492 ELEMENTARY PHYSIOLOGY 



lateral column (Fig. 155, asc. a.l., asc. a.l.) and commences 

 rather lower down in the cord than does the cerebellar 

 tract, but like the latter runs up to the si)inal bulb, and 

 enters the cerebellum by mean of its superior peduncle. 



B. Tracts of descending degeneration. 



(i) The Crossed Pyramidal Tract. — This is a large 

 and conspicuous tract in the inner and hinder part of the 

 lateral column (Fig. 156, Cr.p., Cr.'p.'). 



It extends along the whole length of the cord, passing 

 down into the cord from the spinal bulb. The fibres of 



PF. 



D/py A.F. 



FlO. 156. — DiAOKAM TO SHOW THE POSIIION OF TRAUTS OF DESCENDING 



Deokneration in the Wfiite Matter of the Spinal Cord at the 

 SAME Level as in Fig. I'm. 



Cr.p., Cr'.p'. crossed pyramidal tracts; D'.p'., D.p. direct pyramidal 

 tracts ; dexu. a.l ., desc. a.l. descending antero-lateral tract ; p.p., p.p. 

 prepyrainidal tract. 



this tract are believed to communicate with those cells in 

 the anterior horns of the grey matter who.se process, as 

 previously described (p. 468), gives rise to the nerve fibres 

 which leave the cord as the efferent (motor) anterior roots 

 of the spinal nerves. The communication is made by a 

 small neuron interposed between the two. It may thus 

 be regarded as the path for efferent impulses coming 

 down the cord on their way to outlying parts of the body. 

 But this tract, unlike those we have so far described, 

 does not end, or rather we .sjiould now .say begin, in the 

 spinal bulb. On the contrary, it may be traced up through 



