CHAPTER XVIII 

 THE BRAIN 



1. CONDUCTING TRACTS 



The physiological significance of all the details of the course of 

 the fibres in the brain which anatomy has demonstrated is not 

 known. It is therefore sufficient for us to describe the chief con- 

 ducting tracts. 



I. Course of til e tracts from the spinal cord info the brain. 



A. The motor tract. The crossed pyramidal tract forms 

 in the medulla oblongata the so-called decussation of tJie 

 pyramids, breaking through the anterior horn on its side 

 into the anterior ground bundle of the other side and joining 

 the direct pyramidal tract. From this point upward, the 

 two pyramidal tracts accompany each other, passing through 

 the pons, where they are crossed by cross fibres from the 

 cerebellum, then through the centre of the crusta cerebri, 

 the posterior limb of the inner capsule, and the corona 

 radiata, to the cortex of the cerebral hemispheres. 



On its course from the cerebrum to the decussation, the 

 common pyramidal tract gives off fibres to the cells of the 

 motor fibres of the cranial nerves. The fibres of the pyramids 

 which come from both sides cross shortly before entering in 

 the nerve nuclei to which they go. 



B. Tlie sensorv tracts. 



(ioir s column, in the medulla called the funiculus gracilis, 

 ends mainly in cells of the nucleus of the funiculus gracilis. 

 From there, fibres penetrate forward through the gray sub- 

 stance and cross the fibres from the other side above and 

 behind the decussation of the pyramids. This crossing is 



235 



