VARIETIES OF TUBERCULOSIS 



279 



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 con- 

 firmatory 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 Kerch's communication an enormous amount, 

 of work has been done on this subject, and commissions of; 

 inquiry have been 

 appointed in vari- 

 ous 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 viru- 

 lence — 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. 79). 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. 46) 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 

 appearance, and the growth is much more easily broken up. 

 The difference between the two types is accentuated by the 



Fig. 79. — Bovine tubercle bacilli in milk, x 1000. 



