244 POPULAR SCIENCE MONTHLY 



tions of laboratory experiment, it can not be concluded that they will 

 be equally refractory when exposed to the natural sources and modes 

 of infection. In the laboratory the virulent infectious agent is brought 

 into the animal by injection, under the skin, into the serous cavities 

 or into the circulation, which are avenues through which in the natural 

 disease infection rarely if ever takes place. And while this mode of 

 introduction of the virulent bacilli into the body may, theoretically, be 

 more severe than their introduction into the lungs with inhaled air, 

 or into the stomach through infected stalls and food, yet the profound 

 differences in the defenses of the body with which the bacilli come into 

 conflict, under these different circumstances, may, after all, determine 

 the issue in a manner quite contrary to our expectations. It is, there- 

 fore, of the highest interest to learn that in their later tests Behring 

 and his co-workers exposed vaccinated cattle to stalls and herds which 

 were known to be badly infected, with the result that at the time of the 

 report, they had apparently escaped infection. I am enabled through 

 the courtesy of a private communication from Dr. Pearson to state 

 that cattle vaccinated by himself and Gilliland which were kept for 

 two years under natural conditions of infection have not contracted 

 tuberculosis, while the control animals, exposed to the same conditions, 

 have all developed the disease, some dying spontaneously by reason of 

 the severity of the infection. Dr. Pearson also informs me that their 

 experiments indicate that the degree of resistance bears a rather definite 

 relation to the number of vaccinations given the cattle. No cattle 

 vaccinated three times with their standard vaccine — a living culture 

 of tubercle bacilli of human origin — have developed tuberculous lesions 

 even after two years' severe exposure. In their experience, two injec- 

 tions of Behring's vaccine do not always suffice for such heavy ex- 

 posure as they employed. 



As regards the question of duration of the protection, it may be 

 said that Behring, basing his views on results of vaccination made three 

 years before, expressed the belief in 1901 that it would endure during 

 the life of the animal. As young healthy cattle are vaccinated before 

 they fall victims to infected stalls and herds, it would seem as if in- 

 fected herds might therefore gradually be replaced by healthy ones. 

 The gain, this being true, would be almost incalculable to agriculture. 



I am in the fortunate position of being able to bring before you a 

 critical summary of the subjects just presented by one wholly con- 

 versant with its practical as well as its theoretical aspects. Through 

 the courtesy of Dr. Leonard Pearson I have been enabled to read the 

 advance sheets of a review on immunization in tuberculosis which will 

 soon be issued from the Phipp's Institute. Dr. Pearson concludes that 

 there appears to be no doubt that different cultures of human bacilli 

 have different immunizing values. Some can not be used at all because 



