ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 631 



themselves outward and backward for a little distance, then bend 

 inward to pursue a parallel course again. By this course there is 

 formed a sort of elliptical buttonhole which is inclined obliquely from 

 bottom to top. Traversing this buttonhole are found the crossed 

 pyramidal bundles; both are carried toward the median line, where 

 they decussate with the opposite side to produce the pyramidal decus- 

 sation. Thus, the two principal bundles of the anterior columns 

 have become posterior in the medulla, where they are placed in the 

 deepest part of the pyramids. 



LATERAL COLUMNS. The crossed pyramidal bundle in the 

 medulla bends toward the median line. Here it meets its fellow 

 of the opposite side, with which it decussates in the manner of a 

 twist to arrive in the opposite side of the medulla. At this level, in 

 the same pyramid of the medulla, there exist side by side the direct 

 pyramidal column of the same side of the cord and the crossed pyra- 

 midal bundle of the opposite side. These two bundles now form one 

 and the same group of nerve-fibers. This type of fibers forms the 

 pyramidal, or cerebral motor, tract. Along this course descend 

 motor messages to the voluntary muscles from the brain to the 

 anterior horns of the cord, and then along axis-cylinders to the 

 motor plates in muscles. 



An act incited by an impulse traveling along this course is 

 always crossed, since the left hemisphere of the brain, for example, 

 carries the order of motor power to the right half of the spinal cord 

 by the crossed pyramidal fibers and to the left half of the spinal 

 cord by the direct pyramidal tract. The latter tract decussates 

 throughout the length of the cord with its fellow of the opposite 

 side. Thus, the result is that the decussation is total for the pyra- 

 midal tract in its complete action, and that all of the voluntary parts 

 excited from some part of the cerebral hemisphere end in muscles 

 of the -opposite side of the body. From this the student will deduce 

 that lesions which affect the pyramidal tract above the medulla 

 oblongata have as their direct result a motor paralysis opposite to 

 the lesion; in other words, a crossed hemiplegia. 



POSTERIOR COLUMNS. The columns of Goll ascend to the 

 medulla, where they pass, without decussation, into the postpyra- 

 midal nucleus, or nucleus of Goll. By this nucleus it is carried into 

 the cerebellum, following part of the restiform body; another part 

 is placed in relation with the nuclei of the pons. 



The column of Burdach comprises the longitudinal commissural 

 fibers, the root-fibers of the posterior roots, and the sensory fibers 



