166 VETERINARY PHYSIOLOGY 



sheaths of normal fibres with haematoxyliii, which leaves the 

 degenerated tracts of fibres unstained. The fibres in the 

 central nervous system do not regenerate, probably because 

 they are devoid of the neurileminal sheath. 



B. Outgoing Fibres. The course of these fibres is much more 

 easily determined than that of the ingoing fibres. The effects of 

 experimental or clinical lesions of the cord upon the muscular 

 movements, the downward degeneration which follows such 

 lesions, and lastly the fact that, generally speaking, these 

 outgoing fibres get their medullary sheath at a later date than 

 the ingoing fibres, all enable the position of the outgoing tracts 

 to be defined. They may be grouped in four sets 



1. A very strong band of fibres lying in the dorsal part 

 of the lateral column, just inside the direct cerebellar tract, and 

 becoming smaller as the posterior part of the cord is reached. 

 This is the crossed pyramidal tract (tig. 87, C.Py.'), which comes 

 from the cells of the cortex cerebri of the opposite side, and 

 gives off collateral branches to the cells in the ventral horn 

 of the spinal cord (fig. 41, D, p. 88). 



2. Certain fibres from the cortex cerebri do not cross, 

 but run down, some in the crossed pyramidal tract, some 

 in the direct pyramidal tract (fig. 87, O.Py.), which runs 

 along the margin of the ventral fissure, and extends tail- 

 wards only into the dorsal region. These fibres decussate in 

 the cord. 



3. A set of fibres just inside the ventro-lateral ascending 

 tract, which may be called the ventro-lateral descending tract 

 (fig. 87, Desc.Ant.Lat.}. This comes from Deiters' nucleus (see 

 fig. 87), and as it passes down, gives oft fibres to the cells in 

 the anterior horn of the grey matter of the cord. Deiters' 

 nucleus receives fibres from the cerebellum, and the ventro- 

 lateral descending tract thus carries down impulses from that 

 organ. 



4. From the red nucleus (p. 184) some fibres pass down the 

 cord as the pre-pyramidal tract just ventral to the crossed 

 pyramidal tract. 



C. Fibres not Degenerating beyond the Cord. Proprio-Spinal 

 Fibres. Round the grey matter, a band of fibres the basis 

 bundles (fig. 87, B.B.} and outside of these, scattered through 

 the white matter chiefly of the lateral columns, other fibres 



