MOVEMENTS OF THE LYMPH AND CHYLE. 339 



Influence of the Forces of Endosmosis and Transudation (vis a tergo^.The forces 

 of endosmosis and transudation are undoubtedly the main causes of the lymphatic circu- 

 lation, more or less modified, however, by influences which may accelerate or retard 

 the current ; but this action is capable in itself of producing the regular movement of 

 the lymph and chyle. It is a force which is in constant activity, as is seen in cases of 

 ligation of the thoracic duct, an operation which must finally abolish all other forces 

 which aid in producing the lymphatic circulation. When the receptaculum chyli is rup- 

 tured as a consequence of obstruction of the thoracic duct, the vessel gives way as the 

 result of the constant endosmotic action, in the same way that the exposed membranes 

 of an egg may be ruptured by endosmosis, when immersed in water. 



"We have already alluded to the influence of transudation from the blood-vessels and 

 have compared it to the force with which the secretions are discharged into the ducts of 

 the glands ; and in placing this, with the force of endosmosis, at the head of the list of the 

 agents which effect the lymphatic circulation, its importance is not over-estimated. This 

 conclusion can hardly be avoided when we consider the anatomy of the lymphatic sys- 

 tem. The situations in which the endosmotic force originates are at the periphery, where 

 the single, homogeneous wall of the plexus is excessively thin, and where the extent of 

 absorbing surface is enormous. If liquids can penetrate with such rapidity and 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 numerous valves in the lymphatic system effectually prevent any 

 backward current. The liquid that passes into the lymphatics by endosmosis or by 

 transudation produces movement by displacing an equal bulk of liquid contained in the 

 vessel. We observe with the microscope the rapid filling and rupture of microscopic 

 cells when immersed in water ; and the rough experiments by which the operation of 

 endosmosis is ordinarily illustrated, in which the extent of endosmotic surface is infinite- 

 ly small as compared with that of the lymphatic system, exhibit a current of considerable 

 force and rapidity. When we remember that the infinitely numerous lymphatic radicles 

 are bathed in fluids which undoubtedly pass into their interior with great facility, and 

 when we compare the probable extent of this endosmotic surface with the diameter of the 

 thoracic duct, we can hardly be surprised that this force should be capable of producing a 

 movement in the great trunk at the rate of an inch per second. The great elasticity of 

 the vessels and the fact that they are never completely filled allow of considerable dis- 

 tention of isolated portions of the lymphatic system when there is any obstruction to the 

 curreiit that is not readily overcome. In this way we account for the variations in the 

 flow of the lymph and chyle which are of such constant occurrence. 



Influence of the Contractile Walls of the Vessels. In treating of the anatomy of the 

 lymphatic system, it has already been observed that the large vessels and those of me- 

 dium size are provided with unstriped muscular fibres and are endowed with contractil- 

 ity. This fact has been demonstrated by physiological as well as anatomical investiga- 

 tions. Beclard states that he has often produced contractions of the thoracic duct by 

 the application of the two poles of an inductive apparatus. It is not uncommon 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 con- 

 tractions in the lymphatic system, it is probable that the vessels contract upon their con- 

 tents, when they are unusually distended, and thus assist the circulation, the action of the 

 valves opposing a regurgitating current. This action, however, cannot have any consid- 

 erable and regular influence upon the general current. 



Influence of Pressure from Surrounding Parts. Contractions of the ordinary vol- 

 untary muscles, compression of the abdominal organs by contraction of the abdominal 



