THE LYMPHATIC SYSTEM. 



935 



All the lymphatic vessels terminate directly or indirectly in one of two main 

 trunks, which, as already stated, open respectively into the right and left subclavian 

 veins. The left trunk, the thoracic duct (ductus thoracicusj, is much larger than 

 the right, beginning in the abdominal region and traversing the entire length of the 

 thorax to reach its destination. It recei\'es all the lymph returned from the lower 

 limbs, the pelvic walls and viscera, the abdominal walls and viscera, the lower part of 

 the right half and the whole of the left half of the thoracic wall, the left half of the 

 thoracic viscera, the left side of the neck and head, and the left arm. The other 

 trunk, the right lymphatic duct (ductus lymphaticus dexter), is very short, and, 

 indeed, is frequently wanting, the vessels which typically unite to form it opening 

 independently into the vein. It receives the lymph from the upper part of the right 

 side of the thoracic wall, from the right half of the thoracic viscera and the upper surface 

 of the liver, the right side of the neck and head, and from the right arm (Fig. 785). 



In structure the larger lymphatic vessels are similar to the veins, but, as a rule, 

 their walls are thinner than those of veins of corresponding calibre and their valves 

 are more numerous. The walls of the most robust trunks, particularly those of the 

 thoracic duct, consist of three coats. From within outward these are : (a) the 

 infima, composed of the endothelial lining and the fibro-elastic subendothelial layer ; 

 {b) the media, made up of involuntary muscle interspersed with fibro-elastic tissue ; 

 and (c) the adventitia, consisting of fibro-elastic tissue and, frequently, of longitudinal 

 bundles of involuntary muscle. (,Fig. 784.) 



The Lymphatic Nodes. — Scattered along the course of the lymphatic vessels 

 are to be found in various regions of the body elliptical flattened nodules (Fig. 796") 

 of varying size, some- 

 times singly but more Fig. 786. 

 frequently in chains or 

 groups ( plexus lym- 

 phatici) of from three 

 to six or even ten to 

 fifteen. These are the 

 lymphatic nodes (lym- 

 phoglandulae). As it 

 approaches a node, a 

 lymph - vessel divides 

 into a number of stems, 

 the vasa afferentia, 

 which enter the sub- 

 stance of the node and 

 communicate with a 

 capillary net-work in 

 its interior, from which 

 a somewhat smaller 



Medullary 



cords 



Efferent 



lymphatics 

 emerging 

 from hilum 



Lymph-sinus 



Capsule 



Cortical follicle 



Afferent 



lymphatics 



Lymph-sinus 



Diagram illustrating architecture of lymph-node. 



number of vessels, the 

 vasa efferentia, arise 

 (Fig. 786). These, 

 leaving the node, the 

 surface of which fre- 

 quently presents a 

 slight depression, the 

 hilum, at their point of emergence, unite to form the continuation of the vessel. 

 The lymph conveyed by any of the vessels traverses one or more nodes before 

 emptying into the thoracic or right lymphatic duct, and in those cases in which 

 a plexus occurs in a lymph-path a number of nodes must be traversed. The 

 passage through the intranodular net -work produces a greater or less retardation of 

 the flow of the fluid and affords opportunity for the accumulation of lymphocytes. 

 Moreover, since these possess a phagocytic function, in cases of infection of any 

 part of the body the nodes along the lymph-paths leading from it become more 

 or less engorged with lymphocytes and enlarged, and in case the lymphocytes are 

 unable to contend successfully with the infective material, the nodes may serve as 



