THE NERVE TRACTS. 719 



anterior white commissure. Further, it must be remembered that many fibres 

 which descend in and constitute part of the motor tract decussate before reaching 

 the medulla, and terminate by forming synapses with the nuclei of the cranial 

 nerves situated near the aqueduct of Sylvius, in the pons or in the medulla itself. 

 There is considerable variation in the extent to which decussation takes place in 

 the medulla, the commonest condition being that in which about three-fourths of 

 the fibres decussate in the medulla and the remainder in the cord. 



Other Descending Tracts. 



1. From the cortex of the frontal lobe, anterior to the Rolandic area, fibres 

 arise which descend through the anterior limb of the internal capsule and enter 

 the crusta, where they lie to the inner side of the pyramidal tract ; finally they 

 enter, and end in, the pons. 



2. Descending fibres also take origin in the temporo-occipital cortex and pass 

 through the posterior limb of the internal capsule behind the fibres from the 

 Rolandic area. They pass through the crusta, where they lie to the outer side of 

 the same tract, and end in the pons. 



3. A small tract arises from the cells of the caudate nucleus and descends to 

 end in the substantia nigra or pons. In the crus cerebri it lies immediately above 

 the motor tract, which is on its ventral aspect. 



The Sensory, Afferent, Ascending Tract. 



The course taken by those fibres of the posterior nerve-roots which ascend has 

 been arrived at by dividing the nerve-roots between their ganglia and their entrance 

 into the spinal cord and subsequently examining the degenerated areas. It has 

 been found that the fibres pursue an oblique course, being situated at first in the 

 outer part of Burdach's column ; higher up they occupy the middle of this column, 

 being displaced inward by the accession of other entering fibres, while still higher 

 they enter and are continued upward in the column of Goll. The upper cervical 

 fibres do not reach the column of Goll, but are entirely confined to that of Bur- 

 dach. The degeneration method proves that the localization of these fibres is very 

 precise : the sacral nerves lying to the inner side of Goll's column and near its 

 periphery ; the lumbar nerves to their outer side ; the dorsal nerves still more 

 laterally : while the cervical nerves are confined to the outer part of Burdach's 

 column. 



On reaching the medulla these ascending fibres end by arborizing around the 

 cells in the gracile and cuneate nuclei, and the further upward course of the tract 

 is effected by the axis-cylinder processes of these cells. These new fibres decus- 

 sate in the medulla, dorsal to the crossing of the motor tract, in what is termed 

 the superior pyramidal decussation, the sensory decussation, or decussation of the 

 fillet ; terms which are synonymous. Having crossed the middle line they ascend 

 through the pons and tegmentum of the crus cerebri, and, reaching the ventral 

 surface of the optic thalamus, the majority end either in the subthalamic region or 

 in the optic thalamus, but a small proportion is continued directly into the brain 

 cortex. From the gray matter of the optic thalamus the fibres of the third link 

 in the chain arise. They pass through the internal capsule and end in the cere- 

 bral cortex : those which go to the fronto-parietal cortex being situated in the 

 extreme front part of the anterior limb of the internal capsule, while in the hinder 

 extremity of the posterior limb other fibres pass to their distribution in the tem- 

 poral and occipital cortex. 



Other Ascending Tracts. 



The direct cerebellar tract begins about the level of the second lumbar vertebra, 

 and is the continuation upward of the axis-cylinders of Clarke's column. At the 

 upper end of the cord it passes into the restiform body and through this reaches 



