LYMPHATIC SYSTEM 435 



Aggregations of leucocytes giving rise to follicles are present 

 in the lymphoid or adenoid tissue which occurs beneath the 

 mucous membrane in various parts of the body (e.g. tonsils, ali- 

 mentary canal, bronchi, conjunctiva, urinogenital organs): these cells, 

 owing to their power of amoeboid movement, have a tendency to 

 wander through the mucous membrane to the surface, and thus 

 doubtless to remove useless and harmful material (e.g. broken down 

 particles, inflammatory products, Bacteria): in some cases such 

 leucocytes (phagocytes}, instead of passing out from the surface, 

 possibly serve to carry material from one part of the body to 

 another. 



Lymphoid tissue is very abundant in the body-cavity of Fishes 

 and Amphibians : it occurs in the alimentary canal and round the 

 blood-vessels, but is especially abundant in the neighbourhood of the 

 urinogenital organs. To the same category belong the so-called " fat- 

 bodies " (corpora adiposa) of Amphibia and Reptilia, as well as the 

 mass of lymphoid tissue on the heart of the Sturgeon and possibly 

 also the "hibernating glands" of certain Rodents and Insectivores. 



The agglomeration of a number of lymphoid follicles gives rise 

 to those structures which are known as " lymphatic glands " and 

 as " hcemolymph "or " "blood- glands" which, in spite of certain charac- 

 teristic differences, have much in common as regards the structure 

 of their enclosing capsules and of the network of connective tissue- 

 trabeculse extending through them, and various intermediate 

 forms occur. 1 The spleen belongs to the same category as these, 

 and is in a sense intermediate in structure between the more pri- 

 mitive haemolymph gland and the more highly differentiated 

 lymphatic gland, the three forming an almost continuous series. 

 In the last-mentioned the lymph, and in the two former the blood, 

 passes into a non-tubular network, where it becomes filtered ; the 

 worn-out red corpuscles are thus retained and become taken up 

 and digested by the lining epithelial cells and the leucocytes in 

 the process of phagocytosis. All three of these organs, moreover, 

 like the bone-marrow, have the common function of giving rise to 

 new lymph-corpuscles. The spleen, which is present in all or 

 almost all Vertebrates (?Cyclostomes), arises in the region of the 

 mesentery, and has usually close topographical relations to the 

 pancreas: it corresponds to a specially differentiated portion 

 of a tract of lymphoid tissue primarily extending all along the 

 alimentary canal, and in Protopterus still remains enclosed 

 within the walls of the stomach (Fig. 247). In most other Verte- 

 brates it is situated outside the walls of the canal, but even then 

 may extend along the greater part of the latter (e.g. Siren). 

 In some cases, however, either the proximal or the distal portion 

 of it undergoes reduction, or only the middle part persists 



1 Lymphatic glands are most numerous in Mammals, and occur along the 

 lymph-trunks ; haemolymph glands have been found in numerous Mammals and in 

 certain Birds and Fishes. 



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