12
There is unquestionably good evidence for the conclusion that the cord in the lumbar region
is somewhat inferior in potency to the cord in the cervical region, and that, if it is wished to
secure uniformity of dosage to patients, either emulsions of the cord as a whole should be made
or the pieces should be strictly labelled and care taken to administer to each patient as much
cervical cord as lumbar.
It is equally evident that parts of the brain are of greater infectivity than the spinal cord.
One peculiar feature of the series in which dilutions of the whole cord up to I in 10,000 were
compared with dilutions of the whole brain up to 1 in 50,000 was that, although the incubation
periods were the same for both brain rabbits and cord rabbits, yet the disease was more rapid in
its course by 36 hours on the average in the brain rabbits. The limit of infectivity was not
reached by either dilution.
Taking the three regions of the spinal cord separately in no case did a dilution of above, 1
in 5,000 prove to be infective and, whereas the regions of the cord gave 15 infections out of 33,
i.e. , 45 .4 per cent., the regions of the brain gave 31 out of 43, i.e. , 71 . 8 per cent.
TABLE No. 6.
Region.
Dilution.
Percentage of infections in subdural
i n o c n l a t i o n .
Lumbar cord
100 to 50,000
6 out of 15
40.0 per cent.
Dorsal cord
4
9
44.4.
Cervical cord
5
9
55.5
Cerebellum
7
15
46.6
M otor cortex
Frontal lobe
7
10
70.0
8
9
888
Hippocampus
9
9
1000
The cerebellum seems to be less infective than the cortical parts of the brain.
Taking dilutions of an emulsion of the whole spinal cord of a fixed virus rabbit and of an
emulsion of the whole brain of the same animal we found that dilutions between 1/5,000 and
1/30,000 of the cord gave 25 infections out of 42, i.e. , 59 .5 per cent., and that dilutions between
5,000 and 30,000 of the brain gave 12 out of 14 infections, i.e. , 85 .6 per cent. Putting together all
the experiments performed we found that the spinal cord in dilutions up to 1 in 30,000 had an
infection rate of 41. out of 77, i.e. , 53 .2 per cent., and that the brain in the same dilutions infected
40 out of 53, i.e. , 75 . 4 per cent.
Above 1 in 20,000 the infectivity of the cord is greatly reduced, but the brain between
1/35,000 and 1/100,000 still infected 17 out of 21 or 80 . 9 per cent. of the subdurally inoculated
animals. From brain dilutions between 1/200,000 and 1/1,000,000 only one infection out of 18
resulted. When we examined the duration of the incubation periods in these experiments we
found that the mean period for the cord inoculations in dilutions up to 1 in 30,000 was 9 . 1 days and
that for the brain inoculations in dilutions up to 1 in 30,000 was 7 . 1 days. Further, between
1/30,000 and 1/1,000,000 the brain incubation period was 8 . 6 days.
The percentage of infections after subdural inoculation with dilutions and the shorter mean
incubation period both prove the higher infectivity of the brain substance, but it does not seem
to us to be possible to arrive at a definite ratio between the infectivity of the brain and that of
the spinal cord. It certainly cannot be reached by dilution experiments, for the mechanical
difficulty of making a homogeneous emulsion of either brain or cord which will bear indefinite
dilution without ceasing to be homogeneous appears to be insuperable. When high dilutions are
made it becomes a matter of chance whether a particle of virus happens to be in the minute
volume of fluid introduced beneath the dura mater or not. Hence the higher the dilution the
more survivals there are, not so much on account of the small quantity of virus inoculated but
more probably owing to its entire absence in a considerable proportion. This is borne out by the
fact that when infection does occur after inoculation with a high dilution the incubation period is
generally much the same as with a powerful dose, indicating that a particle of virus of some
magnitude has been injected. There seems to be no sign of a regular lengthening of the incubation
period corresponding to the dilution of the virus.
Failing a method of correlating the infectivities of the various parts of the central nervous
system with any reasonable degree of accuracy, it seems that the sole remaining way of utilising
all the available material provided by each rabbit and at the same time making sure that the dose
given to one patient is, if desired, of the same potency as that given to another is to emulsify
together the whole brain and whole cord of each rabbit. The best practical method of preserving
the emulsion thus procured is now engaging our attention.