THE NERVOUS SYSTEM 



465 



Descending Tracts in the Cord. — These run from the brain to 

 the spinal cord in the ventral columns of the white matter, and 

 give off aborisations to the motor neurones of the ventral or 

 motor roots of the spinal nerves (Fig. 139). Most probably 

 there are other descending paths not yet discovered, but those 

 which are clearly known are as follows : 



Direct Pyramidal Tract. 

 Crossed Pyramidal Tract. 

 Rubrospinal Tract. 

 Proprio-spinal Descending Tracts. 



Direct Pyramidal Tract. — This is only found in man and the 

 anthropoid apes. It is associated with complex, skilled, and 

 delicate muscular movements, such as occur in man, and it is 

 through this channel that these movements are produced and 



A B 



Pyd 



MAN MONKEY 00C 



Fig. 138. — Diagram to illustrate the Relative Size of the Crossed Pyra- 

 midal Tract (Py) in the Dog, Monkey, Man (Foster, after Sherring- 

 ton). 



In B, Py' is an outlying portion of the pyramidal tract separated from the rest 

 by the cerebellar tract. Py.d in A is the direct pyramidal tract only present 

 in man. 



directed. The more highly developed the brain, the larger the 

 direct pyramidal tract. In Fig. 138 the position and relative 

 size of this tract in man is shown. Both pyramidal tracts are 

 spoken of as corticospinal, to indicate that they connect the 

 cortex of the cerebrum with the spinal cord. 



Crossed Pyramidal Tract. — This is the main descending motor 

 path in animals (Figs. 138, C; 139, A). Nevertheless, it is relatively 

 smaller than in man, for the size of this tract bears some relation 

 to the size of the motor areas in the brain. Where these are 

 small, as in the horse, ox, and sheep, the tract is small. Small 

 motor areas suffice for animals such as the horse, where the limb 

 movements are of a simple pendulum type. The combined areas 

 of the two pyramidal tracts in man constitute nearly 12 per 

 cent, of the total cross area of the cord. The following shows 



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