56 CONNECTIVE TISSUE CARTILAGE BONE 



highly refractive appearance. Their digestion in pepsin begins only after 

 an interval of two hours, while white fibers are digested in a few minutes ; 

 they also stain less readily than white fibers and yield reticulin, which 

 differs somewhat from the gelatin of white fibrous tissue. The intimate 

 histologic relation between the reticular and white fibrous tissue is shown 

 by the fact that the two tissues are frequently continuous and exhibit 

 similar staining reactions. 



Mall (Amer. Jour, of Anat., 1902) has attempted to show that reticu- 

 lar tissue should be considered as that form of connective tissue which 

 has been least differentiated from the embryonic mesenchymal type. 

 He accordingly considers the cells of the reticulum as formed by the un- 

 differentiated endoplasm, and the reticular fibers as representing the 

 specialized exoplasm of this most primitive type of true connective 

 tissue. In the liver, the reticulum arises from the endothelial cells of von 

 Kuppfer instead of from mesenchyme (Mall). 



Loose Fibro-elastic or Areolar Connective Tissue. Loose fibro- 

 elastic or areolar connective tissue (Fig. 57) is the most widely distrib- 

 uted of all the varieties; it fills all otherwise unoccupied spaces within 

 the body, and in all microscopical sections areolar tissue is almost in- 

 variably to be found. It is also known as loose connective tissue in. con- 

 tradistinction to the more compact or dense varieties. This tissue con- 

 nects the skin with the underlying structures, maintains the position and 

 relation of adjoining muscles, surrounds the heart and its great vessels, 

 envelops the abdominal viscera as submucous and subserous sheets, occu- 

 pies the spaces of the mediastinum, and fills similar intervals between 

 the various organs in all parts of the body. Areolar tissue of course 

 varies in the degree of its laxity or density. 



The ground substance of areolar tissue is a coagulable fluid, the tissue 

 juice. Solutions of silver nitrate injected into the interstices of areolar 

 tissue coagulate its tissue juice or ground substance and darken it 

 slightly. It is then seen to be permeated by broad lymphatic channels, 

 which are lined by delicate endothelioid mesenchymal cells (W. G. Mac- 

 Callum, Arch. f. Anat., 1902; also Bull. Johns Hopkins Hosp., 1903). 



Both collagenous and elastic fibers occur in areolar tissue, the former 

 being far in excess of the latter. The comparatively loose reticular ar- 

 rangement of the fibers of fibro-elastic tissue affords a most favorable 

 opportunity for the study of these connective tissue elements. . 



The collagenous or white fibers in mature tissues are invariably ar- 

 ranged in bundles which interlace with one another to form an open net- 

 work. Each bundle consists of a number of very fine fibers whose 



