284 



MICROSCOPIC AS ATOMY OF THE ORGANS. 



horns in the region of the motor cell groups (reflex collaterals). 

 Only very few of these collaterals pass through the dorsal com- 

 missure to the opposite side. The descending branches of the 

 dorsal column fibres run only for a short distance, while the 

 ascending branches reach usually as far up as the medulla, where 

 they end in the nuclei of the. columns of Goll and Burdach. 

 The fibres in their courseupward tend to approach the median 

 side of the dorsal column, while the newly entering fibres of 

 the dorsal root are always lateral to those arising in ganglia 



FIG. 214. 



Central canal 



Gray matter 



Developing 

 astrocyte 



Dorsal root 



Ependyma 

 ~7 cells 



Ventral_ 

 root 



Transverse section through the spinal cord of an eight-day chick. Left, nerve cells; 

 uro^lia cells; a and b, motor cells; c, cells of lateral columns; d and e, hecatero- 

 meric cells. X SO. 



lower down. Thus in cross-sections of the cord the fibres enter- 

 ing low down (e, g., those supplying the lower extremities with 

 sensory nerves) are situated always near the septum dorsale 

 in the fasciculus gracilis ; while similar fibres for the upper 

 extremities are placed quite laterally in the fasciculus cuneatus. 

 A slight addition to the fibres contained in the dorsal column 

 is afforded by axones from small cells on the dorsal horn. 

 These fibres after running for a short distance in the fasciculus 

 cuneatus sink into the gray substance. 



