At 12 to 14 years it weighs 1150 grams in the female, and 1300 in the
male;
At 20 to 21 years the weight is 1244 grams in the female and 1374 in
the male.
Growth is slow from this time until at 25 to 35 years the average
weight of the brain is 1269 grams (45 oz.) in the female, 1421 grams
(50 oz.) in the male.
This growth of the brain is due, first to the rapid multiplication of
nerve cells and, secondly, to the individual enlargement of each nerve
cell. These cells arise from the floor of the four primary vesicles
and are each similar to its fellow. They finally show differences in
feature and become characteristic in size and shape which process
continues throughout life. This process of differentiation of nerve
cells results in the peculiar laminated appearance of the brain cortex.
At the period of lamination, the nerve cells throw out delicate
processes which pursue definite directions throughout the brain mass
constituting a system of association fibers which link together in a
most complicated manner all parts of the brain, and are called the
association fibers of Flechsig. Projections from these cells form the
various pathways by which the brain is connected to the various parts
of the body.
Nerve cells in the different parts of the brain mature at different
periods, those areas which have to do with the highest intellectual
functions, viz., the frontal and parietal regions, maturing last.
At the seventh month of intrauterine life the brain cell is a small
round type of neuroblast, undifferentiated, lying in a matrix. The
cells increase in size until about the second week (extra-uterine)
of life, tiny processes begin to develop. At the third to fifth year
these cells are mature and possess axons, dendrons and geminules. These
communicate, forming the above named association system conveying
impulses to and from all parts of the cerebrospinal system. They
multiply and elaborate after puberty into a complicated system up into
middle life after which growth ceases and they slowly diminish.
=Greatest Growth= is between the first appearance of the primitive
brain and the end of the sixth month of life (extra-uterine), hence
it is during this period that any adverse conditions relative to
development of nerve cells may cause the greatest damage.
=Mind and Brain.=—Whatever may be the connection between these two,
we know that the former develops with the growth of brain cells and
fails with their decay. =Amentia= is associated with the incomplete
development of brain cells and =Dementia= is coincident with their
degeneration and death.
=Pathology=—=Brain.=—Structural abnormality of the brain tissue may
exist without variation of mentality or defect. Early observers gave
these gross defects as a cause for amentia. However, it has been
demonstrated beyond doubt by microscopic examination of cerebral
neurosis that cellular changes occur and that imperfect and arrested
development exists and is an essential basis of amentia.
=Histology=—=Blood Cells.=—Cortical blood cells in the ament are