RETICULAR TISSUE. 39 



substance. The protoplasmic processes, however, have been transformed 

 into stiff slender fibrils containing a substance known chemically as 

 reticidin. Whereas ordinary connective tissue may be made to yield 

 gelatin, reticular tissue gives both gelatin and reticulin. Since connective 

 and reticular tissues occur so closely associated that it would be difficult 

 to obtain pure specimens of the latter, the gelatin has been ascribed to 

 a mixture with connective tissue elements. On the other hand, it has been 

 asserted that reticulin is merely a variety of gelatin due to the method of 

 analysis. Reticular fibers, by their greater resistance to pancreatic 

 digestion and by dissolving in dilute acid, differ from the elastic elements 

 of connective tissue. They are said to be more resistant to acids or alkalies 

 than the fibrillar part of connective tissue. Such a distinction is hard to 

 establish, especially since some reticular tissues are more resistant than 

 others. Chemically, therefore, the validity of reticulin 

 is questionable. Histologically reticular tissue is quite 

 clearly defined by the abundance and fluidity of its 

 matrix. Small round cells, the lymphocytes, which 

 may be scattered through ordinary connective tissue, 

 are always abundant in the meshes of reticular tissue. 

 They are so numerous and closely packed that the 

 delicate reticular fibers are mostly hidden, and can be 

 studied to advantage only after the loose cells have 

 been disengaged from their meshes. This may be ac- 

 complished by shaking or brushing the sections, or by 



._..... . . / i i i F IG - 39- RETICULAR 



artificially digesting the specimen (which destroys the TISSUE FROM THE 

 reticular cells along with the others, but leaves the fibers) PIG- 



. . . n., Nucleus; f., fiber, of 



or by the method of rrot. Mall, used in obtaining r ig. 30. reticulin; i.s., in- 



' tercellular space. 



A piece of fresh spleen was distended by injecting 

 gelatin into its substance; then frozen and sectioned. The sections were 

 put in warm water which dissolved out the gelatin, carrying the loose 

 cells with it, and leaving areas of clear reticular tissue. In ordinary sec- 

 tions the student will recognize reticular tissue by the cells in its meshes, 

 but some of its nuclei and fibers can always be detected upon close ex- 

 amination. It may contain cells other than lymphocytes, for it forms the 

 framework not only of lymph glands, but of red bone marrow and the 

 spleen. A layer of reticular tissue is found under the epithelium of the 

 digestive tract, and it has been reported in many organs. 



Mucous TISSUE. 



Mucous tissue forms the substance of the umbilical cord, where it 

 was formerly called Wharton's jelly. There it occurs as a gelatinous 



