LYMPHOID OR ADENOID TISSUE. 85 



Lymphatic vessels contain numerous valves, formed by 

 transverse folds of the inner tunic. Associated with the valves 

 are alternate dilatations and constrictions of the lymphatic 

 vessels, which give them a characteristic beaded appearance. 



The thoracic duct is the largest and best developed of the 

 lymphatic vessels. Its inner tunic contains a considerable 

 amount of subendothelial connective tissue and longitudinally 

 arranged elastic fibres. Its outer coat contains scattered 

 bundles of longitudinal involuntary muscle. 



Lymphoid or adenoid tissue consists of an open connective- 

 tissue network, the meshes of which are crowded with free 

 cells, called lymphoid cells, the whole communicating with 

 lymphatic vessels. The connective-tissue reticulum which 

 forms the basis of typical lymphoid tissue has already been 

 described under the name retiform tissue; it consists of fine 

 interlacing fibrils covered or lined at their intersections with 

 flat stellate connective-tissue or eudothelioid cells. The 

 meshes formed by this structure are filled with lymphoid cells, 

 small free spherical cells with relatively large and prominent 

 deeply staining round nuclei and small cell-bodies. These are 

 ofteu packed in so densely as to obscure and hide the reticular 

 basis. The meshes communicate freely with one another and 

 with the interior of entering or afferent and departing or 

 efferent lymphatic vessels, so that the tissue forms a lymph- 

 space, and through it lymph constantly flows. Adenoid tissue 

 is also supplied with blood-capillaries. 



Lymphoid tissue exhibits all gradations from diffuse masses 

 without definite boundaries to sharply circumscribed nodules. 

 Diffuse lymphoid tissue occurs imbedded in connective tissue, 

 and merges insensiblv into the surrounding tissues. It pre- 

 sents all grades from ordinary connective or areolar or even 

 epithelial tissue infiltrated scantily or densely with wandering 

 lymphoid cells to typical dense adenoid tissue with a retiform 

 basis. 



In other cases lymphoid tissue forms well-defined nodules, 

 more or less spherical in shape, often quite sharply circum- 

 scribed by a condensation of the surrounding fibrous tissue. 

 Such nodules are often called lymph-follicles (Fig. 38). The 

 afferent lymph-vessels are said to open into the periphery 

 of such nodules or follicles. The tissue in the centre of the 



