THE PYRAMIDAL TRACT 37 



by small lesions in the medulla oblongata (see bulbar paralysis, 

 p. 1 08). 



The Path of the Motor Fibres. The motor fibres have 

 already been traced from their origin in the anterior central 

 gyrus, through an intermediate area termed the corona radiata 

 (Fig. 21), into the internal capsule. Lesions of the corona 

 radiata tend to resemble cortical lesions in their distribution, 

 as, unless situated immediately above the internal capsule, 

 they never produce complete hemiplegia (p. 6). Irritative 

 lesions affecting the corona radiata alone differ from similar 

 cortical conditions in that they cause tonic instead of clonic 

 muscular spasms. 



From the internal capsule, the motor fibres pass directly 

 into the basis pedunculi (crusta of the mid-brain, Fig. 21), 

 where they form a compact bundle, which is termed the 

 pyramidal tract. At the lower border of the mid-brain, the 

 pyramidal tract enters the ventral portion of the pons, and 

 is there broken up into a number of small bundles by the 

 grey matter and the transverse fibres of the pons. These 

 separate bundles are reassembled below, and they descend 

 through the ventral part of the medulla oblongata, forming 

 the surface elevation which is termed the pyramid. 



In the lower part of the medulla oblongata, the great 

 majority of the fibres of the pyramidal tract pass backwards 

 and cross the median plane. As they do so, they intersect 

 the corresponding fibres of the opposite side, and the general 

 arrangement is referred to as the decussation of the pyramids. 

 These fibres, now known as the lateral or crossed pyramidal 

 tract, descend in the lateral funiculus (column) of the spinal 

 medulla and terminate by arborising round the cells of the 

 posterior column (cornu) of grey matter. These cells send 

 their fibres forwards to end round the cells of the anterior 

 column, and these in turn give rise to the fibres which 

 constitute the anterior roots of the spinal nerves. Thus, 

 three neurones are concerned in the passage of stimuli from 

 the cerebral cortex to the muscles. 



