98 EXPERIMENTS UPON ANIMALS. 



may be brought under the anaesthetic 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 anaesthetic. 



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 animal. 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 

 microorganisms 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 nqt 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 pathogenic 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 injected is evidently an essential point when 

 the result depends largely upon the toxic products formed in the cul- 



