354 PHYSIOLOGY 



B. ASCENDING TRACTS 



These may be divided according as they are situated in the posterior, the lateral, 

 or the anterior columns. 



(a) THE POSTERIOR COLUMNS. Almost the whole of the fibres making 

 up these columns are exogenous, being axons of cells in the posterior root ganglia. 

 They can be divided into long, medium, and short fibres, all of which, on their way up, 

 give off collaterals, which pass into the grey matter and ramify round nerve-cells, especi- 

 ally in the posterior horns (cp. Fig. 160). The longest fibres pass to the upper end of 

 the cord, where they end in the posterior column nuclei, the nucleus gracilis and the 

 nucleus cuneatus of the medulla. These fibres remain entirely on the side of the cord 

 on which they have entered. As they pass up they are displaced towards the middle 

 line by each incoming and higher placed root. Thus in the cervical region, and indeed 

 from the fifth dorsal segment upwards, two columns can be distinguished in the posterior 

 part of the cord, viz. the postero -median and postero-lateral columns, the division 

 between which is indicated by a small groove on the surface. The postero -median 

 column contains from within outwards the fibres from the sacral region, those from the 

 lumbar region, and those from the inferior dorsal region. The postero-lateral column, or 

 column of Burdach, contains mesially the four upper dorsal root fibres and more laterally 

 the fibres from the cervical nerves. 



(6) THE LATERAL COLUMNS. In these columns are found the two cerebellar 

 tracts, as well as scattered fibres passing to the fore- and mid-brain. 



(1) THE DIEECT OR DORSAL CEREBELLAR TRACT arises from the cells of Clarke's 

 column on its own side. It consists of large fibres, which pass through the grey matter 

 to the lateral columns of the same side, and ascend in the cord immediately ventral to 

 the incoming posterior root fibres, and external to the crossed pyramidal tract. In 

 the medulla they are joined by a bundle of fibres from the opposite inferior olive and 

 pass with the restiforrn body into the cerebellum, where they terminate in the superior 

 vermis of this organ. 



(2) THE VENTRAL OR ANTERIOR CEREBELLAR TRACT, often called the tract of Gowers, 

 arises in cells scattered through the grey matter, chiefly of the posterior horn of the oppo- 

 site side, though a few fibres are derived from cells of the same side. The tract consists 

 of fine fibres which pass upwards in the peripheral margin of the lateral column, extend- 

 ing from the direct cerebellar tract behind to the level of the anterior roots in front ; it 

 passes upwards through the cord, the medulla, and the pons, then turns round to 

 enter the cerebellum through the superior cerebellar peduncle, ending chiefly in the 

 ventral portion of the superior vermis. 



(3 ) THE SPINO-THALAMIC AND SPINO-TECTAL TRACTS. These fibres form a scattered 

 bundle lying internally to the anterior cerebellar tract, and are practically part of Gowers' 

 tract. They may be traced through the cord, medulla, and pons, and end partly in the 

 anterior corpora quadrigemina of both sides, but to a greater extent in the optic 

 thalamuB of the same side. 



(c) ANTERIOR COLUMNS. A number of scattered fibres pass up the anterior 

 columns, mingled with the descending fibres of the tract of Marie in the angle of the 

 anterior fissure. Others pass up partly to end in the olivary body, partly to run on 

 with flu- mrsirtl fillet towards the thalamic region. 



The white matter of the cord can thus be regarded as made up of short 

 and of long tracts, which maintain direct connection between the following 

 parts of the central nervous system : 



(1) Different levels of the cord itself by means of the proprio-spinal 

 fibres, 



(2) Hind-brain and spinal cord, by the anterior and posterior cerebellar 

 tracts, the posterior columns, and t ln> <] linn-olivary fibres among the ascend- 



