REFLEX ACTION. 249 



various levels of the spinal cord some time after destruction of 

 tht- Rolandic area of the cerebrum (see p. 270). The pyramidal 

 fibers are degenerated and they occupy the areas indicated 

 in Fig. 42. Since the degeneration occurs below the destruction, 

 it is called descending degeneration, in contradistinction to as- 

 cending degeneration, which we saw to follow section of the 

 posterior roots between their ganglia and the cord (see p. 246). 



To sum lip, the sensory impulse on entering the spinal cord 

 by the posterior root, by traversing a collateral, may take the 

 shortest possible pathway to the efferent nerve cell of the an- 

 terior horn, or it may avoid this and travel up the posterior 

 columns of the cord to the medulla, thence by the fillet to the 

 cerebral cortex of the opposite side, and thence down the pyra- 

 midal tracts to the anterior horn cells. In this long cerebral 

 route there are at least three places where the impulse must pass 

 by means of a synapsis from nerve fibers on to nerve cells, and 

 then along the nerve fibers arising from these. These three 

 places are: (1) in the medulla, (2) in the cerebral cortex, (3) 

 in the anterior horn. 



This long cerebral route, as it is called, is by no means the 

 only one along which afferent impulses may travel to the brain. 

 Some may be carried by collaterals to certain cells of the grey 

 matter of the cord, and from these cells fibers may run up the 

 cord to the cerebellum or lesser brain. These cerebellar tracts 

 are located in the lateral columns of the cord outside the crossed 

 pyramidal tracts (see Fig. 42). They do not degenerate when 

 the posterior roots are cut, but do so after section of the cord 

 itself (this distinguishing them from the fibers in the posterior 

 columns). The impulses which they transmit to the cerebellum 

 have to do with certain subconscious sensations concerned in 

 the maintenance of the tone of the muscles. There are also 

 certain pathways in the white matter of the cord which trans- 

 mit descending impulses from the cerebellum. 



The main bundles of ascending and descending fibers in the 

 spinal cord are charted in Fig. 42, which should be carefully 

 studied. 



THE EFFERENT FIBER, OR NEURONE. As already explained 



