THE CONNECTIVE TISSUES 33 



epithelium of the mucous membrane of the alimentary canal, particu- 

 larly that of the small intestine, is especially adapted, from its situa- 

 tion, consistency, and properties, to play the chief r61e in the absorp- 

 tion of new materials from the canal. The epithelium lining the 

 air-vesicles of the lungs is engaged in promoting the absorption of 

 oxygen and the exhalation of carbon dioxid. 



3. To form secretions and excretions. Each secretory gland connected 

 with the surfaces of the body is lined by epithelial cells, which are actively 

 concerned in the formation of the secretion peculiar to the gland. 

 Each excretory organ is similarly provided with epithelial cells, which 

 are engaged either in the production of the constituents of the excretion 

 or in their removal from the blood. 



2. THE CONNECTIVE TISSUES 



The connective tissues, in their collective capacity, constitute a frame- 

 work which pervades the body in all directions, and, as the name implies, 

 serve as a bond of connection between the individual parts, at the same time 

 affording a basis of support for the muscle, nerve, and gland tissues. The 

 connective-tissue group includes a number of varieties, among which may 

 be mentioned the areolar, adipose, retiform, white fibrous, yellow elastic, 

 cartilaginous and osseous. Notwithstanding their apparent diversity, they 

 possess many points of similarity. They have a common origin, developing 

 from the same embryonic material; they have much the same structure, 

 passing imperceptibly into one another, and perform practically the same 

 functions. 



Areolar Tissue. This variety is found widely distributed throughout 

 the body. It serves to unite the skin and mucous membrane to the struc- 

 tures on which they rest; to form sheaths for the support of blood-vessels, 

 nerves, and lymphatics; to unite into compact masses the muscular tissue 

 of the body, etc. Examined with the naked eye, it presents the appearance 

 of being composed of bundles of fine fibers interlacing in every direction. 

 In the embryonic state the elements of this form of connective tissue are 

 united by a ground substance, gelatinous in character. In the adult state this 

 substance shrinks and largely disappears, leaving intercommunicating 

 spaces 'of varying size and shape, from which the tissue takes its name. 

 When subjected to the action of various reagents, and examined micro- 

 scopically, the bundles can be shown to consist of extremely delicate, color- 

 less, transparent, wavy fibers, which are cemented together by a ground 

 substance composed largely of mucin. Other fibers are also observed, 

 which are distinguished by a straight course, a sharp, well-defined out- 

 line, a tendency to branch and unite with adjoining fibers, and to curl up 

 at their extremities when torn. From their color and elasticity they are 

 known as yellow elastic fibers. Distributed throughout the meshes of the 

 areolar tissue are found flattened, irregularly branched, or stellate corpus- 

 cles, connective-tissue corpuscles, plasma cells, and granule cells. 



Adipose Tissue. This tissue, which exists very generally throughout 

 the body, though found most abundantly beneath the skin, around the 

 kidneys, and in the bones, is practically but a modification of areolar tissue. 

 In these situations it presents itself in small masses or lobules of varying 



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