1046 



HUMAN ANATOMY. 



Fig. 901. 



pyramidal tracts. Among those whose existence within the antero-lateral ground-bundle may 

 be considered as established, are the following : 



1. Rubrospinal fibres from the cells of the red nucleus within the cerebral peduncles. 



2. Tecto-spitial fibres from the cells of the anterior corpora quadrigemina. 



3. Vestibulospinal fibres from the cells of the lateral vestibular (Deiters') nucleus. 



4. Olivospinal fibres from the cells of the inferior olivary nucleus. 



Of these strands, those from the red nucleus and corpora quadrigemina (tecto-spinal fibres), 

 descend within the lateral ground-bundle, whilst the vestibulo-spinal fibres are particularly 



within the anterior ground-bundle. Although the latter includes 

 the greater part of the vestibulo-spinal fibres, which occupy 

 the ventral margin of the ground-bundle, perhaps similar fibres 

 {tractus vestibulospinalis lateralis) descend within the lateral 

 column mesial to the tract of Gowers. Associated with the 

 spino-thalamic tract are fibres, which probably arise within the 

 thalamus ; hence, thalamo-spinal fibres are recognized. For 

 the most part, the exogenous strands are so intermingled and 

 scattered, that they are without definite boundaries. An ex- 

 ception is presented by the olivary fibres, which constitute a 

 fairly distinct triangular bundle, known as Helweg's tract, 

 situated at the periphery of the cord and just behind the ante- 

 rior root-fibres. Concerning the exact relations of these fibres 

 much uncertainty exists, since by some they are regarded as a 

 descending (olivo-spinal)path and by others as an ascending (spino-olivary) one. It is probable 

 that fibres course in both directions. Collectively, these scattered descending paths are of import- 

 ance, since they bring the ventral horn cells under the coordinating influence of the higher centres. 



Section of spinal cord at level of 

 fifth sacral segment ; anterior cornua 

 small and inconspicuous. Prepara- 

 tion by Professor Spiller. X 8. 



Fig. 902. 



The Fibre-Tracts of the Anterior Column, — According to the simplest 

 classification the anterior column includes two subdivisions : (i) the anterior pyra- 

 midal tract and (2) the anterior ground-bundle. 



The anterior pyramidal tract (fasciculus cerebrospinalis anterior), also called 

 the uncrossed or direct pyramidal tract, stands in complemental relation with the lat- 

 eral pyramidal fasciculus, being composed of the pyramidal fibres that do not undergo 

 decussation in the medulla oblongata. It usually contains about 15 per cent, of the 

 pyramidal fibres, but may include a much larger proportion ; on the other hand, it 

 may be entirely suppressed when, as rarely happens, total crossing occurs. 



The direct pyramidal tract occupies the inner part of the anterior column, 

 forming a narrow area along the median fissure that extends from the white commis- 

 sure behind to near the ventral margin of the cord. Ordinarily the tract ends below 

 about the middle of the thoracic cord, but in exceptional cases, when a larger pro- 

 portion of the pyramidal fibres than usual is included in the tract, it may extend as 

 far as the middle of the lumbar enlargement, with corres- 

 ponding increase in its cross area. If, on the other hand, 

 the number of uncrossed fibres is unusually small, the tract 

 may reach only as far as the cervical enlargement, with a 

 reduction of its sagittal dimension. Although often spoken 

 of as the ' ' uncrossed ' ' pyramidal tract, this characteristic 

 applies only to the relation of the fibres at the decussation 

 in the medulla, since in their downward journey in the cord 

 the great majority of the fibres traverse the anterior white 

 commissure at appropriate levels to end in arborizations 

 about the ventral root-cells of the anterior horn of the 

 opposite side. It is highly probable, however, that some fibres do not undergo 

 decussation, but terminate about the radicular cells of the same side. 



The anterior ground-bundle (fasciculus anterior proprius), following the divi- 

 sion of Flechsig, includes the remainder of the ventral column. In front, where its 

 lateral limits are uncertain, it is continuous with the ground-bundle of the lateral col- 

 umn, the two together being often with advantage regarded as constituting a single 

 antero-lateral tract. What has been said concerning the constitution of the lateral 

 ground-bundle applies in the main to that of the anterior column, since, here as there, 

 the region bordering the gray matter contains chiefiy the short endogenous strands, 

 while the more peripheral parts of the ground-bundle are occupied by the long 

 exogenous paths, intermingled, however, with the longer intrinsic fibres. 



Section of spinal cord at 

 level of lower part of coccygeal 

 segment; differentiation of cor- 

 nua is uncertain. Preparation 

 by Professor Spiller. X 8. 



