=Physiology.=—The normal brain begins its development shortly after fertilization of the germ cell, by the expansion of the anterior end of the rudimentary spinal cord into four primary cerebral vesicles. These develop into a series of elaborate infoldings, each with multiple cells around them. At or about the sixth month of fetal life this embryonic brain assumes the shape of the adult brain, minus the secondary fissures and convolutions which are characteristic of full development. At birth there are sometimes many convolutions and the brain weighs from 280 to 330 grams. Growth is then rapid and at six months it weighs from 560 to 680 grams; At one year, 750 grams. It continues to increase until 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.