/ 



vs 



1044 HUMAN ANATOMY. 



reduction affecting the more deeply placed fibres. In consequence of these variations, the form 

 of the pyramidal tract in cross-section changes from wedge-shape to triangular, with the base 



lying at the periphery and the apex directed 

 inward. During their descent the fibres of the 

 pyramidal tract give off at different levels col- 

 laterals, which bend horizontally inward and 

 forward, enter the gray matter, and end in rela- 

 tion with the anterior horn cells. A similar 

 course is followed by the parent fibres on reach- 

 ing the segment for which they are 'destined, the 

 terminal part of the individual fibres sweeping 

 in short curves through the intervening ground- 

 bundle of the lateral column to gain the radicular 

 < ^ ~ ^'i»^ cells around which they end. By means of its 



''^'^**^"' -^a— =——«-- collaterals, each pyramidal fibre establishes rela- 



■XV, f^5^  ^^""^^SHP^ tion with several cord-segments. The fibres of 



W'^ k^^ ^ this tract are relatively tardy in acquiring their 



medullary coat, which process does not begin 



.^ ^ ^ i ^^ until the last month of foetal life and is not com- 



^°— ' "-^^-^ pleted until after the second year. 

 Section of spinal cord at level of sixtl\, thoracic 



segment; slender posterior cortiua covered with sub- rpi Aire^r^i- r'«»r*aVi«»1lQr 4-»-of-f /' + 



stantia gelatinosa ; postero-lateral angle marks greatest 1 fie UlrcCL CcFcDciiar XiaCl (^IfaC- 



fTsso'^r spiuen"""" ''°''""' ^' ^' ^''^P^'-^t'o" b>' P^^O" tus spino-cerebdlaris dorsalis), is an as- 

 cending path of the second order that 

 establishes communication between the reception sensory cord-nucleus formed by 

 Clarke's cells and the cerebellum. In cross-sections of the thoracic region, the tract 

 forms a superficial flattened comet-shaped field that occupies the dorsal half of the lateral 

 column, extending from the apex of the posterior horn forward along the periphery 

 of the cord, to the outer side of the lateral pyramidal tract, to about the anterior 

 plane of the gray commissure. Its ventral end, particularly in the lower cervical 

 region, is broadest and projects somewhat into the lateral column in advance of the 

 lateral pyramidal field. Although as a compact strand the direct cerebellar tract 

 begins at the tenth thoracic segment, it is represented by isolated fibres in the lumbo- 

 sacral region. The fibres collectively are large and become medullated about the sixth 

 foetal month (Bechterew). In a general way the fibres having the longest course 

 occupy the dorsal part of the tract and those having the shortest the ventral (Flatau). 



Arising as the axones of the cells of Clarke's column, the components of the 

 tract pass in curves almost horizontally outward through the gray matter and lateral 

 column to the peripheral field, on gaining which they bend sharply brainward and 

 ascend without interruption to the medulla. Their further course includes the pas- 

 sage through the dorso-lateral field of the medulla as far as the inferior cerebellar 

 peduncle, by which the fibres reach the cerebellum to end in relation with the superior 

 worm, on, probably, both the same and the opposite sides. 



The tract of Gowers (tractus spino-cerebellaris ventralis) constitutes another 

 pathway of the II order, which connects the cord with the cerebellum and probably 

 also establishes relations with the cerebrum. In cross-sections the tract appears 

 somewhat uncertainly defined owing to the intermingling of its fibres with those of 

 adjoining strands, but in the main it includes a superficial crescentic field that touches 

 the direct cerebellar and lateral pyramidal tracts behind, extends along the margin 

 of the cord for a variable distance, and usually ends in front in the vicinity of the 

 ventral nerve-roots. The inner boundary, separating the tract in question from the 

 lateral ground-bundle, lacks in sharpness and is overlaid by the adjoining strands. 

 Below, the tract appears about the middle of the lumbar region and continues 

 throughout the remainder of the cord. As Gowers' tract ascends, it fails to show 

 the considerable increase in size that might be expected in view of the continual 

 additions that it receives. In explanation of this, the probable mingling of some of 

 its fibres with those of the direct cerebellar tract, rather than their ending in the 

 cord, seems the most plausible (Ziehen). 



The exact origin of the constituents of Gowers' tract is still uncertain, but it is 

 very likely that its fibres are chiefly the axones of the neurones (marginal and inner 

 cells) situated within the posterior horn, partly from the same and partly from the 



