ABSORPTION 225 



compression of the capillary by the contraction of the non-striated muscle- 

 fibers by which it is surrounded. With the entrance of the chyle into the 

 subjacent lymph-vessel there is a distention of the vessel and a rise in pres- 

 sure. When the muscle-fibers relax, regurgitation is prevented by the 

 closure of the valves at the base of the villus. The elastic tissue of the 

 lymph-vessel now recoils and forces the chyle toward the thoracic duct. 

 After the emptying of the lymph- capillary the conditions as far as pressure 

 is concerned are favorable for the absorption of new material. The rhythmic 

 contractions of the intestinal wall undoubtedly aid in the movement of lymph 

 and chyle. It is quite possible that the walls of the general lymphatic system 

 aid the forward movement of lymph by more or less rhythmic contractions 

 of their contained muscle-fibers. 



Inasmuch as the lymph-vessels lie in situations in which they are sub- 

 ject to compression by muscles during contraction, it is probable that the 

 fluid in the vessels will be forced onward toward the thoracic duct at each 

 compression, a backward movement being prevented by the closure of the 

 valves which are even-where present in the vessels. Experimental observa- 

 tions have demonstrated the truth of this supposition. Alternate contraction 

 and relaxation of the muscles of the leg will, in an animal at least, increase 

 considerably the flow as well as the production of lymph from the thoracic 

 duct. Massage has a similar influence. 



The respiratory movements also aid the flow of both lymph and chyle 

 from the thoracic duct and larger lymph-vessels into the venous blood. 

 During inspiration the intrathoracic pressure (that is, the positive pressure 

 exerted by the air in the lungs on the intrathoracic viscera, e.g., heart, veins, 

 thoracic duct, etc., which is less by about 6 millimeters of mercury than 

 the pressure in the lungs) decreases. The decrease is proportional to the 

 extent of the inspiration. With this decrease of pressure, the thoracic duct 

 expands and its internal pressure falls. As the intra-abdominal portion 

 of the thoracic duct and its tributaries are subjected to a higher pressure, 

 practically that of the atmosphere the lymph in these vessels is forced, by 

 reason of the difference in pressure between these two regions, into the 

 intrathoracic portion of the duct. During expiration, the rise of the in- 

 trathoracic pressure to its former value leads to a compression of the thoracic 

 duct and causes the lymph to be discharged rapidly into the blood-stream. 

 A regurgitation of the lymph is prevented by the closure of the numerous 

 valves throughout the course of the duct. 



DIFFUSION. OSMOSIS. FILTRATION 



As these three factors are believed to play an important part in many physiologic 

 processes, it is essential to a better understanding of 'these processes, that certain 

 elementary facts relating to these three factors be known. 



Diffusion. By diffusion is meant the gradual and spontaneous mixture of 

 the molecules of two or more liquids, or of two or more gases, when brought 

 into contact with each other, without the application of an external force. The 

 reason for both processes lies in the fact that the molecules of a liquid and of a 

 gas are in constant motion, in consequence of which a mutual interpenetration of 

 the molecules takes place, which continues until a condition of homogeneity is 

 established. 



Again, when a soluble substance, inorganic or organic, is placed in water, 



15 



