WHITE MATTER OF THE SPINAL CORD. 1047 



The endogenous fibres arise as the axones, chiefly of the inner cells of the posterior horn, 

 as well as from the cells of the intermediate gray matter (Ziehen), and in great measure cross 

 by way of the anterior white commissure to the opposite anterior column. After undergoing 

 T-division, their upwardly directed limbs constitute the ascending paths and those coursing 

 downward the descending ones. While both sets of fibres for the most part pursue only a short 

 path, that of the descending limbs is usually the longer, the fibres entering the gray matter to 

 end in relation with the anterior horn cells of lower levels. They are, therefore, regarded as 

 secondary reflex paths. The termination of the ascending limbs is uncertain, but probably is 

 within the gray matter of the posterior horn. 



The exogenous tracts of the anterior ground-bundle, have been mentioned in connection 

 with those of the lateral column. Certain endogenous fibres claim attention, which ascend 

 partly intermingled with the fibres of the vestibulo-spinal tract and partly within the ventral 

 portion of the anterior ground-bundle, although not grouped as a consolidated tract. These 

 fibres belong to the important spino-thalamic system and take origin from the posterior horn- 

 cells of the opposite side of the cord. After crossing by way of the white commissure, instead 

 of cutting through the adjacent anterior horn and ascending amongst the constituents of 

 Gowers' tract, the fibres in question arch ventrally and pass brainward intermingled with the 

 vestibulo-spinal fibres. This part of the path connecting the spinal cord with the thalamus is 

 sometimes noted as the anterior spino-thalamic tract and, according to some authorities, is 

 concerned particularly in carrying impulses of pressure and touch. The anterior column also 

 probably contains fibres that descend from the roof nucleus of the cerebellum and from the 

 quadrigeminal bodies. Since most of such fibres occupy a ventro-median position, they have 

 been designated the sulco-marginal tract. 



In recapitulation the chief fibre-tracts of the spinal cord may be grouped as follows: 



I. Within the Posterior Column — 

 Ascending Paths : 



Direct ascending posterior root-fibres. 



Ascending endogenous fibres. 

 Descending Paths : 



Descending posterior root-fibres (comma tract). 



Descending endogenous fibres. 



II. Within the Lateral Column— 

 Ascending Paths : 



Direct cerebellar tract. 

 Gowers' tract. 

 Spino-thalamic tract. 

 Spino-tectal tract. 

 Short endogenous fibres. 

 Descending Paths : 



Lateral pyramidal tract. 



Indefinite exogenous tracts (including the rubro-spinal, quadri- 



gemino-spinal and olivo-spinal). 

 Descending endogenous fibres. 



III. Within the Anterior Column — 



Ascending Paths : 



Ascending endogenous fibres from posterior horn cells. 

 . Ascending endogenous fibres from anterior horn cells. 

 Descending Paths : 



Direct pyramidal tract. 



Vestibulo-spinal tract. 



Sulco-marginal tract 



Blood-Vessels of the Spinal Cord. — -The arteries supplying the cord are 

 from many sources — the vertebral, deep cervical, intercostal, lumbar, ilio-lumbar and 

 lateral sacral of the two sides — since the vascular net-work within the pia accompanies 

 the nervous cylinder throughout its length. Above and within the skull, the verte- 

 bral arteries give of? the two anterior and the two posterior spinal arteries, of which 

 the latter retain their independence and descend upon the dorso-lateral surface of the 

 cord, one on each side, in front of the posterior nerve-roots. The two anterior 

 spinal arteries, on the other hand, soon unite (somewhere above the level of the 

 third cervical nerve) into a single trunk, which descends along the ventral surface of 

 the cord, just in front of the anterior median fissure. 



