CONDUCTING PATHWAYS IN THE SPINAL CORD 595 



Here collaterals are given off which again betake themselves to the gray 

 matter and there end in terminal arborizations about other cells. Other fibers 

 of this class run in the posterior columns, being found chiefly in the most 

 ventral section. 



Still other cells, whose axis cylinders break up immediately without pass- 

 ing to any well-defined pathway, serve as connecting links between different 

 elements at the same level. Such cells are found scattered throughout every 

 cross section, but are particularly abundant in the vicinity of the posterior 

 horn. 



It will be apparent from what has gone before that the antero-lateral col- 

 umns of the spinal cord are the most important. In these we have, besides 

 the particularly prominent crossed and direct pyramidal tracts: the dorso- 

 and ventrolateral cerebellar tracts, which are among the most important 

 afferent conducting pathways to the brain; the rubro-spinal, the tecto-spinal, 

 and the vestibulo-spinal tracts; and finally, the most important commissural 

 fibers binding together the different levels of the cord. 



D. EXPERIMENTAL AND CLINICAL OBSERVATIONS ON THE CONDUCTING 

 PATHWAYS IN THE SPINAL CORD 



The question which first confronts us in experimental and clinical investi- 

 gations of the pathways in the spinal cord is, whether or not these pathways 

 cross in the cord itself. The second question to be answered is, in what 

 columns do they run. 



When we consider the difficulties with which investigation of this subject 

 is beset such, for example, as the difficulty in animal experiments of making 

 just the cut intended, and the uncertainties attending the observation of 

 disturbances to sensibility and motility resulting from the operation we can 

 understand why the statements of different authors differ greatly as to the 

 results obtained. Observations on human patients are naturally well calcu- 

 lated to supplement the observations on animals; but here we meet with the 

 difficulty that the lesions occurring as the result of disease or accident are 

 seldom or never limited so exactly as to give us wholly unequivocal results. 

 The following summary must be regarded as largely provisional: 



1. Efferent Pathways. When a hemisection of the cord is made in a dog, 

 immediately after the operation the muscles of the same side, whose nerves leave 

 the cord below the section, are paralyzed, while the muscles of the opposite side 

 remain entirely functional. The hemisection seems therefore to have severed an 

 important pathway of the homonymous side. 



But this paralysis is not final. It gradually disappears more or less com- 

 pletely, the degree of recovery as well as the extent of the primary paralysis 

 depending upon the location of the hemisection. Thus hemisection in the cer- 

 vical cord produces a greater disturbance in the fore paw than in the hind paw. 

 Recovery is made in both extremities, but it is more complete in the posterior 

 than in the anterior. The disturbances to motility in the posterior extremity 

 following hemisection of the thoracic cord last longer and the recovery is less 

 perfect than after hemisection of the cervical cord. The more distally the hemi- 

 section is located, the more profound and the more persistent are the motor dis- 

 turbances which follow, and the less perfect is the subsequent recovery. 



