CELLULAR ELEMENTS OF THE GROUND-SUBSTANCE. 17 



spindle-shaped cells carefully under a high power, one soon finds out 

 that they are only the profile of somewhat flattened branched cells, 

 exactly the same as the other cells of the reticulum. The meshes of 

 the reticulum appear perfectly clear, and contain only here and there 

 a lymphoid corpuscle. We shall return in the chapter on lymphatic 

 vessels to these patches and cords, where we shall be able to recog- 

 nise and understand better their intimate structure. 



We will, however, anticipate, by calling the patches and tracts 

 which we have been considering up to the present lymphangial 

 patches (nodules) and lymphangial tracts. 



We may therefore say that there exist in the omentum of the 

 rabbit two kinds of lymphangial structures : 



a. Patches, the matrix of which consists of groups of ordinary, 

 more or less flattened, more or less branched cells, which on the 

 one hand 'multiply by division, in which way the patch increases 

 in size, and from which, on the other hand, grow up lymphoid 

 cells. The branched cells lie in the lymph-canalicular system, together 

 with the lymphoid cells. At an early stage of development these 

 patches do not contain a special system of blood-vessels ; at a later 

 time they possess a special rich system of mostly capillary blood- 

 vessels. By growing in length these patches join so as to form 

 whole tracts. 



b. Patches and tracts, the matrix of which consists of a recti- 

 culum, the meshes of ivhich contain a variable number of lymphoid 

 corpuscles : they are generally provided with more or less abundant 

 blood-vessels. 



We come now to deal with the omentum of the guinea-pig, cat, dog, 

 and monkey, the omentum of which animals is of a very similar 

 structure, and at the same time perfectly analogous to that of the 

 pleura-mediastini. Consequently, what will be said of the former 

 holds good also for the latter. 



In the tissue of the omentum of these animals similar lymphan- 

 gial structures are to be met with, which, owing to their greater 

 diameter in thickness, we shall not name patches, but nodules. By 

 their size we may distinguish them as nodes and nodules, and we 

 may say as well here as in the rabbit's omentum that they join so as 



