-THE SPINAL CORD AS A CONDUCTOR 399 



(2) RUBRO-SPINAL OR PREPYRAMIDAL TRACT (also called 

 Mouakow's Bundle). This is a fairly compact group of fibres which 

 degenerate downwards after section of the cord. It is situated, in 

 cross-section, ventral to the pyramidal tracts. Its fibres can be 

 traced up to the cells in the red nucleus, a mass of grey matter in the 

 mid-brain lying ventrally to the nucleus of the third nerve. 



(3) VESTIBULO-SPINAL TRACT. This consists of scattered 

 fibres in the antero-lateral column, which degenerate in the downward 

 direction. They were formerly supposed to be derived from the 

 cerebellum of the same side, but it has been shown that they are in 

 all probability derived from Deiter's nucleus in the medulla an 

 important transmitting station between the cerebellum and cord. 



(4) OLIVO-SPINAL AND THALAMICO-SPINAL TRACTS (Bundle 

 of Helweg). This tract is also situated in the antero-lateral column, 

 opposite the head of the anterior horn. It consists mainly of fibres 

 which pass from the thalamus (the fore brain) through the inferior 

 olive of the medulla downwards in the cord as far as the lower cervical 



region. 



(5) COMMA TRACT. This tract lies in the posterior columns at 

 the junction of the postero-niedian and postero- lateral portions. It 

 consists for the most part of the descending branches of the afferent 

 dorsal nerve -roots which enter the cord. These divide as they enter 

 the cord, and their 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 continua- 

 tion of the posterior longitudinal bundle, the connections of which 

 we shall have to study later on. A small tract of fibres, which degene- 

 rate 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 mav contain fibres coming from the mid-brain. 



/ O 



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 not derived from cells in the spinal 

 cord, but are exogenous, being axons of cells in the posterior root 

 ganglia. They can be divided into long, medium, and short fibres, 

 all of which, ascending vertically in these columns, give off collaterals, 

 which pass into the grey matter and ramify round nerve-cells, 



