Among the mechanisms which defend the body from infection with the virus of poliomyelitis is the meningeal-choroid plexus complex, which normally is capable of excluding the circulating virus from the central nervous organs. The complex plays a part also in preventing infection from virus present upon the nasal mucosa.

Aseptic fluids which irritate, inflame, or even slightly alter the integrity of the meninges and choroid plexus diminish or remove their protective function.

Normal monkey or horse serum, isotonic salt solution, and Ringer's and Locke's solutions, when injected into the meninges, promote infection with the virus of poliomyelitis introduced into the blood, the nose, or the subcutaneous tissues.

Simple lumbar puncture and the withdrawal and return of the cerebrospinal fluid in normal monkeys, hemorrhage having been absolutely avoided, do not promote infection with virus injected into the blood; while the replacement of the cerebrospinal fluid of one monkey with that of another does in some instances lead to infection. Simple lumbar puncture attended with even very slight hemorrhage opens the way for the passage of the virus from the blood into the central nervous tissues, and thus promotes infection.

Hence, changes in the structure or function of the meningealchoroid plexus complex, too slight to be detected by chemical and cellular changes in the cerebrospinal fluid or by morphological alterations, suffice to diminish in an essential manner its protective powers.

Of all the irritant fluids tested, immune serum alone injected into the meninges is not succeeded by infection from the virus introduced into the blood.

The protective property of the immune serum is capable of overcoming the promoting action of normal monkey and horse serum and the other irritants mentioned.

The importance first of the meningeal-choroid plexus complex in preventing infection with the virus of poliomyelitis, and next of immune serum in offsetting the disadvantages and dangers arising from defects in the mechanism is apparent, as is the bearing of the experiments reported on the serum therapy of epidemic poliomyelitis.

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