LYMPHATIC SYSTEM AND DUCT/LESS GLANDS. 351 



head and neck. The thoracic duct, thus, brings back to the 

 blood much more lymph than the right lymphatic duct. 



The Serous Cavities. These are great dependencies of 

 the lymphatic system and may be regarded as large lacunae. 

 Each of them (peritoneal, plenral, arachnoidal and pericar- 

 diac) is lined by a definite epithelioid layer of close-fitting 

 polygonal cells. At certain points, however, openings or 

 stomata occur, surrounded by a ring of smaller cells, and 

 leading into tubes which open into subjacent lymphatic 

 vessels. The liquid moistening these cavities is, then, really 

 lymph: in some dropsical diseases it collects in great excess 

 in them. 



Lymphoid or Adenoid Tissue is the name given to cer- 

 tain aggregations of slightly differentiated cells (leucocytes) 

 supported by a peculiar form of tissue and found in con- 

 nection with the lymphatic system in many parts of the body. 

 The cells much resemble white blood--corpuscles, though their 

 nuclei usually have a more distinct network, and they are 

 capable of executing amoeboid movements. Many of them 

 ultimately are carried by the lymph into the blood to be- 

 come pale corpuscles, and from the blood some again pass 

 back into the lymph by migrating through the walls of 

 the blood-capillaries. By amoeboid movement these lymph- 

 corpuscles can take up foreign particles into themselves 

 and creep with the absorbed material along lymph-canaliculi 

 and lymph-capillaries. Lymphoid tissue is extensively devel- 

 oped in the mucous membrane of a great part of the ali- 

 mentary canal. 



The deepest layer of the mucous membrane of stomach 

 and intestines, lying next to the submucous coat is the mus- 

 cular is mucosce, a thin layer of unstriped muscular tissue quite 

 distinct from the proper muscular coats of those viscera. Above 

 it and forming the main bulk of the mucous membrane lying 

 between the glands (o, Fig. 112) and, in the small intestine, 

 the main mass of the villi, is a delicate connective tissue con- 

 sisting of very fine fibres which originated by the branch- 

 ing of cells; in many places the nuclei of these cells have quite 

 disappeared, and the original central part of the cell is only 

 recognizable as the place from which the branches spread : such 

 tissue is reticular connective tissue. Its meshes contain many 

 leucocytes, and the mixture of reticular tissue with these cells 

 constitutes adenoid or lymphoid tissue. At numerous spots, 



