680 FILTERABLE VIRUSES—RICKETTSIA 
longed much more than an hour, and too much pressure (or suction) 
must be avoided. Recent work also indicates that the electrical 
charge on the virus, the electrical charge on the filter, and the pres- 
ence of protein material, all influence the filtration. ^ 
The passage of a virus through a filter of the type mentioned does 
not necessarily indicate that the virus is too small to be visible with 
the highest powers of the microscope, although the filtrates of the 
so-called "ultramicroscopic viruses" are clear and do not contain 
particles demonstrable with the ultramicroscope. There is very little 
known of their true size, however. Filters used for the study of 
filterable viruses should be new, sterile, and tested for permeability 
with suitable known bacteria. A preliminary test, forcing air under 
pressure through the submerged filter,'- will reveal "pin holes." The 
virus to be tested should be forced through at a temperatiu*e of about 
20° C, and the process should be completed within one and a half 
hours, using as little pressure or suction as possible. The filtrate, 
proved to be free from ^•isible particles (best by adding a known organ- 
ism to the fluid to be filtered), should reproduce the disease in suscepti- 
ble animals; the virus should be recovered, again filtered, and again 
reproduce the disease. Some of the filterable viruses will pass only the 
coarser filters, others go through those with finer pores. 
Ultramicroscopic viruses with few exceptions are of unknown 
morphology, and, with the exception of the resistance to desiccation 
and physical agents, but little is known about them. Practically all 
of the known filterable viruses are known to exist only in the closest 
association with living cells, usually those of the host. The viruses of 
pleuropneumonia of cattle, of poliomyelitis and of lethargic enceph- 
alitis may possibly have been cultivated on artificial media; thus far 
the remainder have resisted attempts at cultivation. 
Acute Anterior Poliomyelitis. (Epidemic Poliomyelitis.)— Epidemic 
poliomyelitis is an acute disease observed more frequently in children, 
although adults are by no means immune. The onset is usually 
abrupt, although in some cases the earliest symptom is fever, with 
or without sore throat. The most striking feature is a paralysis of 
one or more limbs, which may be the first clinical indication of the 
disease. The principal lesion of the earlier stages is a hyperemia of 
the vessels of the cord together with thrombosis, and leukocytic infil- 
tration of the perivascular lymph spaces, more commonly in the 
cervical and lumbar regions, and in the spinal fluid as well. The older 
lesions are essentially a degeneration of the ganglion cells, particularly 
of the anterior horn, and eventually their atrophy. The motor nerves 
appear to sufl'er most— there are few, if any, indications of sensory 
disturbance. The relation of the disease to Landry's ascending 
paralysis, if any, is unknown. 
• Mudd: Jour. Bacteriol., 1924, 9, 151. 
2 Bulloch and Craw: Jour. Hyg., 1909, 9, 35. Ferry: Jour. Pathol, and Bacteriol., 
1915, 19, 488. 
