BLOOD AND H^EMOPOIETIC ORGANS 99 



recognized in it are antitoxins, agglutinins, precipitins, 

 ferments, opsonins, and cytotoxins. It would be trans- 

 gressing on the domain of pure pathology to consider 

 most of these in any detail, but a word may be said 

 about precipitins, which are assuming some importance 

 in legal medicine. 



If some human ascitic fluid be injected into the peri- 

 toneal cavity of a rabbit, there develops in the blood 

 serum of the latter a substance (precipitin) which causes 

 a turbidity when added to diluted human blood, but 

 which has no effect on the blood of other animals. The 

 nature of the precipitin is unknown, nor do we know 

 how it is that it causes a precipitate in human blood 

 (possibly this is due to an agglutination of protein mole- 

 cules), but it is obvious that we have here a specific test 

 for human blood which is of great delicacy. The test 

 has already been widely employed in forensic medicine in 

 Germany, and has been the means of bringing about 

 conviction in cases of suspected murder. 



The Spleen. The spleen is usually classed in text- 

 books of physiology amongst the ductless glands. If by 

 a ' gland,' however, one means an organ with secretory 

 functions, then the term is hardly applicable, for whatever 

 its part in the economy of the body, the spleen certainly 

 does not secrete. Indeed, physiology can tell us almost 

 nothing definite about the spleen, except that it is not an 

 organ which is essential to life, and that was already 

 known more than a hundred years ago.* All other 

 statements about this organ are more or less conjectural, 

 and it is for this reason that the part it plays in pathology 



* See the collected works of William Hewson, F.B.S. 



72 



