ADENOID OR LYMPHOID TISSUE 37 



in the thymus, in the tonsils, and in the follicular glands of the tongue; in 

 Peyer's patches, in the solitary glands of the intestines, and in the mucous 

 membranes generally. 



Structure. Adenoid or retiform tissue consists of a very delicate network 

 of minute fibrils, figure 42. The network of fibrils is concealed by being 

 covered with flattened connective-tissue corpuscles, which may be readily 

 dissolved in caustic potash, leaving the network bare. The network con- 

 sists of white fibers, the interstices of which are filled with lymph corpuscles. 

 The cement substance of adenoid tissue is very fluid. 



Neuroglia. This form of connective tissue found in the nervous system 

 is described on page 78. 



Development of Fibrous Tissues. In the embryo the place of the fibrous 

 tissues is at first occupied by a mass of roundish cells, derived 'chiefly from 



FIG. 43. Portion of Submucous Tissue of Gravid Uterus of Sow. a, Branched cells, 

 more or less spindle-shaped; b, bundles of connective tissue. (Klein.) 



the mesoderm, but also from ectoderm and from entoderm. These develop 

 either into a network of branched cells or into groups of fusiform cells, 

 figure 43. 



The cells are embedded in a semifluid albuminous substance derived 

 probably from the cells themselves. Later this formed material is converted 

 into fibrils under the influence of the cells. The process gives rise to fibers 

 arranged in the one case in interlacing networks, areolar tissue, in the other 

 in parallel bundles, white fibrous tissue. In the mature forms of purely 

 fibrous tissue not only the remnants of the cell substance, but even the nuclei, 

 may disappear. The embryonic tissue, from which elastic fibers are devel- 

 oped, is composed of fusiform cells and a structureless intercellular sub- 

 stance. The fusiform cells dwindle in size and eventually disappear so 

 completely that in mature elastic tissue hardly a trace of them is to be found; 

 meanwhile the elastic fibers steadily increase in size. 



Adipose Tissue. In almost all regions of the human body a larger 

 or smaller quantity of adipose or fatty tissue is present. Adipose tissue is 

 almost always found seated in areolar tissue, and forms in its meshes little 

 masses of unequal size and irregular shape, to which the term lobules is com- 

 monly applied. 



