November 22, 1912] 



SCIENCE 



689 



still other routes of infection exist for the 

 monkey. Since nervous tissue is favorable 

 to the parasite it was injected into large 

 nerves — such as the sciatic — in order to as- 

 certain whether these furnished a suitable 

 medium of propagation. The parasite 

 grows along the nerve until the spinal cord 

 is reached and produces injury of the cord 

 first at the point of entrance before it ex- 

 tends to and attacks other parts. The in- 

 jection into the nerve causes no paralysis 

 but paralysis of the innervated muscles ap- 

 pears after the lapse of a time sufficient for 

 the necessary multiplication of the parasite 

 and its passage into the spinal cord. 



Meanwhile the inoculated monkey shows 

 no other signs of illness, and no other organ 

 is severely affected; the injury is centered 

 upon the nervous tissues. And not only 

 does the parasite grow or flow along the 

 nerve but it ascends along the spinal cord 

 from lower to higher levels and eventually 

 reaches the medulla and brain. At last the 

 centers governing respiration are involved 

 and death by paralysis ensues. 



We have now been able to arrive at sev- 

 eral important conclusions. The monkey 

 can be made regularly to develop an ex- 

 perimental disease agreeing in all essential 

 respects with poliomyelitis in man. Inocu- 

 lation is necessary since keeping healthy 

 and paralyzed monkeys together does not 

 lead to infection. The parasitic cause of 

 the disease can traverse the blood, in the 

 monkey, to reach the central nervous or- 

 gans, but with difficiilty, while it easily 

 traverses the peripheral nerves. That the 

 natural, spontaneous disease, so called, in 

 man and the induced disease in monkeys 

 are very much alike is further shown by 

 microscopic study of the spinal cord and 

 brain which exhibit changes that are iden- 

 tical. 



The pathological effects are of two kinds : 

 injury to nerve cells not in the anterior 



gray matter alone but in the posterior gray 

 matter of the spinal cord and in the inter- 

 vertebral ganglia, medulla and brain; and 

 cellular invasion of the pia-arachnoidal 

 membrane of the spinal cord and medulla 

 that follow the blood vessels into these parts 

 and pass into the adjacent gray and white 

 matter. The altered vessels permit an es- 

 cape of albuminous fluid and blood cells 

 into the meshes of the membrane where 

 they mingle with the cerebrospinal liquid, 

 and into the spaces in the tissue composing 

 the solid white and gray matter. Some- 

 times the nerve cells, sometimes the men- 

 inges, vessels and supporting tissues suffer 

 most. When the nerve cells are extensively 

 injured the paralysis is marked; when the 

 meninges are much affected, the symptoms 

 are like those of meningitis. The virus of 

 poliomyelitis displays a high affinity for 

 nervous tissues, but it is the wide involve- 

 ment of the nutritive vascular system in 

 the pathological process that subjects the 

 sensitive nerve cells to so high a degree of 

 injury and destruction. 



The microscopical conditions we observed 

 in the course of oiir experiments were sug- 

 gestive of two things: first, the nature of 

 the parasite itself, and, second, the process 

 of generation of the effects or lesions them- 

 selves. Up to this time no definite parasite 

 could be detected in the nervous tissues 

 either in human beings or monkeys, nor 

 was anything of the kind found in the 

 blood or other organs. The scarcity of 

 polynuclear leucocytes in the altered cere- 

 brospinal liquid and spinal cord itself 

 spoke against a simple bacterial parasite. 

 The large number of mononuclear cells 

 spoke rather for a protozoal parasite. 

 Neither could be found, although the most 

 varied methods of staining and cultivation 

 were employed. There remained the pos- 

 sibility of the parasite being invisible or 

 ultramicroseopic and filterable. This it 



