602 THE NERVOUS SYSTEM 



Motor Impulses. Motor impulses are conveyed downward from the 

 cerebral cortex of the brain along the pyramidal tracts, viz., the lateral 

 and the anterior, chiefly the former. In the lateral pyramidal tract the 

 impressions pass down chiefly on the side opposite to which they originate, 

 having crossed over in the decussation in the medulla. But some motor 

 impulses do not cross in the medulla, but descend in the anterior pyramidal 

 tract to lower levels of the cord, where they cross in the anterior commissure. 

 The motor fibers for the legs partially pass downward in the lateral columns 

 of the same side without decussation. This is also probably the case with 

 the bilateral muscles, i.e., muscles of the two sides that act together, such 

 as the intercostal muscles and other muscles of the trunk. 



It is quite certain, as was just now pointed out, that the fibers of the an- 

 terior nerve roots are more numerous than the fibers proceeding downward 

 from the brain in the pyramidal tracts, the so-called pyramidal fibers. This 

 is because each pyramidal fiber is really a very long nerve process or axone, 

 and is supplied in its course with a large number of collaterals. These go off 

 at different points, and thus put it in relation with different groups of nerve 

 cells in the anterior columns at various levels. Each nerve fiber of the 

 pyramidal tract, by means of its collaterals, can control a number of nerve 

 cells, and can thus co-ordinate the action of impulses sent out through the 

 anterior roots to a number of groups of muscles. In other words, the gray 

 matter of the anterior columns contains an apparatus with various com- 

 plicated co-ordinating powers, which apparatus is under the regulative 

 control of the neurones whose cells of origin are in the cortex of the cerebrum. 

 This is the same apparatus that is also reflexly influenced by sensory im- 

 pressions passing to the cord from the periphery. 



Division of a single anterior pyramid of the medulla at a point just above 

 the decussation is followed by paralysis of voluntary motions in the muscles of 

 the opposite side in all parts below. Disease or division of any part of the 

 cerebro-spinal axis below the seat of decussation of the pyramids is followed 

 by impairment or loss of voluntary motion on the same side of the body. 

 The paralysis is never quite complete, and the opposite side usually shows 

 some slight impairment of function of the muscle. 



When one-half of the spinal cord is cut through in monkeys, the results 

 are as follows (Mott): Motor paralysis of the muscles of the same side 

 (never complete paralysis of the muscles used in bilateral associated action), 

 followed by gradual recovery of muscular movement, except of the finer 

 movements of the hand and foot; wasting and flabbiness of the muscles; 

 partial sensory paralysis of the same side (temperature, touch, pain, and 

 pressure) ; temporary vaso-motor paralysis on the same side. The tempera- 

 ture of the affected side is depressed i to 3 F. 



