THE SPINAL CORD. 



385 



opposite side in the gray commissure, and partly become longitudinal 

 in the gray substance of the same side. 



Connections of the Spinal Cord with the Brain, 



The connections of the spinal cord with the brain take place by a con- 

 tinuation of the fibres of its longitudinal columns, through the medulla 

 oblongata and tuber annulare. But this continuation is not entirely a 

 simple one, since the fibres of the various columns shift their position 

 at the level of the medulla oblongata, and also exhibit at one point or 

 another more or less complete decussations. 



Decussation of the Pyramids. Throughout the greater portion of 

 the cord, its columns are formed almost exclusively of longitudinal 

 fibres, and each column, examined in successive sections, retains its 

 special form and position. But in the upper cervical region a change 

 begins to show itself by which the fibres from the inner and posterior 

 parts of the lateral column are directed obliquely inward and forward, 

 through the base of the anterior horn of gray matter and behind the 

 anterior column of the same side. Above the level of the second cer- 

 vical vertebra this change increases in extent, so that bundles of fibres 

 from the lateral column pass obliquely forward and upward, across the 

 median line, to the opposite border of the anterior median fissure ; thus 

 taking the place, immediately next this fissure, previously occupied by 

 the anterior column. As the 



same thing happens on both FIG. 104. 



sides, there appear, in the lower 

 part of the medulla oblongata, at 

 the bottom of the anterior median 

 fissure, alternating bundles of 

 fibres, successively crossing from 

 left to right and from right to 

 left. This is the decussation of 

 the anterior pyramids; and 

 after its completion these bodies 

 appear as two longitudinal bun- 

 dles, next the anterior median 

 fissure, and running forward to 



the tuber annulare. The ante- TRANSVERSE SECTION OF HUMAN SPINAL CORD ; at 



rior pyramids accordingly are ^ d e s r extremity of the decussa tion of the 

 not continuations of the ante- 

 rior columns of the cord, although placed above them in linear series. 

 Their fibres are derived mainly from the lateral columns of the oppo- 

 site side ; and they form, in the lower part of the medulla oblongata, 

 a decussation which is visible externally because it takes place by distinct 

 bundles of considerable size, alternating with each other on the median 

 line. 



This transfer of fibres from the lateral columns to the anterior pyra- 

 mids, next the median fissure, accounts for the change in form of the 



Z 



