NORMAL HISTOLOGY. 



FIG. 329. 



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 the 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 

 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 (funicutus 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 gray mass of the horns 

 the caput cornu not only reaches the surface, but gradually dis- 

 plays its growth by the formation of the projection known as the 

 funiculus of Rolando, which, higher up, expands into the tubercle 

 of Rolando. The greater size which the tracts of the posterior 

 column assume is produced not only by increased component nerve- 

 fibres, but also by the accession of masses of gray matter, the nu- 

 cleus gracilis and the nucleus cuneatus, derived as extensions of 

 the thickened base of the posterior horn. These gray nuclei are 

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

 until they present the conspicuous masses producing externally the 

 elevations of the clavus and the cuneate tubercle. These nuclei 

 are covered over by a thin sheet of white matter. Embedded within 

 the latter, external to the nucleus cuneatus, lies the small accessory 

 or external cuneate nucleus. 



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 

 Testuf.) 



