BACTERIA AND DISEASE 293 



potato is not visible ; on the white of ^^% it flourishes ex- 

 tremely well. 



It retains its vitality in cultures and sometimes in the 

 throat for months. Three or four weeks is the average 

 length of time for its existence in the membrane, but, owing 

 to the difficulty of killing it in situ, it may live on for as long 

 as a year. All the conditions in the throat — mucous mem- 

 brane, blood-heat, moisture, air — are extremely favourable 

 to the bacillus; but it is very materially modified in viru- 

 lence. It is secured for diagnostic purposes by one of two 

 methods: {a) Either a piece of the membrane is detached, 

 and after washing carefully examined by culture as well as 

 the microscope; ox {b) a " swab " is made from the infected 

 throat and cultured on serum, and incubated at 37^ C. for 

 eighteen hours and then microscopically examined. Both 

 methods — and there is no further choice — present some 

 difficulties owing to the large number of bacteria found in 

 the throat. Hence a negative result must be accepted with 

 reserve. 



We have already referred at some length to the question 

 of toxins in diphtheria, and need not dwell further upon 

 that matter. Still a word or two may be said here sum- 

 marising the general action of the bacillus. Locally it pro- 

 duces inflammatory change with fibrinous exudation and 

 some cellular necrosis. In the membrane a ferment is 

 probably produced which, unlike the localised bacilli, passes 

 throughout the body and by digestion of the proteids pro- 

 duces albumoses and an organic acid which have the toxic 

 influence. The toxins act on the blood-vessels, and nerves, 

 and muscle fibres of the heart, and many of the more highly 

 specialised cells of the body. Thus we get degenerative 

 changes in the kidney, in cells of the central nervous system, 

 in the peripheral nerves (post-diphtheritic paralysis), and 

 elsewhere, these pathological conditions setting up, in ad- 

 dition to the membrane, the signs of the disease. The bacil- 



