SPINAL CORD AS A PATH OF CONDUCTION. 167 



trary, has not supported this view. While results in this direction 

 have varied, as would be expected from the intrinsic difficulties 

 connected with the interpretation of the sensations of an animal, the 

 general outcome has been to show that the sensory conduction is 

 bilateral, but mainly on the same side. That is, if the cord is 

 cut on one side only (hemisected) in the thoracic region, the sensi- 

 bility of the parts supplied below the lesion is impaired upon the 

 same side, but not completely abolished, showing that some 

 crossing has taken place.* It is possible that this crossing is more 

 complete in man than in the lower animals, although later studies 

 in man of unilateral lesions of the cord (Brown-Sequard paralysis) 

 indicate that the contralateral loss of cutaneous sensibility affects 

 chiefly the senses of pain and temperature; the loss of touch is -not 

 complete, and muscular sensibility is affected only on the same 

 side. On the whole, it would seem that the crossing of the sensory 

 fibers in the cord is only partial, and is more extensive in man 

 than in the lower animals. This partial crossing is probably com- 

 pleted in the brain, especially in the great sensory decussation in 

 the medulla. 



The Descending (Efferent or Motor) Paths in the Antero- 

 lateral Column. The main descending path in the cord is the 

 pyramidal system of fibers. In man, as shown in Fig. 70, there 

 are two fasciculi belonging to this system, the direct and the 

 crossed pyramidal tracts. Both tracts arise from the anterior 

 pyramids on the ventral face of the medulla, whence the name of 

 the pyramidal system. At the junction of the medulla and cord 

 the fibers of the pyramids decussate in part, forming a conspicuous 

 feature of the internal structure at this point known as the pyram- 

 idal decussation. According to the general schema of this decus- 

 sation (see Fig. 74), the larger number of the fibers in the pyramid 

 of one side pass over to form the crossed pyramidal tract of the 

 other side of the cord (4, 5) , while a smaller part (3) continues down 

 on the same side to form the direct pyramidal tract. Eventually, 

 however, these latter fibers also cross the mid-line in the anterior white 

 commissure, not, however, all at once, as at the pyramidal decussa- 

 tion, but some at the level of each spinal nerve. These pyramidal 

 fibers have their origin in the cortex of the cerebral hemispheres in 

 large pyramidal cells; some of them cross the mid- line before reach- 

 ing the medulla to end around the cells of origin of the cranial 

 nerves; but the greater number continue into the cord and after 

 crossing the mid-line in the pyramidal decussation or in the anterior 

 white commissure terminate around the motor cells of the anterior 

 horns, which give rise to the motor roots of the spinal nerves. 

 Both tracts, the crossed and the so-called direct, continue through- 

 *Mott, "Brain," 1895, 1. 



