PATHOLOGY OF POLIOMYELITIS 687 



be employed, but to cause the disease by intravenous injection 

 enormous doses must be administered. By means of the inocula- 

 tion method the distribution of the virus in the natural and 

 experimental disease has been determined and has been found 

 to be similar in both cases. The virus is markedly neurotropic 

 and is highly concentrated in the brain and spinal cord. It 

 also occurs in the intervertebral ganglia, the Gasserian ganglion, 

 and in the abdominal sympathetic ganglia. It may be found in 

 the lymphatic glands, especially ' the tonsil and those of the 

 mesentery, and it has been demonstrated in the nasal mucous 

 membrane. It is absent from the solid organs, the blood, and 

 the cerebro-spinal fluid. 



Flexner's view of the pathology of the disease is that infection 

 takes place through the nasal mucous membrane, a catarrh of 

 the buccal and nasal cavities being often the first sign of the 

 disease. In monkeys in which intracerebral inoculation has been 

 practised the virus is eliminated into the nose, and the nasal 

 mucus has been found to be infective in human cases. When 

 an individual is infected by the inhalation of such mucus it is 

 probable that the virus gains access to the brain by the lymph- 

 atics of the olfactory nerves ; this view rests on the observation 

 that when monkeys are inoculated by painting the infective 

 material on the nasal mucosa, the olfactory lobe becomes infected 

 before other parts of the brain. This fact, as well as the size of 

 the dose required to produce infection by intravenous injection, 

 militates against the possibility of the virus being carried to the 

 central nervous system by means of the blood under natural 

 conditions. There is evidence from experimental intravenous 

 injections that the choroid plexus, which is the source of the 

 cerebro-spinal fluid, prevents (so long as it is uninjured, e.g., by 

 inflammation) the passage of virus into the subdural space. It 

 is likewise possible that in natural infection the virus may pass 

 into the mesenteric nodes and thence .be absorbed by the 

 lymphatics of the spinal nerves. All the facts point to the 

 importance of the part played by the peri- and intra-neural 

 lymphatics in the causal agent gaining access to the centra] 

 nervous system. While the virus may be said to be neurotropic, 

 the term must be used in the sense that all the elements of the 

 nervous system — pia-arachnoid, glia, interstitial blood vessels as 

 well as parenchymatous cells — show a special susceptibility. The 

 pathological anatomy in these structures has been described above. 



These observations have furnished important indications 

 of the method by which infection takes place, and by which 

 both the sporadic cases and the epidemic outbreaks occur. It 



