556 THE SPINAL CORD. 



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. 



489. The cerebellar tract (Fig. 127, C.b.) is, as we have seen, a tract of 

 ascending degeneration ; the degeneration in it makes its appearance above 

 the section or seat of other injury of the cord. It begins somewhat sud- 

 denly 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 pos- 

 sibly 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 poste- 

 rior 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 cere- 

 bellum. 



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

 eration 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 deter- 

 mined by the embryological method, by noting the appearance of the me- 

 dulla 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 sec- 

 tion lower down. From this we infer that the fibres composing the tract 

 must start successively from other parts of the cord along its length that 

 is to say, the tract must be fed by fibres coming from other structures in the 

 cord. On the other hand, it is found that the degenerated area following 

 upon a section or injury diminishes as it is traced upward ; when, for instance, 

 a section is made in the mid-thoracic region, the area of degeneration in the 

 tract is greater immediately above the section than it is higher up, say in 

 the cervical region. From this we are led to infer that though the tract is 

 successively fed along its course by fibres coming from other parts of the 

 cord, some of the fibres entering the tract, though like their companions 

 undergoing an ascending degeneration, do not like them continue in the 

 tract right up to the cerebellum, but pass off to other parts of the cord 

 on their way upward. This, however, is equivalent to saying that the 

 tract is not a pure or homogeneous one, but consists of at least two sets of 

 fibres, only one of which is continued on to the cerebellum and strictly 

 deserves the name of " cerebellar." It may, perhaps, here be mentioned 

 that while the fibres composing the tract are, as a whole, conspicuously 

 coarse, large fibres, with these there are mingled, especially in the thoracic 

 region, a number of much finer fibres ; but these apparently undergo a 

 descending, not an ascending, degeneration, and do not, therefore, really 

 belong to the tract ; they may be fibres which have strayed from the pyram- 

 idal tract. 



