MOVEMENTS OF THE LYMPH AND CHYLE. 305 



force through the walls of the blood-vessels, where their entrance is opposed 

 by the pressure of the fluids already in their interior, they certainly must 

 pass without difficulty through the walls of the lymphatics, where there is 

 no lateral pressure to oppose their entrance, except that produced by the 

 weight of the column of liquid. This pressure is readily overcome ; and the 

 valves in the lymphatic system effectually prevent any backward current. 



In describing the anatomy of the lymphatic system, it has already been 

 stated that the large vessels and those of medium size are provided with 

 non-striated muscular fibres and are endowed with contractility. This fact 

 has been demonstrated by physiological as well as anatomical investigations. 

 Beclard stated that he often produced contractions of the thoracic duct 

 by the application of the two poles of an inductive apparatus. It is not un- 

 common to see the lacteals become reduced in size to a mere thread, even 

 while under observation. Although experiments have generally failed to 

 demonstrate any regular, rhythmical contractions in the lymphatic system, it 

 is probable that the vessels contract upon their contents, when they are un- 

 usually distended, and thus assist the circulation, the action of the valves 

 opposing a regurgitating current. This action, however, can not have any 

 considerable and regular influence upon the general current. 



Contractions of the ordinary voluntary muscles, compression of the 

 abdominal organs by contraction of the abdominal muscles, peristaltic move- 

 ments of the intestines and pulsations of large arteries situated against the 

 lymphatic trunks, particularly the thoracic aorta, are all capable of increas- 

 ing the rapidity of the circulation of the lymph and chyle. 



The contractions of voluntary muscles assist the lymphatic circulation in 

 precisely the way in which they influence the flow of blood in the venous 

 system ; and there is nothing to be added regarding this action to what has 

 already been said on this subject in connection with the description of the 

 venous circulation. 



Increase in the flow of chyle in the thoracic duct, as the result of com- 

 pression of the abdominal organs or of kneading the abdomen with the 

 hands, was observed by Magendie, and the fact has been confirmed in all 

 recent experiments on this subject. The same effect, though probably less 

 in degree, is produced by the peristaltic contractions of the intestines. 



When a tube is introduced into the upper part of the thoracic duct, it is 

 frequently the case that the fluid is discharged with increased force at each 

 pulsation of the heart. This was frequently observed by Dalton in his exper- 

 iments on the thoracic duct, and he described the jets as being " like blood 

 coming from a small artery when the circulation is somewhat impeded." 

 This impulse is due to compression of the thoracic duct as it passes under 

 the arch of the aorta. Its influence upon the general current of the lymph 

 and chyle is probably insignificant. 



While the vis a tergo must be regarded as by far the most important 

 agent in the production of the lymphatic circulation, the movements of 

 fluids in the thoracic duct receive constant and important aid from the 

 respiratory acts. This fact has long been recognized ; and in the works of 



