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.