The whitish mass of soft matter (the center of the nervous
system, and the seat of consciousness and volition) which is inclosed
in the cartilaginous or bony cranium of vertebrate animals. It is
simply the anterior termination of the spinal cord, and is developed
from three embryonic vesicles, whose cavities are connected with the
central canal of the cord; the cavities of the vesicles become the
central cavities, or ventricles, and the walls thicken unequally and
become the three segments, the fore-, mid-, and hind-brain.