370 THE LYMPHATIC SYSTEM. 



namely, from the periphery toward the centre. The fluids taken up by 

 the lymphatic capillaries are collected into the larger branches and 

 trunks, and by them conducted from without inward toward the heart. 



The physical cause of the continuous movement of the lymph is pri- 

 marily the force of endosmosis acting at the confines of the lymphatic 

 system. As the volume of fluid accumulates in an endosmometer, in 

 such a manner as to rise perceptibty in the upright tube, so the lymph 

 accumulates by the force of absorption in the lymphatic capillaries, and 

 thence fills the larger vessels of the system. It is evident that the 

 pressure of fluids in a particular direction, due to the force of endos- 

 mosis, may be very considerable, since it is sometimes sufficient, as 

 already shown, to burst the shell-membrane of a fowl's egg when placed 

 in contact with water. As this pressure, in the lymphatic system, is 

 always directed from without inward, and as the main lymphatic trunks 

 finally terminate in the veins, the result is a uniform movement of the 

 lymph, from the peripheral parts of the various organs and tissues 

 toward the centre of the circulation. 



The movement of the lymph is also aided by several secondary causes. 

 As these vessels are provided with valves, even more abundantly than 

 the veins, the alternate contraction and relaxation of the voluntary 

 muscles in the limbs and trunk must facilitate considerably the passage 

 of their fluids in an inward direction. The action of the heart and arte- 

 ries also contributes indirectly to this result. As the thoracic duct 

 passes upward through the chest, it crosses the median line obliquely 

 from right to left, passing between the spinal column and the aorta ; so 

 that at each pulsation of the aorta it is compressed, and its contents 

 urged toward its upper extremity. This effect is often very visible when 

 a canula is inserted, in the living animal, into the thoracic duct at the 

 root of the neck. Under these circumstances we have frequently seen 

 the lymph projected from the extremity of the canula in a distinct jet 

 at each cardiac pulsation, owing to the momentary pressure from the 

 distended aorta. 



Lastly, the thoracic movements of respiration take part in maintain- 

 ing the flow of lymph. At each inspiration the resistance in the inte- 

 rior of the chest is diminished, and the lymph passes more readily from 

 below into the thoracic duct; at each expiration the duct is subjected 

 to compression, and is thus emptied of its fluids in a direction toward 

 its junction with the veins. The influence of the respiratory move- 

 ments, in a reversed form, may often be seen in animals poisoned by 

 woorara, where artificial respiration is kept up through the trachea. 

 If, in such an animal, a canula be inserted into the thoracic duct at the 

 root of the neck, the flow of tymph from its open extremity is percep- 

 tibly increased at each forcible insufflation of the lung, since this pro- 

 duces more or less pressure upon the thoracic duct in the cavity of the 

 chest. 



Of these different physical causes of the lymph-current, the first alone, 

 namely, the endosmotic action, is entirely uniform and continuous. The 



