ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 619 



of the posterior roots. It is formed of fibers which are larger on the 

 surface and much finer in the depths. 



This column comprises five different systems of bundles. They 

 are: (1) the direct cerebellar; (2) the bundle of Gowers, or ascend- 

 ing antero-lateral-ccrebellar tract; (3) the crossed pyramidal tract ; 

 (4) vestibulo-spinal tract; (5) deep lateral, or lateral marginal, zone. 

 The direct cerebellar bundle, or tract, is situated at the posterior 

 and superficial part of the lateral column in the form of a very thin 

 band. It extends from the second lumbar upward to the restiform 

 bodies, into the vermis of the cerebellum. It is formed of a collec- 

 tion of centripetal fibers which unite the cerebellum to different 

 levels of the vesicular column of Clarke. It develops ascending de- 

 generation. About the cells of Clarke arborize the collaterals of the 

 posterior root so that there is an indirect communication between the 

 posterior roots and the cerebellum. 



The bundle of Goivers, or ascending antero-lateral tract, occupies 

 the anterior superficial zone of the lateral column. This bundle com- 

 mences at its inferior part in the lumbar swelling, increasing in size 

 as it ascends by two orders of roots, pome fine, others large. It termi- 

 nates by its fine fibers in the lateral nucleus of the medulla ; by its 

 larger fibers in the cerebellum by way of the superior peduncle. This 

 tract undergoes ascending degeneration. 



The crossed pyramidal tract (motor tract or cerebral crossed 

 tract) is situated inside the cerebellar tract. The term has been 

 applied to that which is contained within the pyramids of the 

 medulla, and which decussates at this level with the opposite tract. 

 It decreases in vo'ume from above downward to terminate in from 

 the second to the fourth lumbar pair. 



It is composed of long, centrifugal fibers which unite the motor 

 regions of the cortex of the brain with the motor cells of the anterior 

 horns of the cord. It undergoes descending degeneration as the re- 

 sult of lesions which seize the cortex, internal capsule, or cerebral 

 peduncle. 



A lesion of the pyramidal tract in the cord produces monoplegia 

 below the lesion and on the same side. Its degeneration, as a result 

 )f lesion of the brain, gives place to a crossed hemiplegia. whose 

 clinical mark is a spasmodic contracture. 



It is well to remember that there is a double decussation of the 

 lotor fibers : one at the level of the neck of the medulla oblongata, 

 ic other much lower the length of the white commissure. From 

 this the student can comprehend why in the majority of hemiplegias 



