the dendrons) form but a minute speck of protoplasm, the nerve fibre which
arises from it, although microscopic in diameter, extends a very long
distance; in some cases it is a yard long; consequently only a minute
fraction of the nerve fibre is represented in the diagram.]

The great brain or cerebrum consists of two halves equal in weight, termed
hemispheres, right and left; and the grey matter covering their surface is
thrown into folds with fissures between, thus increasing enormously the
superficial area of the grey matter and of the neurones of which it
consists without increasing the size of the head. The pattern of the folds
or convolutions shows a general similarity in all human beings, certain
fissures being always present; and around these fissures which are
constantly present are situated fibre systems and communities of neurones
having particular functions (_vide_ fig. 16.) Thus there is a significance
in the convolutional pattern of the brain. But just as there are no two
faces alike, so there are no two brains alike in their pattern; and just as
it is rare to find the two halves of the face quite symmetrical, so the two
halves of the brain are seldom exactly alike in their pattern. Although
each hemisphere is especially related to the opposite half of the body, the
two are unified in function by a great bridge of nerve fibres, called the
corpus callosum, which unites them. The cortical centres or structures with
specialised functions localised in particular regions of one hemisphere are
associated by fibres passing to the same region in the opposite hemisphere
by this bridge.

[Illustration: Fig. 16]

[Description: FIG. 16.--Diagram of the left hemisphere of the brain showing
localised centres, of which the functions are known. It will be observed
that the centres for the special senses, tactile, muscular, hearing, and
vision, are all situated behind the central fissure. The tactile-motor
kinaesthetic sense occupies the whole of the post-central convolution; the
centre for hearing (and in the left hemisphere memory of words) is shown at
the end of the first temporal convolution, but the portion shaded by no
means indicates the whole of the grey cortex which possesses this function;
a large portion of this centre cannot be seen because it lies within the
fissure forming the upper surface of the temporal lobe. Behind this is the
angular gyrus which is connected with visual word memory. The half-vision
cent