114 NORMAL HISTOLOGY. 



Behind each valve the wall of the vein bulges slightly. Single 

 valves of similar structure not infrequently guard the orifices by 

 which the smaller veins discharge into those of larger size. 



5. The Lymphatics. The lymph at first lies in the minute inter- 

 stices of the tissues surrounding the bloodvessels from which it has 

 transuded. In most parts of the body those tissues are varieties 

 of fibrous connective tissue, and contain not only the small crevices 

 between their tissue-elements, but larger spaces also, which have 

 a more or less complete lining of flat endothelial cells, but permit 

 the access of lymph to the intercellular interstices of neighboring 

 tissues. The lymph finds its way into these " lymph-spaces," and 

 thence into the lymphatic vessels. These begin either as a network 

 of tubes with endothelial walls, or as vessels with blind ends, and 

 have a structure similar to that of the blood-capillaries. They are 

 larger, however, and are provided with valves. By their union 

 larger vessels are formed, resembling large veins with very thin and 

 transparent walls, consisting of intima, media, and adventitia. 

 These finally unite into two main trunks, the thoracic duct and the 

 right lymphatic trunk, which open into the subclavian veins. 

 Valves are of much more frequent occurrence in the lymphatic 

 vessels than in the veins, but their structure is the same. 



In its passage through the lymphatic circulatory system the 

 lymph has occasionally to traverse masses of reticular tissue con- 

 taining large numbers of lymphoid cells, called " lymph-glands." 



That portion of the lymphatic system which has its origin in the 

 walls of the intestine not only receives the lymph which transudes 

 through the bloodvessels supplying that organ, but takes up also a 

 considerable part of the fluids absorbed from the contents of the in- 

 testine during digestion. Mixed with this fluid is a variable amount 

 of fat, in the form of minute globules. These globules give the con- 

 tents of these lymphatics a milky appearance, and the vessels of 

 this part of the lymphatic system have, therefore, received the name 

 " lacteals." They do not differ essentially from the lymphatics in 

 other parts of the body. 



Lymph-glands. It is a misnomer to call these structures glands, 

 for they produce no secretion. A better term is " lymph-nodes." 



The lymph-nodes are bodies interposed in the course of the 

 lymphatic vessels through which the lymph-current passes. Their 

 essential constituent is lymphadenoid tissue. 



Each node has a spherical, ovoid, or reniform shape, with a de- 



