MOTOR AREAS, those portions of the brain in which are located the cells that govern the voluntary muscles of the body. The motor areas are confined more distinctly to the cortex or outer surface of the brain on either side of the fissure of Rolando, making up the areas known as the anterior central and posterior central convolutions. In these areas are nu merous motor cells, the stimulating of which results in bodily motion. From these motor cells the impulse passes inward through the substance of the brain. The motor cells may he isolated from adjoining surface area by ex cision, with no effect on the control of the specific motion. When these surface areas are removed or injured there follows an impaired function of the muscle corresponding to the cells or the fibres interfered with. The motor area in the right side of the brain controls the voluntary muscles of the left side of the body and the right hand and leg are controlled by the left side of the brain. The tracts of fibres that come from one side pass the middle line at the lower level of the medulla, the decussation of the pyramids. The motor area itself is sub divided into irregular areas or centres as they are loosely called — an arm-centre, a leg-centre, etc. In these more or less clearly delimited
areas the cells governing the voluntary muscles of a leg or an arm are grouped. In much the same manner the nerves of the muscles of the face and of the eyes, tongue, etc., have their locations of origin in certain portions of this general motor area. There is, however, no evi dent relation between the size of a cortical area and the size of the mass of muscle which is con trolled thereby. Thus the cortical area for the face and head is much larger than for the leg area which controls the many times greater bulk of the leg muscles. Irritation of the motor area usually causes excessive muscular move ments or convulsions and destruction results in loss of motor power or paralysis. The experi ments from which these facts are drawn were performed upon rabbits, dogs and monkeys. Some few of them have been confirmed in the course of surgical practice upon human patients.
(See CONVULSIONS; PARALYSIS ) . Consult Howell, W. H. 'American Text-Book of Physiology) (Philadelphia 1903).