This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
A knowledge of the functions of the various portions of the brain is necessary in order to localize a diseased area. The diseases and injuries to which the brain is exposed oftentimes do not involve the whole brain, but only certain distinct and isolated parts. The brain is not a single, homogeneous organ that acts only as a whole; it is complex. It is composed of a number of separate parts or areas, which may act either singly or in conjunction with other areas. These separate areas have different functions, so that if the disease or injury is limited to one of them, we have its functions abolished, and the symptoms produced indicate the area affected.
These areas are situated on the surface or cortex of the brain in the gray matter. They receive impressions from, and transmit impulses to, all parts of the body through the white matter or fibres of the brain. An injury to the cortex or gray matter destroys the originating and receptive centres. An injury to the white matter destroys the paths to and from these centres and therefore prevents them from receiving impressions or sending out impulses. Thus, we may have a paralysis of the leg and arm caused by an injury to the leg and arm centres in the cortex of the brain, as by a hemorrhage from a fracture, or we can have the same paralysis produced by an injury to the path leading from those centres, the motor tract as it is called, by a hemorrhage, as from apoplexy, involving the corresponding white matter fibres.
The exact localization of the functions of all parts of the brain has not been accomplished, but the functions of many areas have been definitely proven. In cases of brain tumor, abscess, hemorrhage, injury, etc., a knowledge of these areas enables one to localize the seat of the lesion.