TWENTY-FIVE YEARS OF BACTERIOLOGY 127 



the infection itself. Since in epidemic meningitis it is the 

 membranes surrounding the brain and spinal cord and 

 those more delicate ones lining the cavities of the ventricles 

 of the brain which are the seat of infection, it has been 

 found easily possible, through a simple and safe pro- 

 cedure, to inject the serum into the cavity of the spinal 

 meninges, whence it is quickly distributed over all the 

 membranes of the brain and cord. It may be of interest 

 to remark that it is not practicable to reach the inflamed 

 membranes with the serum by way of the blood, since 

 nature, in order to protect the sensitive nerve tissues from 

 injury by any chance deleterious substance in this fluid, 

 has interposed an impenetrable barrier, the choroid plexus, 

 between the circulation and the cerebrospinal fluid which 

 bathes and sustains the nervous organs, and which is 

 itself elaborated from the blood by this plexus with an f 

 accuracy of selectiveness highly remarkable. 



In pneumonia again a beginning success has beeru 

 achieved through a finer discrimination of specific kinds; 

 among the pneumococci, the inciting microbes of the dis- 

 ease. This distinction is independent of ordinary physi- 

 ological and cultural characters displayed by the bacteria, 

 which do not serve to bring out the underlying specific 

 properties of each, and has been accomplished by means 

 of the so-called immunity tests carried out in test tubes 

 or in the animal body. The gain to practical medicine 

 from the detection of the fundamental differences sub- 

 sisting between the three main types of pneumococci ex- 

 isting in this country has been very great. Already a 

 curative serum for one of the specific types of pneumonia 

 has been secured, and through its application many lives 

 have been saved ; while beginnings have been made in re- 

 spect to vaccination against the disease when, as some- 

 times happens in institutions and in communities, epidem- 

 ics prevail and claim many victims, as occurred in the 

 Army training camps during the measles epidemic of 

 1917-18. 



