68 NERVOUS SYSTEM. 



of structure extended to and more or less involved the posterior roots of the spinal 



nerves." 



It is only necessary to add that the views of Todd have been in the mam confirmed 

 in the numerous cases of locomotor ataxia that have lately been so fully described by 

 pathologists ; and, from these facts, it is more than probable that the posterior columns 

 contain fibres connecting the different segments of the cord, and that they play an im- 

 portant part in the coordination of muscular movements. The general function of coor- 

 dination will be considered more fully in connection with the cerebellum. 



Decussation of the Sensory Conductors of the Cord. In hemiplegia due to injury of 

 the brain, the paralysis occurs upon the side of the body opposite to the cerebral lesion. 

 The phenomenon ordinarily observed is simply paralysis of motion ; but in those cases, 

 however, in which both motion and sensation are abolished upon one side of the body, 

 the lesion in the brain is also found to be upon the opposite side. It is evident, there- 

 fore, that there is a decussation of the conductors of sensory impressions as well as of 

 the conductors of the motor stimulus. 



As early as 1822, Fodera made a longitudinal section of the spinal cord in the lumbar 

 region, exactly in the median line. In this experiment, " sensation was destroyed, and 

 in part motion upon the two sides. 1 ' Inasmuch as in this section it is only possible to 

 divide the fibres passing from one lateral half of the cord to the other, it is evident that 

 the sensory conductors must decussate in the spinal cord itself. As far as we know, this 

 is the first experiment pointing to the decussation of sensory fibres in the cord, the ob- 

 servations of Galen, to which we have already referred, being limited to the phenomena 

 of motion. 



The next experiments bearing upon the decussation of the sensory conductors in the 

 cord are those of Van Deen. Among the numerous observations made upon the spinal 

 cord by this physiologist, are one or two in which he noted the fact that, after section of 

 one lateral half of the cord in the frog, at the site to the third dorsal vertebra, " the 

 animal had no real loss of sensibility in the posterior extremity on the side on which the 

 half of the spinal cord had been cut." Although Van Deen did not distinctly state, as a 

 conclusion drawn from these observations, that there is decussation of the sensory con- 

 ductors in the cord, the fact of section of one lateral half of the cord with no loss of 

 sensation on the corresponding side of the body remains as one of the first experimental 

 arguments in favor of the crossed action. 



Experiments upon living animals as well as pathological facts show that, after section 

 or injury confined to one lateral half of the cord, the general sensibility upon the cor- 

 responding side of the body is very much exaggerated, producing a condition of well- 

 marked hypera3sthesia. This remarkable fact was distinctly noted by Fodera, in 1822. 

 This observation has been confirmed, and the experiments very much extended, by 

 Brown-Sequard. Cases presenting the same phenomena have also been observed in the 

 human subject, when one side of the cord has been invaded by disease. 



Physiologists are at a loss to explain the hyperaesthesia which follows section of the 

 sensory conductors of the cord, but the fact nevertheless remains. The exaggeration of 

 sensibility is not due to section of certain fibres, which might be supposed to increase the 

 impressibility of the remaining fibres, for, as was shown by Vulpian, it is sufficient to 

 prick with a pin one of the lateral halves of the cord to observe these remarkable phe- 

 nomena. With these few words, we shall leave the subject of hyperasthesia from injury 

 to the cord, and pass to the crossed action of its sensory conductors. 



In treating of the cord as a conductor of sensory impressions, we have already shown 

 that this function is performed by the gray substance alone. We have also seen, in con- 

 nection with the phenomena of conduction of the motor stimulus, that this is effected by 

 the antero-lateral columns, which do not act as sensory conductors, except by virtue of 

 their gray matter. As it is impossible to divide the gray matter with certainty without 



