IMMUNITY 65 



certain important means by which the body defends 

 itself against bacterial invasion in general. Many of 

 these are so commonplace that their significance is often 

 overlooked : 



(1) The protection afforded to the body by the un- 

 broken skin is undoubtedly one of the most important 

 means of defence. It is well to remember this, and 

 especially to bear in mind that we say "unbroken" skin. 

 In the sterilization of the skin prior to a surgical opera- 

 tion a great deal of harm is sometimes done by too 

 vigorous scrubbing or the application of too concen- 

 trated disinfectants. 



(2) A similar protection, though less powerful, is 

 afforded by intact and healthy mucous membranes. 

 Any condition injuriously affecting these renders the 

 body more liable to bacterial invasion. This is well 

 illustrated by the frequency with which an attack of 

 measles (which affects the mucous membranes to a 

 marked degree) is the starting-point of other and more 

 serious infections. 



(3) TKe~1icid gastric juice undoubtedly destroys 

 large numbers of bacteria which are swallowed. Dis- 

 orders of digestion may, therefore, constitute the decid- 

 ing factor in determining a bacterial invasion, especially 

 of the_intestinal tract. 



(4) It has been found that fresh blood-serum is able 

 to kill a considerable number of bacteria, and this, 

 therefore, constitutes another mode of defence against 

 bacterial invasion. 



(5) The white blood-cells, or leukocytes, as they are 

 called, appear to be designed especially to destroy in- 



