'04 



THE LYMPHATIC SYSTEM 



process be a filtration and diffusion— in which case the causes would lie in the pressure and mo- 

 lecular condition of the tissue fluid outside the lymphatic — or whether it be an active secretion 

 by the endothelium — in which case the driving force would be this secretory power of the 

 endothelium. 



3. THE LYMPHOID ORGANS 



Closely associated with the lymphatic capillaries and vessels is a group of 

 glandular structures known as lymphoid organs. They consist, essentially, of 

 groups of round lymphoid cells, lying in a meshwork of reticulum fibres, and hav- 

 ing often a definite relationship to the blood or lymph vessels. 



The group of lymphoid organs includes, in addition to the lymph-glands 

 [lymphoglandulse] or lymph-nodes, which are particularly related to the lymphatic 

 vessels, the spleen, thymus and bone-marrow, which are also largely made up of 

 lymphoid tissue. The spleen and thymus, however, are considered separately 

 with the Ductless Glands. 



Fig. 551. — Diagram of a Lymph-node. (After Toldt, "Atlas of Human Anatomy, "Rebman, 



London and New York.) 



Capsule 



Mf^dullary cords 



Peripheral sinus 



r" Capsule 



Deep lymph 



Anastomosis between afferent and efferent vessel 



Trabeculae 



Hilus I 



Efferent vessels 



In their most simple form, the lymphoid organs form mere irregular accumulations or patches 

 of lymphoid cells, which have been termed lymphoid infiltrations. Such patches are frequent 

 in mucous membranes especially along the intestinal tract (fig. 549) and the air-passages in the 

 lungs. 



Larger accumulations of lymphoid cells produce definite round nodules, which may occur 

 singly, as solilary follicles or in groups, as aggregated follicles (Peyer's patches) (fig. 548). In the 

 solitary follicle the lymphoid cells are arranged concentrically, with a region in the centre where 

 the cells are less closely packed together. This is called the germinal centre, and contains 

 numerous cells undergoing mitotic division. The sohtary follicle contains blood-capillaries. 

 Lymph-capillaries, however, do not enter the follicle but form a rich plexus about it. 



The lymph-glands or nodes (fig. 551) are larger lymphoid structures, which are developed 

 along the course of tlie lymph-vessels. They vary nmch in size, shape, and colour, and may occur 

 singly or in small or large groups. The size varies from the size of a pin-head to that of an olive, 

 or larger. In shape they may be spherical, oval, or flattened on one or more sides, according to 

 their relations to other organs. Each gland has an indentation or hilus, where the arteries 

 enter, and wliore the veins and efferent ducts emerge. Their colour depends upon position and 

 state of function. The glands along the respiratory tract are black, due to the presence of car- 

 bon granules. The mesenteric glands are milk-white during digestion, and other nodes are pale 

 and translucent when their sinuses are filled with fluid, and pink or even red when red-blood 



