1328 A MANUAL OF ANATOMY 



column, directly in front of the dorsal cornu of grey matter. It is 

 separated from the outer surface of the cord by the dorsal spino- 

 cerebellar tract. It diminishes in size as it descends, and in the 

 lumbar region it becomes superficial. At about the level of the 

 third sacral nerve it terminates. The fibres of this tract have 

 their origin in the pyramidal cells of the Roland ic or motor area of 

 the cortex of the cerebral hemisphere of the opposite side. From 

 this origin they descend through (i) the internal capsule of the 

 corpus striatum, (2) the cms cerebri, and (3) the pons Varolii. On 

 leaving the pons they enter the pyramid of the medulla oblongata 

 on the side from which they have arisen. At the lower part of the 

 pyramid they cross to the opposite side and take up their position 

 deeply in the dorsal part of the lateral column of the spinal cord. 

 The fibres of the crossed pyramidal tract of one side^ therefore 

 come from the cerebral hemisphere of the opposite side, and they 

 form the inner and larger part of the pyramid of the medulla 

 oblongata, also of the opposite side. As this tract descends, fibres 

 leave it in each segment of the cord. These fibres enter the ventral 

 cornu of grey matter and end in close relation with the ventral or 

 motor cells, the axons of manj' of which form the axis-cylinder 

 processes of the fibres of the ventral or motor nerve-roots. 



The direct pyramidal tract {fasciculus cerebro-spinalis anterior B) 

 is also known as the tract of Turck. It is of small size, and is 

 situated in the anterior column, where it lies close to the ventral 

 median fissure. It diminishes in size as it descends, and usually 

 terminates about the centre of the thoracic region, but fibres have 

 been traced as low as the fourth sacral nerve. The fibres of this 

 tract, like those of the crossed pyramidal tract, have their origin 

 in the pyramidal cells of the Rolandic or motor area of the cortex 

 of the cerebral hemisphere, but in this case of the same side. The 

 fibres of the direct pyramidal tract of one side therefore come from 

 the cerebral hemisphere of the same side. They pursue a similar 

 downward path as low as the pjnramid of the medulla oblongata 

 of the same side, of which they form the outer and smaller part. 

 They take no part, however, in the decussation of the pyramids, 

 as do the fibres of the crossed pyramidal tract. Their course is 

 directly downwards into the corresponding half of the spinal cord, 

 where most of them take up their position in the anterior column, 

 close to the ventral median fissure. The fibres of the direct pyra- 

 midal tract, though they take no part in the decussation of the 

 pyramids, cross to the opposite side at regular intervals as they 

 descend in the anterior column of the cord. The crossing takes 

 place in the ventral or white commissure, and, having entered the 

 ventral cornu of grey matter of the opposite side, the fibres end, 

 like those of the crossed pyramidal tract of that side, in close rela- 

 tion with the ventral or motor cells, the axons of many of which pass 

 to the ventral or motor nerve-roots. 



Most of the pyramidal fibres therefore cross from the side on which 



