222 TEXT-BOOK OF PHYSIOLOGY. 



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, 

 the molecules of the substance will at once begin to separate themselves and 

 to diffuse throughout the water until the solution becomes homogeneous, and 

 notwithstanding the fact that the dissolved substance possesses weight, the 

 solution remains homogeneous. The force of gravity is overcome by the force 

 of diffusion. 



The velocity with which the molecules of a substance will diffuse through 

 a solvent like water, varies considerably. The experiments of Graham show 

 that if the molecules of a given weight of hydrochloric acid diffuse completely 

 in a unit of time, the molecules in the same weight of sodium chlorid, cane-sugar, 

 albumin and caramel, will require for their diffusion 2.33, 7, 48, and 98 units of 

 time respectively. 



Osmosis.- Osmosis may be denned as the passage of the molecules of water 

 through an intervening membrane. If the water on one side of the membrane, 

 parchment for example, contains in solution substances such inorganic salts, 

 their molecules will also pass through the membrane though the time required 

 for this to take place may be much longer than in the case of the water molecules. 

 The passage of the dissolved substance through the membrane though usually 

 included under the term osmosis is more properly termed dialysis. 



If the two volumes of water on opposite sides of the membrane are the same 

 in amount, and if the one volume contains a salt in solution, the salt molecules 



