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of the absorption of water, peptones, sugars, salts. The epithelium lining the pulmonary air vesicles is actively engaged in taking up oxygen and giving out carbon-dioxid. 3. As an eliminating agent. Waste products, however produced within the organism, must be taken up by the epithelium of the various excretory organs before being finally disposed of. The secretions of all glands are products of epithelial activity.

Connective Tissue.—The bony skeleton of the body is supplemented by a finer skeleton, composed of connective tissue, which pervades the entire body, and which, under various forms, serves as a bond of connection between its different parts, as a covering and protection for various organs, and as a basis of support for the elements of muscular, nervous, and gland. ular tissues. The connective tissues include several varieties, among which may

be mentioned areolar, adipose, fibrous, elastic, cartilaginous, and osseous. Notwithstanding their apparent diversity, they have many points of similarity. They have a common origin, developing from the same embryonic material; they have much the same structure, passing imperceptibly into each other, and functionally perform the same office, viz., supporting and connecting the specific elements of the tissues or organs.

Areolar Tissue.—This variety is found widely distributed throughout the body in all situations. It serves to unite the skin and mucous membranes to the structures on which they rest, to unite and support blood. vessels, muscles, nerves, etc. When examined with the naked eye it presents the appearance of fine, transparent, colorless fibers, of delicate membranous lamina, which cross each other in every direction, leaving spaces or areolæ between them. Examined microscopically, these fibers are found to be composed of still finer white fibers cemented together by a transparent substance containing mucin. Other fibers are distinguishable by their straight course, their dark outline, their tendency to branch and to unite with adjoining fibers. When torn across they curl up at their extremities, owing to their property of elasticity. Distributed throughout the meshes of the areolar tissue are found flattened, irregularly branched or stellate corpuscles, the connective tissue corpuscles, plasma cells, and granule cells.

Adipose Tissue.—This exists very generally throughout the body, but is found most abundantly beneath the skin, around the kidneys, and in the bones. It is composed almost entirely of small vesicles more or less completely filled with fat globules. The wall of the vesicle is protoplasmic, and contains at some points an oval flattened nucleus. Adipose tissue can arise wherever connective tissue is found. It would appear that the granules of fat are produced by a transformation of the albuminous contents of the connective tissue corpuscles. The vesicles are grouped together to form lobules, which in turn form irregular masses supported by connective tissue and blood-vessels.

Retiform Tissue.—This is also a variety of connective tissue made up very largely of white fibers interlacing in all directions. The spaces or areolæ are wanting in the usual ground substance, but are filled with fluid. Connective-tissue corpuscles are abundant, but elastic fibers are absent. Adenoid tissue is but ordinary retiform tissue, the spaces of which, however, are filled with lymph corpuscles. It is found in lymphatic glands, in the central nervous system, and other situations.

Fibrous Tissue.- White fibrous tissue is exceedingly abundant and important. It forms the ligaments which hold the bones together, the tendons of the muscles, the membranes covering bones, cartilages, the septa of muscles, etc. Fibrous tissue is tough and strong but wholly inextensible and, in consequence, is admirably adapted to fulfil various mechanical functions in the body. It is quite pliant, bending readily in any direction, but difficult to break. When examined microscopically it is found to be composed of white fibers, resembling in all respects those of areolar tissue. Treated with acetic acid they swell up and become indistinct. When boiled they yield gelatin, a derivative of collagen.

Elastic Tissue.—The elastic tissue is also an important member of the connective-tissue group. It is almost invariably associated with white fibers in some proportion, but in some tissues, as the ligamentum nuchae, the ligamenta subflava, the coats of the large blood-vessels, it exists almost alone. In its pure state it presents a distinctly yellow appearance. The fibers of which it is composed are transparent, but present a distinct outline; they run almost parallel, but give off branches which unite to form a reticulated structure. As the name implies, these fibers are very extensible and elastic. Cartilage and bone have been considered in connection with the skeleton.

Physical and Physiological Properties of Connective Tissue.Among the physical properties may be mentioned consistency, which varies from the semi-liquid to the solid state. This variation depends upon the quantity of water in the individual tissues. Their cohesion, with the exception of the softer varieties, is considerable, and offers great resistance to traction, pressure, torsion, etc. In the various movements of the body, in the contraction of muscles, in supporting weights, in diminishing the effects of shocks, the properties of consistence and cohesion play important parts. Wherever the various forms of connective tissue are found, their chemical



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