LYMPHATIC VESSELS 



703 



general pressure of the organs on the lymph-ducts. Since these are provided with valves, all 

 preventing the lymph from flowing backward, any such pressure causes the lymph to move on- 

 ward. As accessory agents must also be reckoned the smooth muscle and elastic tissue which is 

 present in the walls of the lymph-vessels and in the lymph-gland. That these forces, however, 

 are not primary is shown by numerous facts. There is an active circulation in the lymphatics of 



Fig. 550. — A. The Adventitial and Supra-muscular Nerve Plexuses, together with 

 Sensory Endings in the Thoracic Duct of a Dog. (Methylene-blue method.) B. 

 Nerve-fibres on the Endothelium of a Lymphatic Capillary of a Dog. (After 

 Kytmanoff.) 



( 



A J 



embryos long before valves develop. In many lower animals no valves develop save at the 

 entrance of the lymphatics to the veins. That neither valves nor muscular movements are es- 

 sential is shown by the fact that, in the tails of frog larviE, where no valves are present and 

 where the muscle movements have been completely paralysed by an anesthetic, the circula- 

 tion of lymph continues unchecked. 



The primary cause, therefore, for the movement of lymph is to be sought in the capillary 

 region, in the force produced by the passage of lymph through the endothehal wall, whether this 



