18 TISSUES AND ORGANS 



Macrophages. These peculiar elements have been the subject 

 of most intensive study during recent years ; here the account 

 given by Maximow will be followed fairly closely, because no 

 other worker has made such outstanding contributions to this 

 complex subject. It is not possible to define the subject of this 

 section more nearly than by saying that these elements form a 

 widely scattered system that plays an important role in many 

 defence mechanisms, particularly in inflammatory processes. It 

 is now realised that the so-called resting wandering cells of the 

 body really belong to one great, although scattered, cell-group, 

 which can be demonstrated by the method of intra-vitam staining 

 with a variety of more or less non-toxic colloidal substances. The 

 series of cells so made obvious is often known as the reticulo- 

 endothelium, although there are objections to the name because the 

 ordinarv endothelium of blood vessels is a structure entirely 

 different from the histiocytes and the reticular cells. The name 

 " histiocyte " is in common use, although its significance is no 

 longer accepted in its original form. It was supposed that these 

 tissue cells were totally and entirely distinct from the leucocytes 

 of the blood, but it is now commonly admitted that their histories 

 are closely intertwined. The modern convention is to term 

 histiocytes those cells stainable with certain colloidal dyestuffs 

 injected into the blood stream, but of course, in human pathology 

 and physiology, this test is but rarely possible. There are there- 

 fore many cells allocated to the histiocytic or reticulo-endothelial 

 system simply because they occupy the position or have the 

 appearance of elements known to be vitally stainable in other 

 animals. The interest in these cells, which lie so close to or, 

 indeed, are part of the supporting reticulum of the tissues, has 

 stimulated research into this reticular stroma itself, and, as a 

 result method, technical methods of great value have emerged. 

 These procedures are almost all based upon the neuro-histological 

 methods introduced by the Spanish school of Cajal. that is, they 

 are dependent upon impregnation with silver. Well-made pre- 

 parations show many surprising features, and supply some good 

 reason for the use of the name reticulo-endothelium so long as it 

 is realised that the lining of ordinary blood vessels is not included 



