THE LYMPHATIC SYSTEM. 321 



The cause of this centripetal movement of the lymph is primarily 

 the force of endosmosis acting at the confines of the lymphatic system. 

 As the volume of fluid accumulates in an endosmometer, the ingre- 

 dients of the lymph penetrate by absorption into the lymphatic 

 capillaries, and thence into the larger vessels of the system. It is 

 evident that the pressure of a fluid from endosmotic action may be 

 very considerable, since it can sustain a column of mercury at the 

 height of 600 millimetres, and may burst the shell' membrane of a 

 fowl's egg placed in contact with water. As this pressure, in the 

 lymphatic system, is always from without inward, and as the main 

 lymphatic trunks terminate in the veins, the result is a uniform move- 

 ment of the lymph, from the peripheral parts toward the centre of the 

 circulation. 



As the lymphatic vessels, like the veins, are provided with valves, 

 opening toward the centre and closing toward the periphery, the con- 

 traction and relaxation of the voluntary muscles in the limbs and 

 trunk must facilitate the passage of the fluids in an inward direction. 

 The pulsations of the heart and aorta also contribute to this result. 

 As the thoracic duct passes obliquely through the chest, between the 

 spinal column and the aorta, at each aortic pulsation it is compressed, 

 and its contents propelled upward. This effect is often visible in the 

 experiment of collecting lymph from the thoracic duct at the root of 

 the neck ; the lymph being often projected from the extremity of the 

 canula in a distinct jet at each cardiac pulsation. 



Lastly, the respiratory movements of the chest take part in main- 

 taining the flow of lymph. At each inspiration the resistance in the 

 thorax is diminished, and the lymph passes more readily from below 

 into the thoracic duct ; at each expiration the duct is subjected to com- 

 pression and thus emptied toward the veins. When artificial respira- 

 tion is kept up through the trachea after the chest Has been opened, 

 the influence of the respiratory movement is reversed. The flow of 

 lymph from the thoracic duct is then perceptibly increased at each 

 insufflation of the lung, since this produces a momentary pressure 

 within the chest. 



Of the forces above enumerated for the production of the lymph- 

 current, the most important and continuous is endosmotic action. The 

 remainder are more or less irregular or intermittent, but they con- 

 tribute by mechanical aids to the same result ; and the effect of the 

 whole is an incessant transportation of the lymph from the periphery 

 to the centre, where it is mingled with the returning current of venous 

 blood. 



Daily Quantity of the Lymph and Chyle. The quantity of fluid 

 passing through the thoracic duct varies according to the condition of 

 abstinence or digestion. In the fasting condition it is comparatively 

 moderate, but becomes more abundant soon after the commencement 

 of digestion, to diminish again during its later stages. We have found, 

 at various periods after feeding, in the dog, the following quantities 



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