COURSE OF NERVE-FIBRES. 25 



more developed on one side than on the other, or the tract on one side may be wholly unde- 

 veloped. The direct tract is said to be wanting in 15 per cent, of cases in man. In different 

 animals there is also much variation in the position and size of the pyramidal tracts. A 

 well-marked direct pyramidal tract appears to be absent in most animals, even in monkeys. 

 In some (mouse, rat, guinea-pig) the pyramidal tracts are in the posterior columns, but in 

 most animals (rabbit, cat, dog) they run in the lateral columns as in man. The fibres of the 

 pyramidal tract are probably connected with the anterior horn by collateral fibres, which ramify 

 amongst the large cells that give origin to the anterior nerve-roots (figs. 1 6, 20). 



The pyramidal tracts are undoubtedly the paths by which voluntary impulses pass from 

 the brain to the various spinal segments. All the fibres within the area embraced by the 

 " tract " are not, however, of the same nature, although fibres of the one function predominate : 

 and this is probably correct of all the so-called " tracts " of conduction. 



There are at least two descending tracts in the antero-lateral column, besides the 

 direct and crossed pyramidal. One, the antero-lateral descending cerebellar 

 tract (anterior marginal bundle of Loewenthal), consists of fibres which are connected 

 with cells in the cerebellar cortex of the same side, and which undergo degeneration 

 on removal of the corresponding half of the cerebellum (Marchi). These fibres form 

 an extensive circumferential tract in the anterior three-fourths of the antero-lateral 

 column, spreading inwards in front of the crossed pyramidal tract to reach the 

 intermedio-lateral tract of the grey matter. The tract which is thus marked out 

 (see fig. 26, p. 32) embraces (in the dog) the part of the anterior column which 

 in man is occupied by the direct pyramidal tract, and also the whole region of the 

 tract of Gowers (see below), the fibres of these two tracts being intermingled. Some 

 of the fibres of the anterior roots also exhibit degeneration after removal of the 

 cerebellar hemisphere, and are therefore probably directly continued from fibres of 

 this tract. 



In the monkey a few fibres in this column degenerate after lesions of the cerebral 

 hemisphere of the opposite side. They are intermingled with those of the descending 

 cerebellar tract and with those of the tract of Gowers, and are connected with cells 

 in the Rolandic region of the cerebral cortex, as shown by the fact that they 

 degenerate after lesions of that region. These fibres may perhaps be regarded as 

 belonging to the system of the direct pyramidal tract, which in the monkey docs 

 not exist as a well-marked tract as in man : it has not yet been ascertained whether 

 they occur in man as well as the direct pyramidal. 



Ascending tracts in the antero-lateral column. The dorso-lateral 

 ascending cerebellar tract (direct lateral cerebellar tract of Flechsig) (fig. 14) lies 

 between the lateral pyramidal tract and the outer surface of the cord, occupying a 

 somewhat narrow area of the transverse section, which in the upper regions of the 

 cord reaches to the tip of the posterior horn, but lower down becomes more limited, 

 and is separated from the horn by the intervention of the adjoining pyramidal tract. 

 It begins to appear at the lower dorsal region in man, and is then seen in all sections 

 of the cord and lower part of the bulb, passing eventually by the restiform body 

 into the cerebellum (middle lobe) (see fig. 27, p. 33). 



It is found that there are a few fibres scattered through the neighbouring parts 

 of the lateral column which, from their development simultaneously with those of 

 the cerebellar tract, should be apparently reckoned with it. The axis-cylinder 

 processes of the cells of Clarke's column are said to give origin to the fibres of the 

 dorso-lateral cerebellar tract. The fibres of this tract acquire their medullary sheath 

 somewhat earlier than those of the pyramidal tract. They are also considerably 

 larger. 



The ventro -lateral or antero-lateral ascending cerebellar tract (antero- 

 lateral ascending tract of Gowers) occupies a position in the sectional area of the 

 lateral column which is anterior or ventral to the dorso-lateral cerebellar tract, and 

 It has in section an arched shape curving from immediately in front of the crossed 



