334 - Multicellular Animals, Especially Man 



The Lymphatic Circulation. In the body it 

 is difficult to find the lymph vessels, or lym- 

 phatics, because they are so delicate. How- 

 ever, the network of lymph vessels (Fig. 17-17) 

 is very extensive and reaches all parts of the 

 body. The lymphatics drain lymph directly 



Fig. 17-17. Part of the lymphatic system of man. On 

 left, thoracic duct (a) opening into left subclavian vein 

 (b); c, internal jugular vein. On right, lymphatics of 

 hand and forearm; d, lymph nodes. (From The Living 

 Body, by Best and Taylor. Holt, Rinehart and Winston, 

 Inc.) 



from the tissue spaces, and carry the lymph 

 slowly from the outlying parts toward the 

 thorax. 



In the thorax the lymphatic network drains 

 into the two main lymph vessels of the body: 

 (1) the thoracic duct and (2) the right lym- 

 phatic duct. The thoracic duct is the largest 

 lymph vessel in the body, since it drains 

 lymph not only from the digestive tract, but 

 also from the whole left side of the bod) and 

 from the right leg and trunk. The lymph 



vessels of the digestive tract pass into the 

 mesentery where they form larger vessels, 

 called the lacteals. The lacteals converge to- 

 ward the root of the mesentery, just below 

 the diaphragm, where they flow together into 

 the thoracic duct. This large vessel passes 

 upward through the thorax, where it finally 

 terminates by emptying into the left sub- 

 clavian vein (Fig. 17-17). Meanwhile the 

 thoracic duct receives tributaries from all of 

 the left and part of the right sides of the 

 body. The smaller right lymphatic duct has 

 an equivalent course and position on the 

 right side of the thorax. It collects lymph 

 from the lymphatic network of the right 

 arm, the right sides of the head, neck, and 

 thorax, and delivers this lymph into the 

 right subclavian vein. Thus the lymph, which 

 filters from the blood in all the capillaries of 

 the body, returns to the blood via the sub- 

 clavian veins. 



The Flow of Lymph. Lymph flows very 

 slowly, as compared to blood. A given sam- 

 ple of blood returns to the heart within less 

 than a minute after it is pumped forth from 

 the heart. But an hour or more elapses before 

 a sample of lymph, formed in the leg, finds 

 its way back into the blood stream. Lymph 

 pressure, which sustains this sluggish flow, 

 originates from the fact that new lymph is 

 continually forced from the capillaries into 

 the already filled tissue spaces. Lvmph pres- 

 sure amounts to only about 8 mm of mer- 

 cury; but there are many valves in the lym- 

 phatics, and these valves prevent any back- 

 flow. Moreover many parts of the lymphatic 

 network lie between the muscles of the body, 

 and thus whenever a muscle contracts and 

 presses against its neighbors, the lymph is 

 driven forward in a considerable part of the 

 local network. 



The Lymph Nodes (Glands). These oval 

 masses of lymphoid tissue are situated in 

 strategic positions in the lymphatic network, 

 especially at points where the smaller lvmph 

 vessels converge to form larger ones (Fig. 17- 

 17). There are several groups of unusually 

 large nodes: in the knee and elbow joints; 



