XI " TRACTS IN SPINAL CORD 493 



tlie bulb into the higher parts of the brain and is found 

 to start from a certain portion of wliat we shall describe 

 later on as the cortex of the cerebral hemispheres. 

 It is upon the cells of this i)art of the cortex that the 

 fibres of the pyramidal tracts depend for their nutrition ; 

 hence injury to this portion of the cortex leads to a 

 degeneration which extends right down to the lowest end of 

 the spinal cord. These facts still further confirm the idea 

 that this tract provides a path for efferent (motor) im- 

 pulses in the cord, since, as we shall see, that part of the 

 cerebral cortex of which we are now speaking, is 

 specially concerned in the development of etierent 

 (motor) impulses. 



The fibres of this tract which enter the spinal bulb 

 from, say, the left side of the brain, cross over in the 

 bulb, in what is known as the decussation of the pyramids 

 (p. 520) just above the origin of the first cervical nerve, and 

 then pass down the right side of the spinal cord. For 

 this reason it receives the name of the "crossed" 

 pyramidal tract. 



(ii) The Direct Pyramidal Tract. — This is a small tract 

 in the median part of the anterior white columns, adjacent 

 to the anterior fissure. (Fig- 156 D'.p'., D-P-) It really 

 consists of a small portion of those fibres which passed 

 into the bulb as the main pyramidal tract coming from 

 the cortex of the cerebral hemispheres, but which have 

 not yet crossed over in the bulb. Instead of crossing in 

 the brain they cross in the cord. Hence the tract grows 

 small as it descends and ultimately disappears. Thus in 

 Fig. 156 the direct tract D.j). comes from the same side 

 of the brain as the crossed pyramidal tract Ch:p., and a 

 similar remark applies to D'.p'. and Gr'.p'. 



(iii) The Descending Antero-lateral Tract.— This 

 tract is not very clearly marked, in fact the fibres of the 

 ascending and descending antero-lateral tract intermingle 

 to some extent. It forms part of an alternative route 

 from the cerebral cortex to the anterior horn cells. 

 (Fig. 156, desc.a.l., desc.a.l.) 



