THE WHITE MATTEE OF THE SPINAL MEDULLA. 



535 



osterior nerve-root have three main modes of distribution : (1) the majority take part in 

 ie formation of the fasciculus cuneatus, and pass upwards or downwards to end in the 

 :ray matter at some other level in the central nervous system ; (2) some fibres, and many 

 ollaterals of fibres in the fasciculus cuneatus, lie close to the posterior column and 

 escribe a series of graceful curves as they pass forwards, prior to turning laterally into 

 11 regions of the gray matter to end at the same level as they enter the medulla spinalis ; 

 3) a third series form the pos'tero-lateral fasciculus and end in connexion with the cells of 

 he substantia gelatinosa and other cells in the posterior and anterior columns of gray 

 latter (Fig. 473). 



The fibres derived from the posterior nerve-roots which ascend in the posterior 

 uniculi of the medulla spinalis to the medulla 

 blongata of the brain constitute a direct sensory 

 : ract ; other fibres are described which give rise to a 

 rossed sensory tract termed the fasciculus spino- 

 halamicus. These latter fibres arise as the axons of 

 ertain of the cells in the posterior column in con- 

 lexion with which fibres from the posterior nerve- 

 oots have ended, and crossing to the opposite side 

 )f the medulla spinalis through the anterior commis- 

 ,ure they ascend in the antero-lateral funiculus to 

 ,he brain, where they ultimately reach the thalamus. 

 Vs the spino-thalamic tract ascends in the spinal 

 nedulla its fibres are not gathered into a compact 

 strand, but are more or less loosely scattered in the 

 .ateral funiculus. 



Association Fibres in the Posterior Funiculus. But 



;he whole of the fibres of the posterior funiculus are not 

 lerived from the posterior nerve-roots. A few fibres exist 

 in this funiculus which have a different origin. They are 

 derived from certain of the cells of the gray matter, and, 

 entering the posterior funiculus, they divide into ascend- 

 ing and descending branches which pass upwards and 

 downwards in the funiculus for a varying distance, before 

 they finally turn in to end in the gray matter at higher 

 and lower levels. These fibres, therefore, constitute 

 links of connexion between different segments of the 

 spinal medulla, and they constitute the fasciculus posterior 

 proprius. Our information regarding these fibres at 

 present is somewhat defective ; but it is believed that the 

 deepest part of the funiculus, i.e. the part next the 

 posterior gray commissure, and the fasciculus septo- 

 marginalis of Bruce, placed in apposition with the 

 posterior-median septum and in the adjoining part of the Fm 

 surface, belong mainly to this category. 



TO SHOW THE 



MANNER IN WHICH THE FIBRES OF 

 THE POSTERIOR NERVE-ROOTS ENTER 

 AND ASCEND IN THE POSTERIOR 

 FUNICULUS OP THE SPINAL 

 MEDULLA. (From Edinger. ) 



Funiculus Lateralis and Funiculus Anterior. 



-It is convenient to consider the anterior along 

 with the lateral funiculus and to call the whole mass 



white substance that is left, after eliminating the posterior funiculus, the antero- 

 lateral funiculus. In contact with the surface of the gray columns there is a broad 

 band of white matter the parts of which are known respectively as the fasciculus 

 proprius anterior and lateralis (O.T. the ground bundles of the antero-lateral 

 iuniculus). It is composed wholly of fibres which spring from nerve-cells in the 

 gray columns, and, after passing for varying distances upwards or downwards, end 

 in the gray matter of the spinal medulla. Thus they constitute an intrinsic 



tern of fibres linking together different levels of the spinal medulla. They 

 >ecome medullated before any other fibres, except the root-fibres and their con- 



tuations in the posterior funiculus. When cut across some of the fibres degenerate 



ove, others below, the injury, and the degeneration extends for varying distances 

 upwards and downwards respectively. 



