MACROPHAGES 19 



in the conception. In lymphatic glands the sinuses are lined by 

 elongated cells usually known as endothelium in spite of many 

 structural differences between its cells and those of vascular endo- 

 thelium. In this section there will be no discussion of the lining 

 cells of ordinary blood vessels, and therefore the term " endothelial 

 cell " will be applied without further apology to elements of 

 the type that line lymphatic sinuses in lymphatic glands. The 

 boundary of such sinuses has long been recognised as not being 

 composed of a complete and continuous layer of cells, but there are 

 nevertheless no distinct gaps in the continuity of the walls. The 

 completeness of the boundary has been supposed to result from 

 the inter-digitation of the elongated processes of the endothelial 

 cells, but to-day this is not regarded as being correct, because two 

 types of cell can be found in the wall. First there is the endo- 

 thelial cell, which does not seem to have any very obvious processes, 

 and secondly, there is the so-called reticular cell which has only a 

 small cell body and extensive branching processes. In many 

 pathological conditions the lining of the lymphatic sinuses differs 

 very much from normal. For example, in Hodgkin's disease, 

 when seen in an early stage, there are surprisingly few reticular 

 cells, whereas endothelial elements are very greatly increased in 

 number. Later, in the stage of so-called fibrosis, the appearances 

 are very differer^. Lymphocytes and endothelial cells are very 

 scanty, whereas fine fibrils are numerous and almost completely 

 fill the tissue. Giant cells which were fairly numerous in the 

 earlier cellular stage are now difficult to find, so that there is an 

 impression as of a diffuse fine fibrosis. The two histogenetic 

 possibilities that immediately present themselves are either that a 

 new cell type has ousted the endothelial elements or that there has 

 been conversion, that is, metaplasia, of the earlier type of elements 

 into the final variety of tissue. There is no evidence that the 

 former process is possible, whereas the evidence for the latter is 

 quite sound. Suitably stained and impregnated sections of the 

 tissues in the earlier stages of the disease show peculiarly intimate 

 relations between the endothelial cells and the fairly scanty 

 argentaffin fibrils, inasmuch as the former are obviously sur- 

 rounded bv a close network of fine fibrils. It is only on verv 



