THE SPINAL CORD. 



275 



boundaries and illy defined, adjoining tracts often overlapping, and depend 

 for their presence upon the fact that nerve-fibres having the same origin. 



FIG. 321. Diagram of spinal cord, showing position of chief tracts and relations of their component 

 fibres to nerve-cells; 1-5, posterior root-fibres entering root-zone (R.Z.) and Lissauer's tract (L.), open 

 circles (o) indicate that fibres pass up and down; c, c, collaterals from long ascending tracts (i, 2) to 

 anterior root-cells ; 3, fibres ending around cells of Clarke's column ; 6, fibres forming direct cerebellar 

 tract ; 7, 8, fibres forming Cowers" tract ; 9, 10, fibres from lateral and direct pyramidal tract ; n, n, n, an- 

 terior root-fibres; 12, association tracts ; 13, rubro-spinal ; 14, tecto-spinal ; 15, spino-thalamic ; 16, olivo- 

 spinal tracts ; 17, vestibulo-spinal. 



function and destination usually proceed along a similar path. In addi- 

 tion to being provided with 

 paths of conduction necessary 

 for the performance of its func- 

 tion as a centre for independent 

 or reflex impulses in response 

 to external stimuli, the cord 

 contains tracts that connect it 

 with the brain, as well as those 

 that bring the various levels of 

 the cord itself into association. 

 The white matter contains, 

 therefore, three classes of nerve- 

 fibres: (i) those entering the 

 cord from the periphery and 

 other parts of the body; (2) 

 those entering it from the 

 brain; and (3) those arising 

 from the nerve-cells situated 

 within the cord itself. It is 

 evident that some of these fibres 

 constitute pathways for the 

 transmission of impulses from 

 lower to higher levels and hence form ascending tracts, while others, which 

 conduct impulses in the opposite direction, form descending tracts. 



FIG. 322. Section of spinal cord at level of second cer- 

 vical segment ; formatio reticularis fills bay between pos- 

 terior and anterior cornua ; substantiagelatinosa caps apex 

 of posterior cornu. (Drawn from Weigert-Pal preparation 

 made by Professor Spiller. ) X 6. 



