72 



ANATOMY OF THE CENTRAL NEKTOUS SYSTEM. 



plished. The secondary sensory tract, — i.e., tract of the second order, — 

 which arises from the cells of the gray matter and passes toward the brain 

 in the antero-lateral colnmn, is, in all probability, a part of the fillet, ending 

 in the roof of the midbrain. The complete system will, in future, be called 

 the Tradus tedo-spinalis in the cord and Tradus tedo-bulharis where it 

 comes into relation with the medullar or bulbar nuclei. 



In mammals there are, associated with those above mentioned, still 

 other tracts. Of these the most important of all is the tract from the cere- 

 bral cortex: Tradus cortico-spinalis. This tract, as yet demonstrated only 

 in mammals, has a different position in the cord in different species. It has 

 been longest known in the human anatomy as the pyramidal trad of the 

 lateral column, or crossed pyramidal tract. In the mouse and guinea-pig it 

 lies in the posterior columns quite near the commissura dorsalis. In the dog 

 and in all apes it lies in the dorsal segment of the lateral columns. In man 

 a part of the tract passes also in the anterior columns. This part is, in man, 

 more highly developed than in any other mammal; in the lower mammals. 



Fig. 35. — A, Spinal cord of a clog whose Tr. cortico-spinalis has degenerated 

 in consequence of removal of the cerebrum. B, Human spinal cord in which the 

 anterior and lateral pyramidal tracts (Tr. cort.-spin. lat. et Tr. eort.-spin. ant.) 

 have suffered degeneration in consequence of a hemorrhage in the left cerebral 

 hemisphere. 



in fact, this part of the tract is represented by only a few fibers. This part is 

 called the anterior or direct portion of the pyramidal tract, or the column 

 of Tiirck. 



One has the impression that these cortico-spinal tracts are developed in 

 proportion to the measure in which the cerebral activity is called into play 

 in those functions of the extremities which are in no way instinctive, but are 

 learned and cultivated by the individual. Fig. 35 shows a human spinal 

 cord in which these tracts are functionally destroyed through disease; also 

 a dog's spinal cord which has suffered a loss of the corresponding tracts 

 through removal of the hemispheres. The difference in the development of 

 these tracts in man and in the dog is apparent. 



It has already been stated that fibers pass from the white substance into 



