THE MOTOE PATHS 



523 



the pons, however, they are broken up by the transverse pontal 

 fibres into numerous small bundles of large medullated fibres. At 

 the lower border of the pons these bundles reunite to form a com- 

 pact bundle, on either side of the median line, which enters the 

 pyramids of the medulla oblongata. 



Thus far the course of the motor tracts has been entirely ven- 

 tral. In the lower part of the medulla a marked change in this 

 condition is produced as a result of the 

 motor decussation. About ninety per 

 cent of the motor fibres which have been 

 traced into the pyramids of the medulla 

 now change their direction, pass obliquely 

 inward, dorsalward, and spinalward, de- 

 cussate in the median line with their fel- 

 low of the opposite side, and enter the 

 lateral columns of the spinal cord, where 

 they form the crossed pyramidal tracts. 



Not more than ten per cent of the fibres 

 of the pyramidal tracts in the medulla,* 

 on the other hand, continue straight into 

 the ventral columns of the spinal cord as 

 the direct or uncrossed pyramidal tract. 

 This uncrossed fasciculus takes a position 

 on either side of the anterior median 

 fissure. However, the fibres of this tract 

 constantly decussate through the ventral 

 or white commissure of the spinal cord in 

 the cervical region, so that the tract be- 

 comes progressively smaller and smaller 

 in its passage down the cervical cord, and 

 below this region is seldom found. Its 

 fibres end in arborizations about the ven- 

 tral horns of the opposite side. Thus all 

 the fibres of the motor paths to the spinal cord reach the opposite 

 side as compared with the cerebral hemisphere in which they arise. 

 The fibres of the crossed as well as those of the direct pyramidal 

 tracts end in arborizations about the ventral horn cells of the spinal 

 cord. 



The neuraxes of the motor cells of the ventral horns in each 



* Usually less than ten per cent ; the volume of decussation within the me- 

 dulla oblongata is subject to very great individual variation. 



FIG. 403. DIAGRAM OF THE 



INTERNAL CAPSULE. 

 No, caudate nucleus; Nl, 

 lenticular nucleus ; Th, optic 

 thalamus ; 7 7 A, anterior stalk 

 of the thalamus; 1, fronto- 

 pontal path ; 2, cortico-bulbar 

 path ; <?a, cortico-cervical path ; 

 8b, cortico-lumbar path ; ^, 

 path of muscular sense; 5, 

 temporo-pontal path ; , optic 

 radiation. (After Obersteiner.) 



