THE NERVOUS SYSTEM 153 



(Fig. 40, (7.). 2. A thin layer of fibres round the margin of the 

 lateral column degenerates upwards, and has been traced as 

 far as the cerebellum (Fig. 78, D.C. ; Fig. 40, E.). It 

 consists of two sets of fibres. Those behind constitute the 

 direct or dorsal cerebellar tract ; those in front which take 

 a somewhat different course forming the ascending antero- 

 lateral or ventral cerebellar tract (Fig. 78, Asc. Ant. Lat.). 

 They both take origin in Lockhart Clarke's column of cells, 

 which is specially well developed in the dorsal and upper 

 lumbar region of the cord. 



B. Fibres degenerating downwards. 1. A very strong 

 band of fibres lying in the posterior part of the lateral 

 column, just inside the direct cerebellar tract, and becoming 

 smaller as the lower part of the cord is reached. This is 

 the crossed pyramidal tract (Fig. 78, 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 anterior horn 

 of the spinal cord (Fig. 40, D.). 



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. 78, O.Py.), which runs along the 

 margin of the anterior fissure, and extends downwards only 

 into the dorsal region. These fibres decussate in the cord. 



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

 tract, which may be called the antero-lateral descending 

 tract (Fig. 78, Desc. Ant. Lat.). This comes from Deiters' 

 nucleus (see p. 89), and, as it passes down, gives off fibres to 

 the cells in the anterior horn of the grey matter of the cord. 

 Deiters' nucleus receives fibres from the cerebellum, and 

 these fibres probably carry down impulses from that organ. 



C. Fibres not degenerating for any distance. Round the 

 grey matter, a band of fibres the basis bundles (Fig. 78, 

 B.B.) do not degenerate far, and seem to be commissural 

 between adjacent parts of the grey matter. 



Other tracts of fibres have been described, such as Lis- 

 sauer's tract and the septo-marginal tract, but their relations 

 have not been satisfactorily investigated. 



2. Developmental Method. The development of the cord 

 also helps to demonstrate the various tracts, since it has been 

 found that the fibres of outgoing tracts become functionally 



