WHITE MATTER OF THE SPINAL CORD. 1041 



sections of the cord in the cervical region the long fibres entering by the lower nerve- 

 roots occupy the inner part of Coil's column. In the lumbar cord, they are excluded 

 from the median septum by a narrow hemielliptical area, which with that of the 

 opposite side forms the oval field of Flechsig. The fibres entering by the lower 

 thoracic nerves lie more laterally, while those entering by the upper thoracic and 

 cervical nerves appropriate the adjoining part of Burdach's tract, the lateral area of 

 which, next the posterior horn, is occupied chiefly by the posterior root-fibres. 



It must be understood that while in a general way the fibres of the long ascending tracts 

 have the disposition just indicated, they are so intertwined and mingled with the strands passing 

 to and from the gray matter that the definite outlines of their conventional area, as represented 

 in diagrams, are wanting. Collectively the 



fibres composing this tract are of medium or p- I(; 



small size, but acquire their medullary coat 

 very early, myelination beginning about the 

 fourth foetal month, although not completed 

 until the ninth (Bechterew). 



The termination of the long ascend- 

 ing fibres is chiefly in relation with the 

 neurones within the lower part of the 

 medulla the fibres of Goll's tract end- 

 ing about the cells of the nucleus gracilis 

 and those of Burdach's tract about the 

 cells of the nucleus cuneatus. From 

 these stations paths of the II order 

 convey the impulses to the cerebel- 

 lum, by way of the inferior cerebellar 

 peduncle, and to the higher sensory 

 centres by way of the mesial fillet, as 

 later described (page 1115). Whether 

 certain of the component fibres of these 

 ascending tracts are directly continued 

 to the cerebellum, and perhaps to the 

 mesial fillet, without undergoing inter- 

 ruption in the nuclei of the medulla is still uncertain, although supported by the 

 statements of Hoche, Kolliker, Solder and others. 



The root-fibres passing to Clarke's column occupy the middle and median 

 part of Burdach's tract, mingled with those of the long ascending paths. After cours- 

 ing longitudinally, usually for some distance, within the posterior column, they bend 

 outward, and, sweeping in graceful curves, enter the gray matter to end about 

 Clarke's cells. It is noteworthy that the level at which they end is often considerably 

 higher than that at which the root-fibres enter the cord, an arrangement which 

 explains the fact that lesions of the lowermost of these strands may be followed as 

 ascending degenerations into the thoracic region (Mayer). On entering the gray 

 matter the terminal arborization of a single root-fibre usually ends in relation with 

 several neurones of Clarke's column (Lenhossek). The important sensory path of 

 the II order, known as the direct cerebellar tract (page 1044), arises as the axones 

 of these neurones. 



The anterior reflex fibres to the ventral horn are all collaterals, not continu- 

 ations of the stem-fibres, far the greater part of which come from the fibres of the 

 long ascending posterior tract. These collaterals penetrate the gray matter princi- 

 pally at the median border of the head of the posterior horn, behind Clarke's 

 column, but partly also through the substantia Rolandi, and thence pass ventrally or 

 ventro-laterally, with a slightly curved or sigmoid course, towards the anterior horn. 

 As they enter the latter, the collaterals diverge more and more and are distributed 

 to the various groups of the anterior horn cells, chiefly in relation with the lateral 

 groups of radicular cells from which the ventral root-fibres arise ; they thus establish 

 direct reflex paths by which sensory impulses conveyed by the posterior root-fibres 

 impress the motor neurones, while, at the same time, these impulses are transmitted 



66 



Section of spinal cord at level of second cervical seg- 

 ment ; formatio reticularis fills bay between posterior and 

 anterior cornua; substantia gelatinosa caps apex of pos- 

 terior cornu. Drawn from Weigert-Pal preparation made 

 by Professor Spiller. X 6. 



