cus (fissure of Rolando). The upper limit of the motor area reaches
on to the mesial aspect of the paracentral lobule, and the lower limit
stops short of the lateral cerebral fissure (fissure of Sylvius) (Fig.
179).

[Illustration: FIG. 179.--Relations of the Motor and Sensory Areas to
the Convolutions and to Chiene's Lines.

(After Cunningham.)]

Each group of muscles has its own regulating centre, the size of the
area representing any group depending upon the character and
complexity of the movements performed by the muscles, rather than upon
the amount of muscular tissue that is governed by the centre--for
example, the centre for the mouth, tongue, and vocal cords is larger
than that for the muscles of the trunk.

The motor centres have been localised on the surface of the brain with
approximate accuracy. For example, above the superior genu of the
praecentral gyrus, the centres governing the hip, knee, and toes are
grouped; opposite the genu are the centres for the movements of the
trunk; between the superior and middle genua lie the centres for the
upper extremity; opposite the middle genu, those for the neck, and
below it, those for the face, jaws, and tongue, pharynx and larynx.

#The Motor Tracts.#--It is now generally accepted that there are two
paths by which motor impulses pass from the brain: one--the
_rubro-spinal tract_--which controls the more elemental movements of
the body, such as standing, walking, breathing, etc.; the other--the
_pyramidal tract_--developed later in the evolution of the nervous
system, and concerned with the finer and more skilled movements.

The pyramidal tract is the more important clinically. From the
pyramidal cells in the cortex of the Rolandic area, the axis cylinders
pass through the centrum ovale towards the base of the brain. They
converge at the internal capsule, and pass through the anterior
two-thirds of its posterior limb (Figs. 180 and 195). The fibres for
the eyes, face, and tongue lie farthest forward, and next in order
from before backward, those for the arm and the leg.

From the internal capsule, the motor fibres pass as the _pyramidal
tract_ through the crusta of each crus cerebri, the pons and the
medulla oblongata. Throughout this part of its course, numerous axons
leave the tract, and enter the mid-brain, pons, and medulla in which
lie the nuclei of the motor cranial nerves.

At the _decussation of the pyramids_ in the lower third of the
medulla, the