THE CORNEA 323 



Morphology. Rods, either straight or slightly curved, lying very 

 often in pairs ; individuals are 0'7 /* long, and 0'6 fj. broad. Smaller 

 individuals and filaments are rare. Their ends are rounded. Clearer 

 areas (vacuoles) can be seen at their ends, and sometimes also at 

 their centres, when they are faintly stained. In many features the 

 bacilli resemble xerose bacilli. Gram negative. No chain or capsule 

 formation. 



Cultures The colonies on agar are 2 to 7 millimetres in diameter 

 after twenty-four hours, and by transmitted light appear bluish and 

 translucent ; they are slightly raised, with round and sharp margins. 

 They tend to run together, and form a thick mucous slime. When 

 the medium is poured out in Petri dishes or plates, small round or 

 whetstone-shaped colonies develop in the substance ; on the surface, 

 however, they are large, round, and have a small dark central area. 

 In transmitted light they are concentrically marked. In the deep 

 layers of the agar they do not grow at all ; they are obligate aerobes. 



Colonies on gelatine plates are transparent, bluish, and homo- 

 geneous. In gelatine stab culture growth takes place only in the upper 

 part, with the formation of a flat nailhead. 



In sugar-agar there is no formation of gas, but the medium becomes 

 acid. 



Cow's milk is coagulated. 



In bouillon slight growth, without indol formation. 



On potatoes a thick, yellowish-brown growth. 



On human blood-serum or Loffler's serum a thick greyish-white 

 scum. 



No motility. 



The temperature optimum is that of the body ; still, there is a 

 scanty growth at 10 and 40 C. After three-quarters of an hour 

 at 55 C. the bacillus is killed. 



Resistance against dryness is very slight. 



Differential Diagnosis. On account of the characteristics given 

 above, Zur Nedden rightly differentiated his bacillus from those of 

 the Coli group, the Bacillus typhosus, the Bacillus dysenteric, the 

 aerogenous group, and rightly also from the other bacilli found in the 

 eye. In their cultures and morphology the Diplobacilli are quite 

 different. The Zur Nedden bacilli may lie in pairs here and there, as, 

 indeed, may be the case in any bacillus. The bacilli are readily seen 

 in smear preparations from the central, rarer ulcers, which resemble 

 ulcera serpentia, but without their tendency to spread out on the 

 surface. The drawing on Plate III. is made from such a case. In 



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