Chap. 7 TISSUES 113 



oxygen, food, water — and the metabolic products which tissues pass on to 

 the blood and lymph must all go through connective tissues. Like epithelium 

 it is a screen through which substances pass to and fro. The characteristic 

 of the body known as its constitution is probably connected with properties 

 of the loose connective tissue. Abnormal growths such as tumors persist or 

 fail to develop, depending to some extent on the reactions of this tissue. In 

 its defensive response the phagocytic cells called macrophages (large eaters) 

 which originate in it are the main actors. These cells are scattered throughout 

 the body and are ordinarily quiet, but if properly stimulated, as by infection, 

 they become mobilized like an army, enlarged, and active. 



The structure of loose areolar connective tissue is typical of all connective 

 tissue (Fig. 7.5). It is composed of: (1 ) cells, such as macrophages, fibroblasts 

 (associated with the formation of fibers); (2) nonliving collagenous white 

 and elastic yellow fibers; (3) a thin jellylike ground substance. Collagenous 

 fibers are so-called because they contain a protein, collagen, which on boiling 

 yields glue and gelatin. In areolar tissue they run in all directions, are very 

 flexible and resistant, but are not elastic. They are really bundles of very, 

 very fine cross-striated fibrils, but these are invisible except by special tech- 

 niques. Elastic fibers appear as single strands, branched and like rubber 

 bands; when a pull is released they return to their original length. Areolar 

 tissue pulls the skin into place after it has been pinched up from the back of 

 the hand, more quickly in a younger than an older person; it also surrounds 

 organs. Dense areolar tissue, the dermis of the skin, is the fibrous part of 

 leather. 



In many ligaments and tendons collagenous fibers are predominant and 

 compactly arranged according to the strains put on them. They are densely 

 woven like felt in the sclerotic coat commonly called the white of the eye. 

 Connective tissues often contain very few collagenous white fibers and many 

 yellow elastic ones, the latter so abundant that the whole tissue is elastic. 

 This is the case in the nuchal ligament of grazing animals: a strap of ex- 



FiG. 7.5. Connective and supporting tissues. Top, cross section through the human 

 tailor's or sartorius muscle showing how muscle cells are held together by a web 

 of interlacing strands of connective tissue, the white lines in most cross cuts of 

 meat. This muscle is the longest in the body originating on the hip, crossing the 

 thigh obliquely, extending down the leg, and attached to the inner side of the shin 

 bone. Bottom, microscopic structure of the loose areolar connective tissue of a 

 kitten, spread out and stained to show its parts. This tissue tears like paper as one 

 skins an animal, a tissue with many open spaces, c, non-living collagen (or pro- 

 tein) white; e, elastic yellow fibers; /, fibroblasts, the cells associated with produc- 

 tion of the fibers; /, lymphocytes; m, macrophages, the cells that consume bacteria 

 and foreign particles; m^, mast cells, function unknown. [Top, courtesy, Maximow 

 and Bloom: Textbook of Histology, ed. 6. Philadelphia, W. B. Sanders Co., 1952. 

 Bottom, courtesy, Nonidez and Windle: Textbook of Histology, ed. 2. New York, 

 McGraw-Hill Book Co., Inc., 1953.) 



