VARIETIES OF TUBERCULOSIS 



285 



necessary to take any measures against it. Previously to this, 

 Theobald Smith had pointed out differences between mammalian 

 and bovine tubercle bacilli, the most striking being that the 

 latter possess a much higher virulence to the guinea-pig, rabbit, 

 and other animals, and in particular that human tubercle bacilli, 

 on inoculation into oxen, produce either no disease or only local 

 lesions without any dissemination. Koch's conclusions were 

 based chiefly on the result of his inoculations of the bovine 

 species with human tubercle bacilli, the result being confirmatory 

 of Smith's, and also on the supposition that infection of the 

 human subject through the intestine is of very rare occurrence. 



Since the time of 

 Koch's communi- 

 cation an enor- 

 mous amount of 

 work has been 

 done on this sub- 

 ject, and Commis- 

 sions of inquiry 

 have been ap- 

 pointed in various 

 countries. We 

 may summarise 

 the chief facts 

 which have been 

 established. Prac- 

 tically all observers 

 are agreed that 

 there are two chief 

 types of tubercle 

 bacilli, which differ 

 both in their cul- 

 tural characters and in their virulence a bovine type and a 

 human type. The bacilli of the bovine type, when cultivated, 

 are usually shorter and thicker and more regular in size ; whilst 

 their growth on various culture media is scantier than that of 

 the human type (Fig. 85). From the latter character the British 

 Royal Commission have applied the term dysgonic to the bovine 

 and eugonic to the human type. For distinguishing the growth 

 characters of the two types egg media (p. 45), are especially 

 suitable. On Dorset's medium the human type produces an 

 abundant, dry and wrinkled or verrucose growth, which has often 

 a yellowish or pinkish tint ; while the bovine type forms a thin 

 whitish layer, smooth or somewhat granular, rather moist in 



FIG. 84. Bovine tubercle bacilli in milk, x 1000. 



