354 PHYSIOLOGY 



descending branches pass down for two or three segments in the comma tract 

 before turning into the grey matter. The tract, however, contains fibres of 

 other origin, some of which begin and end in the spinal cord itself. 



(6) TRACT OF MARIE. This, also in the anterior column, contains 

 both descending and ascending fibres and is largely a continuation of the 

 posterior longitudinal bundle, the connections of which we shall have to 

 study later on. A small tract of fibres, which degenerate in the descending 

 direction, is also found in the posterior part of the cord adjoining the posterior 

 longitudinal fissure. 



(7) SEPTO-MARGINAL BUNDLE. This is largely proprio-spinal, but 

 may contain fibres coming from the mid-brain. 



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, especially 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 dis- 

 tinguished 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 DIRECT 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 resti- 

 form 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 



