244 THE LYMPH 



lymphatic system. No doubt, the greatest part of the original driving force is used 

 up in its passage through the lining cells of the capillaries, because the pressure 

 in the tissue spaces is very slight. Scarcely 10 mm. Hg are left for the return of 

 the lymph from the tissue to the blood-stream. 



2. The structure of the lymphatic tub ales is very similar to that of the blood- 

 capillaries. Their walls are composed of a single layer of elongated cells with 

 sinuous outlines which are joined at their edges into delicate tubules. While the 

 small lymphatic ducts are generally larger than the blood-capillaries, they do not 

 retain a constant diameter throughout, because constrictions are formed here and 

 there which impart a segmented or bead-like appearance to the entire channel. 

 The diameter of the thoracic duct, for example, varies between 8 mm. and 4 mm. 

 Of greatest importance to us at this time is the fact that these narrow places are 

 usually beset with streamer-like valves which open only in the direction of the 

 venous channels and close as soon as the pressure in the more central lymphatic rises 

 above that in its tributaries. In this way, a backward flow of lymph is prevented. 

 3. The flow of lymph is also furthered by the contractions of the skeletal mus- 

 culature. Under ordinary conditions, practically no lymph escapes from the rest- 

 ing limbs, while copious amounts of it are derived from the abdominal lymphatics. 



Fig. 121. — Cross-section of Lymphatic Vessel to Show Arrangement of Valves. 



The muscles, however, are well supplied with fluid so that their contraction is 

 immediately followed by an appreciable discharge of lymph. Quite naturally, as 

 the lymphatics are equipped with valves, the pressure which is thus brought to 

 bear upon their outside surfaces, must effect a quick onward rush of lymph in the 

 direction of the more central ducts. The respiratory movements also facilitate 

 the onward movement of lymph in an indirect way, because during inspiration the 

 intraabdominal pressure is increased, while the intrathoracic pressure is decreased. 



4. The importance of gravity as a factor favoring the flow of lymph varies 

 with the position of the body. Under normal conditions, this force cannot exert 

 an unfavorable influence upon dependent parts, for the same reason that it cannot 

 hinder the flow of the blood. The lymphatics possess a certain degree of tonicity 

 which tends to counteract the effects of gravity. 



5. In amphibia, reptilia and birds, the flow of the lymph is also aided by lymph- 

 hearts which contract rhythmically at the rate of about fifty beats in a minute. 

 These pulsating saccules receive several lymphatics and give off a single tube 

 which in most cases is directly connected with a blood-vessel. The orifices of the 

 tributary tubules, as well as that of the central channel, are guarded by valves which 

 insure a flow in the direction from the tissues toward the veins. In the frog, the 

 pulsations of the lymph-hearts may be observed dorsally in the fore part of the 

 body as well as next to the coccyx. 



