ormations of the brain, except very gross conditions which are
incompatible with extra-uterine existence, are not very common. At
birth those parts of the brain which are the seat of memory and what
are understood as the higher faculties are very imperfectly developed.
Variations in structure are extremely common, there are differences in
different individuals in the nerves and in the number, size, form and
arrangement of the nerve cells, and so complex is the structure that
considerable variation can exist without detection. The tissue of the
central nervous system has a considerable degree of resistance to the
action of bacteria, but is, however, very susceptible to injury by
means of poisons. Serious injury or destruction of tissue of the brain
and spinal cord is never regenerated or repaired, but adjustment to
such conditions may be effected by reciprocity of function, other
cells taking up the functions of those which were destroyed.

Certain parts of the brain are associated with definite functions;
thus, there are areas which influence or control speech and motion of
parts as the arm or leg, and there are large areas known as the silent
areas whose function we do not know. All activity of the central
nervous system, however expressed, is due to cell activity and is
associated with consumption of cell material which is renewed in
periods of repose and sleep. Fig. 13 shows a nerve cell of a sparrow
at the end of a day's activity and the same after the repose of a
night.

Diseases of the nervous system have a special interest in that they so
often interfere with man in his relations with his fellows. In
diseases of other organs the disturbances set up concern the
individual only. Thus, others need not be disturbed save by the
demands made on their sympathies by an individual with a cold in the
head or a cancer of the stomach. Disease of the nervous system is
another affair, instead of those reactions and expressions of activity
to which we are accustomed and to which society is adjusted, the
reactions and activities are unusual and the individual in consequence
does not fit into the social state and is said to be anti-social.
There are all possible grades of this, from mere unpleasantness in the
social relations with such an individual, to states in which he is
dangerous to society and must be isolated from it. Insanity is an
extreme case. There is no disease signified in the expression, but it
is merely a legal t