LYMPH FORMATION AND CIRCULATION 117 



therefore in many ways the most important of the fluids of the body. 

 In the tissue cells themselves there is also the fluid in which the various 

 colloids and crystalloids that enter into the composition of protoplasm 

 are dissolved. This can be removed from cells only by mechanical means, 

 such as grinding with fine sand in a mortar and subjecting the mass 

 to a pressure of several thousand atmospheres in a hydraulic (Buchner) 

 press. This is known as the tissue juice. The ultimate exchange of 

 foodstuffs occurs between the tissue fluids and the tissue juices across 

 the cell membrane. The extent and character of this exchange depend 

 on many circumstances, some affecting the cell wall, others, the osmotic 

 and other properties of the two fluids. Obviously, the function of the 

 circulation is to maintain the tissue fluids of correct composition, the 

 blood plasma serving to carry food materials and dissolved oxygen to 

 them (see page 393), but being assisted in the opposite function of re- 

 moval of effete products by the lymph. The lymph is purely a scavenger ; 

 the blood is both purveyor and scavenger. 



The above description of the lymphatics is not universally accepted 

 by anatomists, certain of whom believe that the lymphatics are developed 

 from tissue spaces and are consequently much more extensive than they 

 appear to be from injected specimens. The above conclusions are based on 

 reconstruction models, made from serial sections of embryonic tissues, 

 in which the lymphatics frequently appear as isolated vesicles without 

 visible connections. The failure of injections to penetrate into the re- 

 moter parts of such a lymphatic system in the embryo is attributed to 

 the discontinuity of spaces, which is, however, removed at later stages 

 of development. 



The manner of absorption of injected fluids does not, however, sup- 

 port the view that the lymphatics are directly connected with the tis- 

 sue spaces. When all the structures of a part are ligated except the 

 main artery or vein, injected poisons which affect central structures, 

 such as the nerve centers, develop their action as quickly as in the in- 

 tact animal (e.g., strychnine). Similarly, when pigments such as meth- 

 ylene blue are injected into the pleural cavity or subcutaneously, they 

 appear in the urine long before the lymph of the thoracic duct. Such 

 results indicate the pathway of absorption to be the blood rather than 

 the lymph vessels. Through this latter channel absorption proceeds 

 more slowly, but can be greatly assisted by massaging the site of injec- 

 tion. When colored solutions, such as India ink or carmine, are injected 

 subcutaneously, however, a very perfect injection of the neighboring 

 lymphatics may ultimately occur, and through the same pathways mi- 

 croorganisms spread from an infected area. 



