1870.] BRITISH ASSOCIATION. 335 



independent existence out of the body, it seems to me that the 

 shadowy boundary between morbid growth andXenogenesis would 

 be ejffaced. And I am inclined to think that the progress of 

 discovery has almost brought us to this point already. I have 

 been favoured by Mr. Simon with an early copy of the last published 

 of the valuable ' Reports on the Public Health,' which, in his 

 capacity of their Medical Officer, he annually presents to the Lords 

 of the Privy Council. The Appendix to this Report contains an 

 introductory essay ' On the intimate Pathology of Contagion,' by 

 Dr. Burdon Sanderson, which is one of the clearest, most compre- 

 hensive, and well-reasoned discussions of a great question which has 

 come under my notice for a long time. I refer you to it for details 

 and for the authorities for the statements I am about to make. 



You are familiar with what happens in vaccination. A minnte 

 cut is made in the skin, and an infinitesimal quantity of vaccine 

 matter is inserted into the wound. Within a certain time, a vesicle 

 appears in the place of the wound, and the fluid which distends 

 this vesicle is vaccine matter, in quantity a hundred or a thousand- 

 fold that which was originally inserted. Now what has taken 

 place in the course of this operation ? Has the vaccine matter by 

 its irritative property produced a mere blister, the fluid of which 

 has the same irritative porperty ? Or does the vaccine matter 

 contain living particks, which have grown and multiplied where 

 they have been planted ? The observations of M. Chauveau 

 extended and confirmed by Dr. Sanderson himself, appear to leave 

 no doubt upon this head. Experiments, similar in principle to 

 those of Helmholtz on fermentation and putrefaction, have proved 

 that the active element in the vaccine lymph is non-difi"usible, and 

 consists of minute particles not exceeding -zoojis of an inch in 

 diameter, which are made visible in the lymph by the microscope. 

 Similar experiments have proved that two of the most destructive 

 of epizootic diseases, sheep-pox and glanders, are also deiDcndent 

 for their existence and their propagation upon extremely small 

 Uving solid particles, to which the title of microzi/mcs is applied. 

 An animal sufi'ering under either of these terrible diseases is a 

 source of infection and contagion to others, for precisely the same 

 reason as a tub of fermenting beer is capable of propagating^ its 

 fermentation "by infection," or "contagion," to fresh wort. In 

 both cases it is the solid living particles which are efficient ; the 

 liquid in which they float, and at the expense of which they live, 

 being altogether passive. 



