BY R. GREIG SMITH. 157 



— but now we have a third, the opsonin. It is, perhaps, on 

 account of this complexity that bacteriologists have refrained from 

 regarding digestion and lysis as being analogous in other than 

 a half-hearted manner, although it has recently been shown that 

 the secretion of digestive enzymes is anything but a simple pro- 

 cess. Emmerich and Loew have certainly emphasised the tryptic 

 nature of their pyocyanase, but the}^ practically stand alone in so 

 emphatically regarding a digestive enz3ane as being a source of 

 immunity. Metchnikoff,* after mentioning Delezenne's work 

 upon the antiseptic action of the intestinal juice, says that it 

 proves for the first time the great analogy that exists between the 

 mechanism of intestinal digestion and the bactericidal and hfemo- 

 lytie effect of the blood sera. Wright and Douglas f think it 

 probable that the bacteriolytic, bactericidal and bacterio-opsonic 

 effects are each in their degree manifestations of a digestive 

 power exerted by the blood fluids upon bacteria. 



When a substance is introduced or finds its way into the tissues 

 or into the body cavity the animal endeavours to get rid of the 

 intruder. If the substance is soluble and diffusible it may be 

 eliminated by way of the kidneys. If not, it may be altered into 

 other bodies that can be so eliminated. Ignoring the case of 

 substances that are absolutely insoluble and indigestible, we are 

 left with organic bodies that are digestible. To this class belong 

 the bacteria. When they are digested they are called harmless, 

 but when they are not digested and, multiplying, produce toxines 

 that interfere with the health of the animal, they are pathogenic. 

 The difference between the two is that the pathogenic cell 

 encounters no enzyme capable of dissolving it. It is a stranger, 

 and has not recently been within the animal. Had the animal 

 been immunised either by the introduction of weakened cells or 

 of blood serum from immune subjects, the pathogenic cell would 

 not have been a stranger and would have been dissolved like a 

 harmless microbe. Although the cells of the body respond to 



* Bull. I'Inst. Past. i. 228. 

 t Proc. Roy. Soc. Ixxii. 369. 



