LYMPHATIC SYSTEM 335 



(e.g., alimentary canal, bronchi, conjunctiva, urinogenital organs) 

 and is particularly abundant in Fishes, Dipnoans, and Am- 

 phibians (pp. 267, 352, 363). 



The migration of the amoeboid leucocytes to the surface (p. 267) is due 

 to various causes. It may simply result in getting rid of superfluous 

 material, or may be of considerable importance in removing broken-down 

 substances and harmful bodies (e.(/., inflammatory products, Bacteria), the 

 particles being ingested by leucocytes (hence often called phagocytes) 

 before the latter are got rid of. 



The mass of lymphoid tissue 011 the heart of the Sturgeon, and possibly 

 also the so-called fat-bodies (corpora adiposa) of Amphibia and Reptilia 

 (pp. 368, 370), and the "'hibernating gland" of certain Rodents, may be 

 placed in this category ; they consist of lymphoid and fatty tissue, and serve 

 as stores of nutriment. 



The agglomeration of a number of lymphoid follicles gives rise 

 to those structures which are spoken of as "lymphatic glands 9 ' 

 or adenoids. These are always interposed along the course of 

 a lymphatic trunk so that afferent and efferent vessels to each 

 can be distinguished. They probably appear first in Birds, and 

 are most numerous in Mammals, where they are present in 

 abundance in various regions of the body ; they differ greatly in 

 size. 



The spleen which is present in almost all Vertebrates, is 

 closely related to these structures. It corresponds to a specially 

 differentiated portion of a tract of lymphoid tissue primarily 

 extending all along the alimentary canal, and in Protopterus 

 it still remains enclosed within the walls of the stomach 

 (Fig. 209). In other Vertebrates it is situated outside the 

 walls of the canal, but even then may extend along the greater 

 part of the latter (e.g., Siren). Usually, however, either the 

 proximal or the distal portion of it undergoes reduction, and the 

 organ is generally situated near the stomach, though it is occa- 

 sionally met with in other regions of the intestinal tract, as, for 

 instance, at the commencement of the rectum (Anura, Chelonia). 

 In some cases (e.g., Sharks) it is broken up into a number of 

 smaller constituents. 



The tonsils are also adenoid structures. They are most 

 highly developed in Mammals, where they give rise to a paired 

 organ lying on either side of the fauces that is, in the region 

 where the mouth passes into the pharynx, and usually also to a 

 mass situated more posteriorly on the walls of the pharynx 

 itself (pharyngeal tonsils); the latter are phylogenetically the 

 older organs and are present in Reptiles, Birds, and most Mam- 

 mals. 1 The tonsils consist of a retiform (adenoid) connective- 

 tissue ground-substance enclosing a number of lymph-corpuscles, 

 which are arranged in so-called follicles, and are capable of mi- 

 grating to the surface. 



1 Tonsil-like organs are also present in Amphibians. 



