34 Cincinnati Society of Natural History. 



examination, and can take very little count of future changes. 

 They include the important matter of family history or 

 heredity and predispositions, and form their conclusions. 

 The army standard is intended to secure healthy men : and is a 

 reliable one. Both of these standards fail in properly estimat- 

 ing the personal e(juation pertaining to every one, or the 

 amount of reserve force available, or how far conclusions 

 based on heredity may be subject to variations. It is neces- 

 sary, therefore, that your constitution should be kept up to its 

 highest point, and also that you should hold a stock of reserve 

 force, for in time of sickness it will hold you in good stead. 

 If your health account is on the debtor side when the epidemic 

 comes, or pneumonia pri-vails, you may easily become irre- 

 trievably bankrupt. 



In the way of protective influence for the individual we 

 may refer to a well-known fact that in families there are .some 

 members, or even all of them, who have an amount of exemp- 

 tion from prevailing disea.se, by which they escape infection 

 altogether, or receive it in perfectly safe quantities, so that 

 their illness is without danger. This is called natural immun- 

 ity, in contradistinction to acquired immunity, which is 

 brought about by artificial means. 



This question is now the .subject of much experimental 

 work and thought. It is admittedly not yet solved. Instances 

 of natural immunity among animals may be mentioned. No 

 undisputed case of typhoid fever transmitted to animal from 

 the human being has appeared. Injections of human saliva, 

 which contain the micro-organisms of pneumonia, will kill a 

 house mou.se in a few hours, as stated above, but will not 

 affect a field mouse. Probably the healthier habits of the 

 latter, his nomadic life, are the cause of this diflerence. 

 Inoculations with the glanders bacillus will destroy a guinea 

 pig. but will produce only slight eflect on a rabbit. Cattle are 

 immune against glanders. In the serum, or what nia,\- be 

 called the watery portion of the blood, is supposed to reside 

 that power of producing immunity from certain infectious 

 disea.ses. As for instance, in tuberculosis. The serum of the 

 dog has been used in a human subject as l)eing antidotal to 

 the bacillus of tuberculosis and its products. The (piestion 

 as to what element in the blood sernm it is which produces 



