THE SPINAL CORD. 277 



and, with few exceptions, undergo a >- or h- like branching into ascend- 

 ing and descending limbs, which assume a longitudinal course and pass up 

 and down in the cord for a variable distance. During their course both 

 limbs give off collateral branches which bend sharply inward and pass hori- 

 zontally into the gray matter to end chiefly in relation with the celL of the 

 posterior horns, from which cells new secondary paths arise. The main 

 stem-fibres of the descending and of the short ascending limbs also end in 

 the manner just described. In addition to the short collaterals destined for 

 the cells of the dorsal horn, others, the ventral reflex collaterals, traverse 

 the posterior horn and intermediate gray matter to end in arborizations 

 around the ventral radicular cells, and thus complete important reflex arcs 

 by which impulses transmitted through the dorsal roots directly impress the 

 motor neurones. With possibly the exception of certain fibres said to pass 

 directly to the cerebellum, all the sensory root-fibres end around neurones 

 situated either within the gray matter of the spinal cord or within the nuclei 

 of the medulla. Thence the impres- 

 sions are conveyed by axones of 

 these secondary neurones to higher 

 centres, to be taken up in turn by 

 tertiary neurones, in the sequence 

 of the chain required to complete 

 the path for the conduction and 

 distribution of the impulse. 



The long ascending poste- 

 rior tract includes the dorsal root- 

 fibres that pass uninterruptedly up- 

 wards within the posterior column 

 (within the tracts of Goll and of 

 Burdach) as far as the nuclei (graci- 

 lis and cuneatus) of the medulla. 

 Many other root-fibres however FJG 324 ._ Section of cord through middle of thoracic 



ascend for Only a short distance and region. X 6. (Preparation by Professor Spiller.) 



then bend inwards to end around 



the cells of the posterior horn. Among the most important of such fibres 

 are those that pass to the neurones of Clarke's column (page 271), around 

 which they end in telodendria. The axones of these neurones continue the 

 path for the impulses received from the dorsal root-fibres by cutting across 

 the gray matter and lateral column to the periphery where, bending brain- 

 wards, they form the important direct cerebellar tract {fasciculus spino- 

 cerebella ris dorsalis ) . 



Among the many neurones of the posterior horn around which the 

 dorsal root-fibres end, some send their axones into the lateral columns to 

 form the ascending tract of Gowers {fasciculus spino-cerebellaris ven- 

 tralis*), which occupies the periphery of the cord immediately in advance of 

 the direct cerebellar tract. Others send their axones into the antero-lateral 

 column of the opposite side and ascend as the spino-thalamic tract, a 

 sensory pathway of importance but diffuse. 



The foregoing tracts are all concerned in transmitting, directly or 

 indirectly, the impulses brought by the dorsal root-fibres to higher levels; 

 they are all, therefore, ascending paths. In order, however, that the spinal 

 cord with its long series of motor nerves shall be brought under the influence 

 of the volitional and coordinating impulses arising within the cerebrum and 

 cerebellum respectively, it is evident that descending tracts composed of 



