296 NORMAL HISTOLOGY. 



elements, since the tissues of the cord are prolonged into the medulla, 

 where the increased importance of parts before relatively incon- 

 spicuous, together with the addition of new masses of nervous matter, 

 brings about the redisposition of the structures continued from 

 the cord. The changes taking place in the transition of the cord 

 into the medulla consist primarily in a modification of th^ gray 

 matter ; the principal factors are the gradual increase in the size 

 of the tracts of the posterior column and the decussation of fibres 

 from the lateral column destined to aid in forming the anterior pyra- 

 mids. The changes wrought by the first 

 FlG - 3 2 9. factor are earliest indicated, and affect par- 



ticularly the posterior cornua of the gray 

 substance, while the second modifies the 

 anterior horns. 



An intimation of the changes to follow 

 is seen in sections as low as the first, or 

 even second, cervical nerve in the thick- 

 ened club-shaped accumulation of gray 

 matter representing the posterior cornu, 

 connected by an extended and attenuated 

 stalk with the chief mass. With the pro- 

 gressive increase in the size of the columns 

 of Goll (funiculus gracilis) and the col- 

 umns of Burdach (funiculus cuneatus} 

 the posterior horns are displaced more and 

 more laterally and ventrally until the cornua with their supporting 

 necks lie nearly horizontally, forming almost a right angle with the 

 posterior median septum. The increased mass of the substantia 

 gelatinosa not only approaches the surface, but gradually displays its 

 growth by the formation of the projection known as the funiculus 

 of Rolando, which, higher up, expands into the tubercle of Ro- 

 lando. The greater size which the tracts of the posterior column 

 assume is produced by the accession of masses of new gray matter, 

 the nucleus gracilis and the nucleus cuneatus. These gray 

 nuclei are at first narrow, but become more robust as the medulla is 

 ascended. The ascending fibres of the posterior cord columns (Goll 

 and Burdach) end in arborizations closely related to the cells of the 

 nucleus gracilis and cuneatus ; from these nuclei new fibres spring 

 which pass partly to the cerebellum of the same and the opposite 

 side, as the posterior and the anterior external arcuate fibres, and 

 partly to higher levels by way of the mesial fillet, as the decussating 

 deep arcuate fibres. 



With the opening out of the central canal of the cord into the 

 fourth ventricle the gray matter lying originally dorsally to the canal 



Diagram of spinal cord indicating 

 the paths taken by fibres of crossed 

 pyramidal tract (6) to gain the an- 

 terior columns (a), and by fibres of 

 posterior column (s) higher up to 

 form sensory decussation. (After 

 Testut.) 



