_Motor Functions and Mechanism._--The nerve centres, which together make up the motor area, and govern the voluntary muscular movements of the body, are situated in the grey matter of the præcentral or ascending frontal gyrus, and of the frontal aspect of the central sulcus (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 præcentral 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 main mass of the motor fibres crosses the middle line, and enters the lateral column of the spinal cord as the _crossed pyramidal tract_. The remaining fibres pass down as the _direct pyramidal tract_, and decussate in the cord near their termination.