82 THE NERVOUS SYSTEM. 



temporal lobe, the speech center in the third left frontal 

 convolution. Thus the impulses of the senses have 

 been located, though the function of many parts, the 

 so-called silent areas, are still in obscurity. 



The motor center, that is, the center for motion of the 

 skeletal muscles, is situated about the fissure of Rolando 

 and is divided into three parts, one for the legs, one for 

 the face, and one for the arms, the one for the legs being 

 uppermost and the others below in the order mentioned. 

 Fibers from these cells extend down through the brain 

 and cord to the muscles, the fibers being collected into 

 well-recognized bundles and the whole known as the motor 

 tract. There may be one long fiber from a cell in the 

 brain down through most of the cord or there may be a 

 succession of shorter fibers that are not actually con- 

 nected but are in close contact with each other. In the 

 upper pons the fibers for the face cross to the opposite 

 side, while the rest keep on down through the medulla, 

 and as they emerge from the medulla they too cross to 

 the other side and keep on down in the crossed pyramidal 

 tract. A few fibers do not cross but come down the 

 direct pyramidal tract, which, however, disappears part 

 way down. The crossed pyramidal tract is the true motor 

 tract and in it the fibers are continually sending branches 

 to the cells in the gray matter, where they connect with 

 the anterior horn. 



The anatomy of the sensory tract is not so well under- 

 stood. By it impulses are sent to the brain by the 

 peripheral organs, practically the surface of the body. 

 The sensory fibers connect with the sensory cells in the 

 posterior horn, from, which fibers are sent to the brain, 

 practically the reverse of motor action. There are 

 three chief sensory tracts, which are supposed to trans- 

 mit different sensations, one pain, one muscular sensa- 

 tions, and the third sensations of touch. All these 

 tracts, of which the chief is the direct cerebellar tract, 

 in passing up the cord pass to the opposite side at dif- 

 ferent levels and then go on to the cortex of the brain. 



