92 



THE INDIVIDUAL ORGANISM 



(Fig. 6.6), closed at the ends but communicating freely with each other 

 and fusing to form larger and larger vessels. The latter resemble veins 

 and, like them, are provided with valves that permit the movement of 

 lymph only in one direction, toward the heart. The large lymph vessels 

 finally converge to the region of the left shoulder, where the main thoracic 

 duct empties the lymph into a large vein of the blood circulatory system. 

 This duct carries the lymph from all parts of the body except the right 

 side of the head, neck, and thorax and the right arm; the lymph from 



blood capillaries 



lymph capillaries 



tissue spaces cells 



Fig. 6.5. Diagram of the relationships between blood capillaries, tissue spaces, cells, and 

 lymph capillaries in a tissue. 



those regions enters the corresponding vein on the right side of the body. 

 Movement of the lymph in the vessels is brought about by muscular 

 movements of the body that incidentally compress the lymph vessels and 

 squeeze the lymph in the direction determined by the valves. 



The intimate contact between the lymph and the tissue cells supple- 

 ments and completes the functioning of the circulatory system proper by 

 permitting a much more complete interchange between the circulating 

 medium and the body tissues. The lymphatic system also provides an 

 important means of defense against the invasion of the body by disease- 

 producing organisms. Most of the lymph vessels have, at intervals, 

 small filterlike enlargements, the lymph nodes, where numerous phagocytes 

 are concentrated. Here invading bacteria, as well as worn-out cell frag- 



