irtue of the law already
quoted, that the evolution of force at one centre of the nervous system
is incompatible with an evolution of equal intensity at another, since
it diminishes the sum of resources distributed to the nervous system as
a whole. Hence, relatively to the amount of power left in the brain, the
same exertion becomes very much more fatiguing, and may easily lead to
exhaustion with all its consequences.

Nothing seems more simple than this proposition when thus stated. But
all physiological problems are complicated by the element of
quantity--circumstance which almost indefinitely limits our power of
making absolute assertions. The comparison already made between the
process of digestion and that of menstruation should suffice to show
that there is no absolute incompatibility between the evolution of nerve
force at the ganglionic centres and at the cerebro-spinal. For if so the
process of digestion would necessitate such absolute torpor of the brain
and spinal cord as certainly would be quite incompatible with the
exigencies of civilized life. There is a certain alternation between the
periods of activity of the two systems, but this varies in infinite
gradation; from the digestive torpor of the savage, analogous to that of
ruminating animals, up to the unconscious digestion of healthy men of
temperate habits and marked intellectual and physical activity, to whom
all hours of the day are nearly equally suitable for exertion. As
previously said, up to a certain point, the incompatibility diminishes
with every increase in the development of the cerebral system.

But again, the evolution of nerve force required by ovulation should not
normally be comparable in intensity with that effected in cerebral or
spinal action. Whenever it is so the activity of the ganglionic system
must be in excess, or that of the cerebro-spinal system must be
deficient. It is true that among the women of highly civilized
societies, one or both of these conditions very frequently exist, but it
is then as truly abnormal as is the dyspepsia and spleen--equally
prevalent.

Although, for certain purposes, it is necessary to consider the
ganglionic and cerebro-spinal system together, as parts of a single
apparatus, it is important also to remember the boundaries that lie
between them. It is much easier, by intense muscular exertion, that
necessitates evolution of force in the spinal cord, to render the brain
incapable of function, tha