100 EXPERIMENTS UPON ANIMALS. 
may be brought under the anzsthetic by placing them in a covered 
jar into which a pledget of cotton wet with ether has been dropped. 
Before making injections into the anterior chamber of the eye it is 
well to use a two-per-cent solution of cocaine as a local aneesthetic. 
Mice which have been inoculated are usually kept in a glass jar 
having a wire-gauze cover. A quantity of cotton is put into the jar 
to serve as a shelter for the little animal, and it is well to partly fill 
the jar with dry sawdust. Larger animals are kept in suitable cages 
of wire or wood, and, as a rule, each one should be kept in a separate 
cage while under observation after an inoculation experiment. 
In experimenting upon animals the following points should be 
kept in view and noted : 
(a) The age and weight of the antmal. Young animals are, as 
a rule, more susceptible than older ones, and with many pathogenic 
bacteria the lethal dose of a culture bears some relation to the size 
of the animal. 
(b) The point of inoculation. Injections into the circulation 
are generally more promptly fatal and require a smaller dose than 
those into a serous cavity or into the connective tissue. Pathogenic 
bacteria introduced into the abdominal cavity reach the circulation 
more promptly than those injected subcutaneously. But certain 
microérganisms owe their pathogenic power to the local effect about 
the point of inoculation and the absorption of toxic products formed 
in the limited area invaded, and do not enter the general circulation, 
or at least do not multiply in the circulating fluid, and quickly dis- 
appear from it. 
(b) The age of the culture injected. Old cultures sometimes 
have greater and sometimes less pathogeni¢ potency than recent cul- 
tures. Some kinds of virus become ‘‘ attenuated” when kept. But 
when the pathogenic power depends chiefly upon toxic products 
formed during the growth of the bacteria, old cultures are, as a rule, 
more potent than those recently made. 
(d) The medium in which the pathogenic bacteria are sus- 
pended, Cultures in albuminous media, like blood serum, are in 
some cases more potent than bouillon cultures ; and the virulence of 
several pathogenic bacteria is greatly intensified by successive cul- 
tures—by inoculation—in the bodies of susceptible animals. Ogston 
found that pus cocci cultivated in the interior of eggs had an in- 
creased virulence. According to Arloing, Cornevin, and Thomas, 
the activity of a culture of the bacillus of symptomatic anthrax is 
doubled by adding one-five-hundredth part of lactic acid to the cul- 
ture fluid. 
(e) The quantity 7njected is evidently an essential point when 
the result depends largely upon the toxic products formed in the cul- 
