THE CENTRAL NERVOUS SYSTEM 793 



involving the destruction of one lateral half of the cord in man 

 and experimental semisections in some mammals, are followed 

 by symptoms which suggest that some kinds of sensory impulses 

 decussate chiefly in the spinal cord viz., diminution or loss of 

 sensibility to pain and to changes of temperature on the opposite 

 side below the level of the lesion, with little or no impairment, and 

 often increase of sensibility (hyperaesthesia) on the same side. 

 Tactile sensibility is lost on the side of the lesion, and likewise 

 the muscular sense. 



The first general description of this symptom-complex was given 

 by Brown-Sequard, although long after he saw cause to retract this 

 interpretation of the facts. While it may be true that in man it has 

 not been rigidly demonstrated that the symptoms are associated with 

 a clean-cut lesion precisely limited to one-half of the cord, clinical 

 observation has on the whole tended to confirm the view that an 

 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 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. 719) 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 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, 



