which can be seen also in Fig. 18. "O.
T." is the thalamus, about which more later.]

Now it appears that the essential work in producing these higher
cooerdinations of skilled movement is performed not by the motor area,
but by neighboring parts of the cortex, which act on the motor area in
much the same way as the motor area acts on the lower centers. Some of
these {57} skilled-movement centers, or super-motor centers, are
located in the cortex just forward of the motor area, in the adjacent
parts of the frontal lobe. Destruction of the cortex there, through
injury or disease, deprives the individual of some of his skilled
movements, though not really paralyzing him. He can still make simple
movements, but not the complex movements of writing or handling an
instrument.

It is a curious fact that the left hemisphere, which exerts control
over the movements of the right hand and right side of the body
generally, also plays the leading part in skilled movements of either
hand. This is true, at least, of right-handed persons; probably in the
left-handed the right hemisphere dominates.

Motor power may be lost through injury at various points in the
nervous system. Injury to the spinal cord, destroying the lower motor
center for the legs, brings complete paralysis. Injury to the motor
area or to the pyramidal tract does not destroy reflex movement, but
cuts off all voluntary movement and cerebral control. Injury to the
"super-motor centers" causes loss of skilled movement, and produces the
condition of "apraxia", in which the subject, though knowing what he
wants to do, and though still able to move his limbs, simply cannot
get the combination for the skilled act that he has in mind.

Speech Centers

Similar to apraxia is "aphasia" or loss of ability to speak. It bears
the same relation to true paralysis of the speech organs that hand
apraxia bears to paralysis of the hand. Through brain injury it
sometimes happens that a person loses his ability to speak words,
though he can still make vocal sounds. The cases differ in severity,
some retaining the ability to speak only one or two words which {58}
from frequent use have become almost reflex (swear words, sometimes,
or "yes" and "no"), while others are able to pronounce single words,
but can no longer put them together fluently into the customary form
of phrases and sentences, and still others can utter simple sentences,
but not any connected speech.

[Illust