FUNCTIONS OF THE SPINAL CORD 895 



important portion of the sensory path decussates in the cord. But it 

 is a curious circumstance that experimental physiologists have for the 

 most part obtained contradictory results. Thus Mott, working with 

 monkeys, found that the different kinds of sensation, far from being 

 abolished, are as a rule impaired in a smaller degree on the side opposite 

 to the semisection than on the same side, while Ferrier and Turner 

 obtained on the whole a contrary result, and one that corresponded 

 closely with Brown-Sequard's original description. The discovery that 

 no ascending degeneration, or only a trifling amount, is to be found on 

 the opposite side of the cord, either after semisection or after division 

 of posterior roots, does not of itself enable us to decide the question. 

 For while this latter fact shows that few or none of the afferent fibres 

 cross the middle line to enter the long conducting paths before being 

 interrupted by nerve-cells, it by no means proves that afferent impulses 

 do not decussate in the cord. The long paths of the posterior column, 

 indeed, do not decussate below the level of the bulb. The dorsal and 

 ventral spino-cerebellar tracts are also, in the main at least, uncrossed 

 spinal paths. A portion of the afferent impulses must therefore be 

 carried up to the cerebrum and the cerebellum without decussating in 

 the cord. But nobody can tell how massive a link between the two 

 halves of the cord may be formed by the grey matter and the endogenous 

 fibres of the white columns and their collaterals. We know that some 

 afferent impulses do decussate far below the level of the medulla. For, 

 (i) A part of the action current (p. 838) crosses the middle line and 

 ascends in the opposite half of the cord when the central end of one 

 sciatic is stimulated (Gotch and Horsley). (2) Crossed reflex move- 

 ments are possible ; and when excitation of the central end of the sciatic 

 is followed by contraction of the muscles of the opposite fore-limb, the 

 afferent impulses must either decussate in the lumbar cord, and then 

 run up on the opposite side to the level of the brachial plexus, or must 

 ascend on the same side and cross over somewhere between the plane 

 of the sciatic and the brachial nerve-roots. The only other hypothesis 

 on which crossed reflex action can be explained but a hypothesis for 

 which there is not a tittle of evidence is that the afferent impulse 

 always acts on the few motor cells whose axis-cylinder processes pass 

 over to the opposite side, and there enter anterior nerve-roots. But 

 while, for these reasons, it cannot be denied that some afferent impulses 

 decussate in the cord, it would be an error to conclude that all do so in 

 any animal, or that all animals are in this respect alike. It is indeed 

 extremely probable that in different species of animals, and even in 

 individuals of the same species, there are considerable differences in 

 the extent of the sensory decussation in the cord, just as there are in the 

 extent of the motor decussation in the bulb. In some animals the 

 greater part of the sensory path may decussate in the cord ; in others the 

 greater part may decussate in the bulb, or higher up. The lack of 

 agreement in the experimental results may be due partly to this cause. 

 When it is further remembered how difficult it sometimes is to interpret 

 the account which a man gives of his sensations, and to recognize 

 precisely the degree and nature of sensory defects produced by disease 

 in the human subject, it will not be thought surprising that experi- 

 ments on animals, from the time of Galen onwards, should have yielded 

 evidence which, although perhaps now at length tending to a definite 

 result, is still unfinished and in part conflicting. 



If, leaving them out of account, not as valueless but as still 

 difficult of interpretation, we attempt to draw any general conclusion 



