1902.] Descending Spinal Tracts in the Mammalian Corel. 119 



short fibres ; but analysis of our material makes us hesitate to positively 

 affirm that it is true for all of them. A small proportion of the short 

 fibres may decussate, at least in the sense that short fibres arising in 

 perikarya belonging to one lateral half of the cord may find their 

 way into the white ventrolateral columns of the crossed half. We do 

 not affirm, however, that any of even our " short " fibres do decussate, 

 we simply affirm our present inability to deny that a small proportion 

 of them may do so. 



Some of the " long " fibres are very long, both in the lateral and in 

 the ventral columns of the cord. Thus, some of those arising from 

 perikarya in the 6th and 7th cervical segments we have traced into the 

 sacral region, i.e., through nearly thirty spinal segments, both in the 

 lateral and in the ventral columns. The rule pointed out by one of us 

 in a previous paper,* that the long fibres in the spinal cord tend to lie 

 nearest the surface of the cord, is well exemplified in these intrinsic 

 spinal systems. 



Besides fibres in the ventrolateral columns the aborally-running 

 fibre-systems of spinal origin include fibres in the dorsal columns. 



Fig. 2. 



Cross-section of the spinal cord of the dog at the level of the 1st sacral segment ; 

 Marchi preparation. The section reveals the topography at that level of the 

 aborally-running fibre-system of the nerve-cells of the 1st and 2nd lumbar 

 segments. A short lengtli of the 13th thoracic segment of the cord had been 

 completely exsected and ablated. A partial translesion (rather more than a 

 semi-section) was then made through the right side of the 2nd lumbar segment 

 in its anterior levels 290 days subsequent to the total exsection. The extent of 

 this second lesion was accurately determined later by microscopic examination 

 in serial preparations ; its exact limits will be described in a fuller communica- 

 tion. The dots indicate, in the way mentioned in the text, the density and 

 extent of the tracts of degenerate fibres. L = left side; B, = right side. 



* ' Jo urn. of Physiology,' vol. 11, p. 298. Cambridge and London, 1893. 



