174 PRINCIPLES OF VETERINARY SURGERY 



some other animal, the serum will remain clear. If we wish 

 to determine the actual source . of the meat, we inoculate 

 rabbits with blood from all the probable animals from 

 which the suspected meat might come (using only one kind 

 of serum for each rabbit). After a sufficient number of in- 

 oculations, usually about eight or ten, place a little serum 

 from each into as many test-tubes, and also into each test- 

 tube a little of the aqueous extract of the unknown meat. 

 The test-tube giving a precipitate indicates the source of 

 the meat. 



The precipitin test can be used also for spermatozoa, 

 and all tissues of the bodies, even the urine. 



OPSONINS. 



Certain bodies called opsonins represent a property of 

 serum which renders bacteria more susceptible to the phag- 

 ocytes and are especially associated with infections caused 

 by streptococci, staphylococci, pneumococci, etc. The 

 study of immunity against these last-named microbes has 

 led to the greatest diversity of' opinion, some claiming anti- 

 toxic propei"ties, others lytic, and others phagocytic proper- 

 ties for the immune sera. A very remarkable advance to- 

 ward the definite solution of this controversy has been made 

 by Wright and Douglas (1903), who have shown that the 

 serum of the blood forms a definite and independent role in 

 connection with phagocytosis and is brought about by the 

 action of certain substances which have been designated 

 opsonins, whose function in some way is to so change the 

 microbe that it will fall an easy prey to the leucocyte. If 

 the opsonins are removed from the blood-serum, the leuco- 

 cytes will not manifest any phagocytic action. Opsonins 

 are very susceptible to heat and also, like complement, dis- 

 appear spontaneously from drawn serum in a short time. 

 They seem to have a structure like that of toxin. 



