890 THE FEATURES OF DIFFERENT REGIONS. [BOOK in. 



cord, cross over in the cord itself before they make connections with 

 the fibres of the anterior roots. Probably the crossing is effected 

 by means of some of the decussating fibres which form the 

 anterior white commissure. A part only, indeed a small part, of 

 the commissure can serve this purpose ; most of the fibres of the 

 commissure, and in the lower regions of the cord, where the direct 

 tract no longer exists, all the fibres must have some other functions. 

 Some of the fibres of this great pyramidal tract, leave the tract, 

 as we have said, to join some of the cranial nerves before the 

 pyramids of the bulb are reached; and the impulses passing 

 along these fibres also cross over to the opposite side before they 

 issue along the cranial nerves. Hence we infer that these fibres 

 decussate above the decussation of the pyramids just as those of 

 the direct tract decussate below it. So that of the whole strand 

 as it leaves the cerebral cortex, while the main mass of fibres 

 crosses over at the decussation of the pyramids, the rest of the 

 fibres cross the middle line in succession from the level of the 

 third cranial nerve to the level of the lower limit of the direct 

 tract; below the decussation of the pyramids the crossing takes 

 place by means of the anterior commissure of the cord, above the 

 decussation by means of what we shall later on learn to speak of 

 as the raphe of the bulb, or by structures corresponding to this 

 higher up. 



576. The cerebellar tract (Fig. 104, Cb) is as we have seen a 

 tract of ascending degeneration ; the degeneration in it makes its 

 appearance above the section or the seat of other injury of the 

 cord. It begins somewhat suddenly at the level of the second 

 lumbar nerve region, being absent at least as a distinct tract 

 below; injury of the cord at the level of the middle and lower 

 lumbar nerves leads to no marked tract of degeneration (though 

 possibly scattered single fibres may degenerate), while injury 

 higher up does. The tract lies, as we have said, close to the 

 surface of the cord in the posterior part of the lateral column just 

 outside the crossed pyramidal tract, and while varying somewhat 

 in the shape of its section from level to level remains throughout 

 a somewhat narrow crescentic patch. At the top of the spinal 

 cord it passes, as we have said, from the lateral columns into the 

 restiform bodies of the bulb, and so to certain parts of the 

 cerebellum. 



When the section or lesion is limited to one side of the cord, 

 the degeneration is similarly limited to the same side, and that 

 along its whole course up to the cerebellum ; there is no evidence 

 of any of the fibres decussating in the cord. 



The area of the tract increases from below upward. This has . 

 been determined by the embryological method, by noting the 

 appearance of the medulla in the fibres, as well as by comparing 

 the extent of the degeneration following upon a section high up 

 in the cord with that following upon a section lower down. From 



