182 SURFACES AND MEMBRANES 



energy of the expanding cell, as it progressively ingests the particle, drops 

 to a minimum at some specific depth of penetration, where the cell and 

 particle are then in equilibrium. Under these conditions, whether the 

 particle is partly or completely ingested, the surface energy between 

 coccus and cell is at a minimum. If as a result of ingestion the cell- 

 particle interface changes, it is possible that further ingestion goes on at 

 the expense of the internal energy of the cell, or that the particle is 

 ejected. 



Phagocytosis a Phenomenon of Spreading 



The above analysis is supported by experimental evidence (Mudd 

 [1933]), which proves that the ability of a phagocyte to spread over the 

 surface of a particle undergoing ingestion is one of the principal factors 

 in determining phagocytosis. It has been found that, in the presence 

 of dilute sensitizing serum, adhesion of particles to the phagocytes may 

 take place with little evidence of complete ingestion. If the sensitizing 

 serum is more concentrated, however, the particles adhere to the phago- 

 cytes and are subsequently covered by their cytoplasm. The conclu- 

 sion that may be drawn is that the phagocytosis-promoting substances of 

 immune sera, opsonin or bacteriotropin, resurface the particles with at 

 least a monomolecular layer of an interfacial-surface-energy depressant, 

 a surface deposit upon which phagocytes can spread very readily. 



Phagocytosis in the body can be promoted by the deposition, on the 

 particle or on a bacterium, of a film of serum globulin which gives the 

 surface a low interfacial tension against the leucocyte and a high inter- 

 facial tension against the medium in which the particle is suspended. 

 Since it is practically impossible to measure the three interfacial tensions 

 in a phagocyte-particle-liquid system, inferences must be drawn from 

 such theoretical situations and indirect experimental evidence as out- 

 lined above (Mudd, McCutcheon, and Lucke [1934]). 



The influence of sulphanilamide upon phagocytic activity is still 

 uncertain. Recent work, however, indicates that sulphanilamide and 

 sulphapyridine are more effective therapeutically in conjunction with 

 immune serum than if either drug or serum is given alone, a result sug- 

 gesting that phagocytosis may play a part in the final disposal of the 

 infective agents. 



Stalagmometer 



A rather usual method for measuring surface energy is by determining 

 the weight of the drops which detach themselves slowly from the tip of a 

 calibrated vertical glass tube of small bore. A rather crude formula is 



