LYMPHATIC SYSTEM 333 



tained throughout life, but usually only one persists. This begins at the cistern 

 of chyle in the lumbar region and empties into the left brachiocephalic vein 

 near the entrance of the single cephalic lymph duct. The thoracic duct receives 

 the lymph-vessels from the limbs and others (lacteals) from the alimentary 

 canal. In those cases where there is but a single thoracic duct in front, its 

 representative on the right side is a much smaller vessel connected with the 

 right side of the venous system. No lymph hearts are known in the mammals. 

 The jugular lymph sacs of the embryo have been regarded as such, but the 

 absence of valves and muscles in the walls renders such an interpretation 

 doubtful. 



As a part of the lymphatic system are to be enumerated l3miph 

 follicles, lymph glands (or knots) and blood lymph (haemolymph) 

 glands. The follicles (often called Peyer's patches) appear in the 

 intestinal wall from reptiles upward. They consist of reticular 

 connective tissue crowded with leucocytes (lymphocytes). In birds 

 and mammals (their existence in sturgeon and crocodiles is doubted) 

 the lymph glands are found. These are aggregates of lymph follicles, 

 enclosed in a common capsule, and in close relation to lymph-vessels 

 which send afiferent and efferent branches into them, these breaking 

 up into a network of sinusoids on the interior. They occur in the 

 neck region of birds, and in the mesenteries of mammals. 



The spleen is in some respects intermediate between the lymph 

 glands and the blood-lymph glands. It lacks the afferent and 

 efferent lymph-vessels and has a rich blood supply. It arises in the 

 wall of the alimentary canal as an aggregation of mesenchyme cells, 

 and the statement that there are entodermal contributions to it has 

 been contradicted. The differences in its position in the different 

 groups leads to the view that it originally belonged to a greater 

 length of the digestive tract, and that different portions have de- 

 veloped in the separate classes of vertebrates. In the cyclostomes 

 there is no distinct spleen, but cavernous tissue in the intestinal wall 

 has been interpreted as splenic. In many fishes it is a compact 

 organ, while in others it is composed of separate segments distrib- 

 uted along a considerable length of the canal. In turtles and frogs 

 it lies along the intestine, while in urodeles, reptiles, birds and most 

 mammals it is a compact body near the stomach. In monotremes 

 it is three-lobed, the posterior lobe extending back along the intes- 

 tine. The spleen serves, apparently, as a place for the disintegra- 

 tion of the red blood corpuscles as well as functioning as a leucocyte- 

 forming organ. 



The tonsils (p. 266) belong to the category of lymphoid struc- 



