layers of stratified and transitional epithelium, as in the skin, cornea, vagina, and mucous membranes of the buccal cavities, or in a single layer (pavement) chiefly lining the alveoli of the lungs, on the posterior surface of the cornea, and on the anterior surface of the iris. The cells often vary somewhat in thickness and peripheral outline, and have been variously described as roundish. ovoid, polygonal,

Fig. 15.-Epithelial cells from various localities: a, Squamous cell; b, prickle cell from skin; c, columnar ciliated cells; d, goblet cells; e, stratified epithelium.

and stellate. The stellate forms have numbers of very irregular prolongations, frequently branching. These cells frequently contain granules of pigment.

Columnar cells are longer than they are broad, and are placed with their long diameters vertical to the surface. According to their cross-section, they will be cylindric, prismatic, or cuboid. As their vertical sec

tions vary, they may appear pyriform, fusiform, gobletshaped, or ciliated.

Columnar cells may occur in a single layer, as lining ducts, glands, and in the intestine; or they may be stratified, as in the trachea. In some localities the free ends of the cells are covered by numbers of very delicate hair-like processes, called cilia. These cilia wave rapidly to and fro in a definite direction, with a whiplike 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. this arrangement the name of pavement epithelium may be given.

To

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

ments at each extremity, placed end to end. The spaces between the contracting elements are filled by a clear substance, called sarcoplasm. As the sarcostyles run parallel with their composing elements, opposite each other, the appearance of the contiguous thick centers will be that of the dark band. The thin ends, surrounded by clear sarcoplasm, will show as the light band. The enlargements at the thin ends will produce a row of granules, which are so closely placed as to appear as a line-the membrane of Krause. The median line of Hensen is produced by a slight thinning of the contracting elements just at their centers, and by refraction of light. The individual fibers of voluntary muscle are bound together by very delicate fibers of white fibrous tissues, called the endomysium, to form bundles (the fasciculi). These fasciculi are surrounded and bound to other fasciculi by a larger band of white fibrous tissue, called the perimysium, and these fasciculi are grouped together to form the large muscles as found in dissection, and are surrounded by a still larger mass of fibrous tissue (the epimysium), which constitutes the sheath of the muscle. These connective tissues serve also to support numerous nerves, bloodvessels, and lymphatics. The blood-vessels break up into capillaries, which form a fine network around the individual muscle-fibers. The nerves terminate in special end-organs under the sarcolemma.

SMOOTH MUSCLE.

Smooth, plain, involuntary, or nonstriated muscle is found chiefly in the gastro-intestinal tract, the arteries around the various ducts, in the genitourinary tract, and in the skin and capsules of organs. It contracts slowly and is not subject to the will. Smooth

SMOOTH MUSCLE.

39

muscle is composed of spindle-shaped cells, covered by a fine sheath, which, while varying considerably in size in various locations, may be said to average about 250 of an inch in length and of an inch in width. In the center of each fiber is an elongated, rod-shaped

nucleus. Very fine longitudinal striations may be seen in especially favorable sections. The fibers are held together by cement substance and by interlacing of their ends. Occasionally, extremely fine spicule branch at right angles to the fibers and aid in uniting them.