PATHOLOGY: S. FLEXNER 
417 
too small to determiae whether young persons who are the more suscepti- 
ble yield secretions which are mmierically inferior in neutralizing power 
to those supplied by older persons. 
Probably the failure of this external defensive measure is not in itself 
decisive, because of the existence of the second or inner defensive mech- 
anism. It consists of the membranes about the brain and spinal cord 
and attached secreting organ of the choroid plexus. This meningeal- 
choroid complex is remarkably efficient in excluding from the cerebro- 
spinal fluid, and hence from the substance of the brain and spinal cord, 
almost everything present in the circulating blood, except water and a 
few inorganic salts. The fluid is also almost free of cells. Only when 
the complex has been injured in some way and its integrity impaired 
does it permit even protein and cells to pass through from the blood 
into the cerebrospinal fluid. 
The fact had previously been determined,^ that the virus of poliomye- 
litis passes with great difficulty from the blood into the nervous organs, 
unless the choroid plexus and meninges had previously been injured, say 
through setting up an aseptic inflammation by injecting sterile horse 
serum into the meninges by means of lumbar puncture. What the 
present studies have brought out is the extraordinary sensitiveness of 
those structures to the injurious action of othewise bland fluids,^ for not 
only is their permeability affected by irritants such as horse and even 
normal monkey serum, both of which produce visible signs of inflamma- 
tion, but also by sterile physiological salt. Ringer's or Locke's solutions 
which set up only evanescent inflammatory changes, and of the cere- 
brospinal fluids from other monkeys which produce no detectable 
inflammatory changes whatever. The injury produced by the last fluid 
mentioned is so slight as possibly to be regarded in Cohnheim's sense 
as merely molecular. 
Possibly pohomyeHtis arises during the prevalence of the malady when 
both sets of defensive measures fail. This probably would occur only 
in exceptional instances in individuals among populations of any size. 
It is for the moment not difficult to conceive of reasons to account for 
the failure of the external mechanism, and more difficult to account for 
failure of the internal mechanism of defense. Not improbably the neu- 
tralizing power of the nasal secretions tends to reduce the carriage of 
the virus upon the nasal mucosa of persons exposed to and having suf- 
fered from infection with the virus of poKomeyHtis. It becomes, there- 
fore, an essential agency in diminishing public danger through reduction 
in the number of the potential virus carriers which arise. 
There is one irritating fluid only so far detected which does not pro- 
