1068 PHYSIOLOGY 



makes it way through the thoracic duct. Movement, active or passive, 

 of the limbs at once causes a flow of lymph from them. Since the lymphatics 

 are all provided with valves (Fig. 490), the effect of external pressure on them 

 is to cause the lymph to flow in one direction only, i. e. towards the thoracic 

 duct and great veins. Hence we may look upon muscular exercise as the 

 greatest factor in the circulation of lymph. The flow of lymph from the 

 commencement of the thoracic duct in the abdominal cavity to the main 

 part of it in the thoracic cavity is materially aided by the respiratory move- 

 ments ; since, with every inspiration, the lacteals and abdominal part of the 

 duct are subjected to a positive pressure, and the intrathoracic part of the 

 duct to a negative'pressure, so that lymph is continually being sucked into 

 the thorax. 



. 



FIG. 490. A lymphatic vessel laid open to show arrangement of 

 the valves. (TESTUT.) 



THE ABSORPTION OF LYMPH AND TISSUE FLUIDS 



On injecting a coloured solution or suspension into the connective tissues 

 of Jany part of the body, and gently kneading the part, it is found that 

 the fluid fills all the lymphatic channels running from the part ; and we can 

 in this way inject the lymphatics of the limb and trace their course on to 

 the thoracic duct. The same path is taken by micro-organisms as they 

 spread in the tissues, or by particles of carmine or Indian ink which have been 

 introduced in tattooing. It is on account of these facts that the lymphatics 

 are often spoken of as the ' absorbent system.' 



This process of lymphatic absorption, except in the case of the pleural 

 and peritoneal cavities, is however a slow one unless aided to a large extent 

 by passive or active movements of the surrounding parts, and cannot therefore 

 account for the rapid symptoms of poisoning which supervene within two or 

 three minutes after the hypodermic injection of a solution of strychnine or 

 other poison. That this absorption is not dependent on the lymphatics is 

 shown by the fact that the symptoms occur almost as quickly when all the 

 tissues of the limb have been severed, with the exception of the main artery 

 and vein. In the same way, after injecting methylene blue or indigo carmine 

 into the pleural cavity or subcutaneous tissues, the dyestuff appears in the 

 urine long before any trace of colour can be perceived in the lymph flowing 

 from the thoracic duct. The absorption in these cases is by the blood vessels, 

 and consists in an interchange between blood and extravascular fluids, 

 apparently dependent entirely upon processes of diffusion between these 

 two fluids. So long as any difference in composition exists between the 

 intra- and extravascular fluids, so long will diffusion currents be set up, 

 tending to'equalise this difference. 



