THE COURSE AND PHYSICS OF THE CIRCULATION 243 



quill. If a vein of this size were cut it would yield a 

 large outflow of blood in a short time. By contrast, the 

 flow of lymph from the cut thoracic duct is a mere dribble. 

 The question arises why the lymph flows at all. The 

 fundamental fact is that it is formed under some pressure 

 in the tissues while an outlet into the great veins is pro- 

 vided at a place where there is practically no opposing 

 pressure. It is also permissible to say that the continu- 

 ous formation of new lymph is bound to crowd along in 

 the lymphatics the fluid that entered them previously. 



An important factor contributing to the movement of 

 the lymph is muscular contraction. The larger lym- 

 phatics are provided with simple valves which allow an 

 advance toward the thorax but forbid a return. When 

 the skeletal muscles throw pressure upon the lymphatics 

 the lymph is made to slip along toward its distant outlet 

 and when the pressure is removed the refilling of the 

 emptied vessels can occur only from their small branches. 

 There are valves in the veins of the limbs which similarly 

 promote the venous circulation during exercise. 



Lymph Nodes. Lymph which is moving from an 

 extremity toward the thorax must pass somewhere on its 

 way at least one of the structures known as lymphatic 

 glands or, better, as lymph nodes. These are small 

 kernels of fairly dense tissue interposed in the course of 

 the lymphatics, particularly in the abdomen, the neck, 

 armpits, and groins. Microscopic study shows that 

 they must act mechanically as filters, the lymph working 

 through their narrow interstices. Certain cells from 

 these nodes may become detached and pass on with the 

 lymph to the blood, furnishing one type of colorless 

 corpuscle. The lymph nodes probably constitute de- 

 fenses against the march of infection along lymphatic 

 pathways. 



The Circulation before Birth. The arrangement of 

 the circulatory system in the unborn child is so different 

 from the permanent condition that the changes occurring 

 at birth seem almost miraculous. The critical character 



