76 NORMAL HISTOLOGY. 



the Whartonian jelly of the umbilical cord. It does not occur in 

 the adult under normal conditions, except, perhaps, in the vitreous 

 humor of the eye. 



2. Reticular Tissue (Fig. 62). The fibres of this variety of ele- 

 mentary tissue are disposed in extremely delicate bundles, which 

 anastomose with each other to form a fine meshwork. The spaces 

 between the fibrous bundles are filled with lymph, which is usually 

 so crowded with cells similar to the white blood-corpuscles that the 

 structure of the tissue is masked by their presence. The cells of 

 this tissue are flattened and closely applied to the surfaces of the 

 bundles of fibres, which are so fine that they simulate delicate 

 branching processes emanating from the cells. The cement- or 

 ground-substance is reduced to a minimum, only a small amount 

 lying between the fibres and the cells of the reticulum. The tissue is 

 bounded by denser forms of fibrous tissue, with the fibrous bundles 

 of which the reticulum is continuous. It is possible that reticular 

 tissue contains stellate cells of the third variety mentioned as occur- 

 ring in fibrous tissues, as well as the thin cells already described, 

 which belong to the first variety. Where this is the case it is 

 probable that the branching processes of those cells take part in the 

 formation of the reticulum. 



Where the meshes of the reticulum are crowded with lymphoid 

 cells i. e., cells identical with some of the white corpuscles of the 

 blood the tissue has received the name " lymphadenoid tissue." 

 This tissue is the chief constituent of lymph-glands and follicles, 

 and is also found in a more diffuse arrangement in many of the 

 mucous membranes (Fig. 107, L). 



3. Areolar Tissue. This is the most widely distributed variety 

 of fibrous tissue. It contains all three kinds of cells mentioned at 

 the beginning of this section, though not always in the same relative 

 abundance. The intercellular substance consists chiefly of bundles 

 and laminse of fibres, which interlace in all directions. The white 

 fibres predominate over the elastic, but there are always some of the 

 latter which either form a wide-meshed reticulum, interlacing with 

 the bundles of white fibres, or are applied to the latter in a sort of 

 open spiral, binding them together. In the developing tissue the 

 cement- or ground-substance at first fills all the interspaces between 

 the cells and the fibres ; but as development proceeds spaces appear 

 in the tissue, which are occupied by lymph and intercommunicate 

 throughout the tissue. The ground-substance is then restricted to 



