ETIOLOGY 19 
among other reasons, by the fact that they commonly 
exert a specific action on particular organs, and even as 
the bacterial toxins of diphtheria and tetanus exert their 
influence on the nervous system, so apparently also does 
that of B. bronchisepticus on the respiratory organs. 
Another almost constant feature is the occurrence of 
a period of incubation between the moment of infection 
of the animal and the appearance of symptoms. In 
contradiction to this exotoxin theory, however, it must 
be recorded that M‘Gowan has expressed it as his 
opinion that no exotoxin is elaborated by the B. 
bronchisepticus. 
Filtration Experiments with Bacillus Bronchisepticus.— 
In the year 1915 Ferry, realising that the bacillus in 
question was a very minute organism, determined to 
prove, if possible, its ability to pass through the pores of 
filters used for the demonstration of the presence of 
known filterable organisms: The only difficulty encoun- 
tered was in finding a proper and satisfactory means of 
determining the integrity of the filters. The usual test 
—that of proving their ability to retain micro-organisms 
of ordinary size—was obviously out of the question, as 
the B. bronchisepticus is one of the smallest of known 
organisms. A test known for several years, and well 
described by Bulloch and Craw (1909), was finally 
decided to be the most reliable. This test depends upon 
the measure of the pressure of air as it is allowed to 
pass through the pores of the filter while immersed in 
water. A leakage at any point, or a variation in the 
texture or thickness of the walls, can be detected at 
once, the efficiency of the filter measured to a fraction of 
a pound, and the porosity of various filters compared, 
merely by observing the pressure necessary to produce 
a general flow of air through the pores of the candles 
proper. 
