THE SPINAL CORD 553 



have their origin in nerve-cells situated for the most part in the" cortex 

 of the cerebral hemisphere and in that restricted region known as 

 the precentral convolution. From this origin the fibers descend 

 through the corona radiata, the internal capsule, along the middle of 

 the ventral surface of the crus cerebri, behind the pons to the medulla 

 of which they constitute the ventral pyramid. At this level the pyra- 

 midal tract of each side divides into two portions of unequal size, a 

 large and a small portion. 



(a) The larger portion, containing from 85 to 90 per cent, of all the 

 pyramidal axons decussates or crosses the median plane of the 

 medulla to the opposite side for which reason it has been termed 

 the crossed pyramidal tract. As it descends the cord it comes to 

 occupy the posterior portion of the lateral funiculus though its 

 exact position varies in different regions of the cord. In the 

 cervical and thoracic regions it is covered by a layer of fibers. 

 In the lumbar region, however, it comes to the surface. From 

 above downward this tract gradually diminishes in size, for the 

 reason that its fibers and their collaterals enter the gray matter 

 at successive levels. The terminal branches of these fibers are 

 in close physiologic relation either directly or indirectly through 

 correlation neuron cells with the dendrites of the cornual cells. 

 These fibers being the continuations of fibers which take their 

 origin in cerebral cells located in the cortex undergo descending 

 degeneration when divided. 



(ft) The smaller portion containing from 15 to 10 per cent, of all the 

 pyramidal axons does not decussate at this level but descends 

 the cord on one side for which reason it is termed the direct 

 pyramidal tract. It is also known as the tract of Tiirck. From 

 the medulla this tract descends in the inner portion of the ventral 

 funiculus from the upper extremity of the cord as far down as the 

 mid- thoracic region. From above downward this tract dimin- 

 ishes in size, for the reason that its fibers or their collaterals cross 

 at successive levels to the opposite side of the cord by way of the 

 anterior commissure to enter the gray matter. The terminal 

 filaments of these fibers or axons are in physiologic relation either 

 directly or indirectly through correlation neuron cells with the 

 dendrites of the cornual cells. When divided in any part of their 

 course, these fibers also undergo descending degeneration, a fact 

 discovered by Tiirck for which reason the tract bears his name. 

 Thus all the fibers of the cortico-spinal or pyramidal tract which 

 reach the spinal cord are eventually brought into relation with 

 the cornual cells of the opposite side of the cord. 

 The Rubro-spinal Tract, or Tract of Monakow. The fibers of this tract 

 have their origin in the cells of the red nucleus and in the tegmentum 

 of the midbrain. After emerging from the nucleus the fibers cross 

 the median plane and then descend into the cord and come to occupy 

 a region internal to the dorsal spino-cerebellar tract and ventral to the 

 crossed pyramidal tract. The terminal branches of these fibers enter 

 the gray matter and come into relation with the cornual cells. 



