THE CENTRAL NERVOUS SYSTEM. 55 



THE PHYSIOLOGICAL PATHS IN THE SPINAL CORD. 



Division of the posterior spinal nerve roots of the nerves supplying 

 a limb results in the immediate loss of all sensation in the limb. There 

 is also loss of muscle tone, and the limb is paralysed owing to the ab- 

 sence of afferent impressions from it. Later, the afferent fibres undergo 

 degeneration centrally to the lesion, if the latter is between the root 

 ganglion and the spinal cord. 



Complete section of the spinal cord in the lower thoracic region is 

 followed by immediate loss of movement and sensation jm the hind limbs. 

 There is also loss of vascular tone witn passive dilatation of the bloocf- 

 vessels (passing off in twenty-tour hours J, ancf the hind limBs become poiki- 

 lothermic, that is, their temperature varies with that of the surrounding 

 medium.^ Further, the reflex visceral centres in the lumbar region are 

 cut off from the inhibitory impulses from the higher centres, so that mic- 

 turition and defamation become simple reflexes. Later, the motor fibres 

 below the point of section and the afferent fibres above the section, which 

 have entered by posterior roots below the lesion, undergo degeneration. 



Hemisection of the spinal cord in the lower thoracic region results 

 in motor paralysis of the homolateral- hind limb, accompanied by loss 

 of muscle sense and tactile discrimination on the same side. There is 

 also loss of tactile localisation, and of the senses of heat, cold, and pain 

 in the contralateral hind limb, the fibres for these senses having 

 crossed in the spinal cord shortly after their entrance by the posterior 

 roots. Immediately below the lesion, however, this sensory paralysis 

 occurs in a narrow zone on the side of the section, depending on fibres 

 which have been divided before their crossing. The remote effects of 

 hemisection consist in ascending and descending degeneration of the 

 divided fibres above and below ihe lesion respectively. 



Transection at various levels shows that the motor paths and the paths 

 for muscle sense and for tactile discrimination cross the middle line above 

 thelevel dfths spinal cord, whereas the paths for tactile localisation, 

 the senses of heat and cold, and the sense of pain cross in the spinal 

 cord itself in the vicinity of the posterior roots conveying these impulses 

 from the periphery. Further, in the disease known as syringomyelia, 

 in which the central canal of the spinal cord is dilated and" there 

 is pressure on the adjacent structures and interference with their 

 functions, it is found that the sense of pain may be lost while those of 

 temperature and tactile localisation are unimpaired, or the temperature 

 sense may be lost while the other two senses are intact. There are, 

 therefore, separate bundles of fibres for the transmission of the different 

 impulses which give rise to these various sensations. s 



