702 A MANUAL OF PHYSIOLOGY 



on the side opposite to the hemisection than on the same side, while 

 Ferrier and Turner obtained the contrary result. The discovery 

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

 found on the opposite side of the cord, either after hemisection 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. And, indeed, 

 we know that some afferent impulses do decussate far below the 

 level of the medulla. For, (i) A part of the negative variation 

 (p. 646) 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 movements 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 motor cells whose axis-cylinder pro- 

 cesses 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 (in the monkey, for instance), 

 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 recognise precisely the degree and nature of sensory defects pro- 

 duced by disease in the human subject, it will not be thought sur- 

 prising that experiments 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 from the clinical observations which, however 

 imperfect, are in such questions our surest guide, it can only 

 be this, that in man many of the sensory impulses, and particu- 

 larly those connected with the cutaneous sensations of pain and 



