THE CENTRAL NERVOUS SYSTEM 795 



And there seems no other tenable hypothesis than that in such 

 cases the pathological change has picked out a particular group of 

 fibres, either collected into a single strand or scattered among 

 unaltered fibres of different function. For example, in syringo- 

 myelia, a condition in which cavities are formed in the grey matter 

 of the cord secondary to a new growth of the neuroglia surrounding 

 the central canal, a frequent symptom is the loss in a certain region 

 of sensibility to pain and to changes of temperature, while tactile 

 sensibility is unaffected (dissociation of sensations). Again, in loco- 

 motor ataxia, a disease in which inco-ordination of movement and 

 derangement of the mechanism of equilibration are prominent symp- 

 toms, degeneration in the posterior column of the cord is a most 

 constant lesion. And there is strong evidence that afferent impulses 

 from muscles and tendons, which either give rise to impressions be- 

 longing to the group of tactile sensations, or produce no effect in 

 consciousness, and which, according to the most widely accepted 

 doctrine, serve as the basis of the muscular sense, and play an impor- 

 tant part in the maintenance of equilibrium (p. 835), pass up in the 

 posterior column. It may also conduct tactile impressions from the 

 skin. A case has been observed where a man received a stab which 

 divided the whole of one side of the cord and the posterior column of 

 the other side. Sensibility to touch was lost on both sides of the body 

 below the level of the injury, sensibility to pain only on the side 

 opposite to the main lesion. In another case, in which some small 

 syphilitic tumours (gummata) in the lateral column on the left side 

 caused marked degeneration in the left direct cerebellar tract, the 

 tract of Gowers, and the crossed pyramidal tract, without affecting 

 the posterior columns, tactile sensibility was only slightly impaired 

 in the opposite leg, while the sensibility for pain and temperature 

 was much enfeebled. In the left leg, which was paralyzed, there 

 was slight hyperaesthesia. These observations indicate that impres- 

 sions of pain and temperature pass up in the antero-lateral column, 

 either in the tract of Gowers, or in the direct cerebellar tract, or in 

 both (Dejerine and Thomas). 



But it does not follow that they cannot ascend by other paths as 

 well. It appears indeed that the grey matter of the cord, or, rather, 

 short endogenous fibres arranged in series in the antero-lateral column 

 so as to connect the grey matter at different levels, constitute such a 

 path, and that impulses which give rise to pain can be propagated 

 along a cord in which hardly a vestige of white substance remains 

 uncut. In man the path for pain and temperature impressions along 

 these short endogenous fibres seems to be mainly or entirely a crossed 

 path. The afferent paths for such vaso-motor reflexes as are elicited 

 by stimulation of the central end of the sciatic ascend in the lateral 

 column, and the impulses largely cross the middle line in the cord. 

 The posterior columns have nothing to do with the conduction of 

 painful impressions, for division of them causes not anaesthesia, but 

 rather hyperaesthesia, while if they are left intact, and the rest of 

 the cord, including the grey matter, divided, the animal is insensitive 

 to pain below the level of the lesion. Just as man differs from lower 

 animals in the completeness with which certain of the sensory 

 impressions decussate in the cord, so differences exist in the degree 

 of localization of the different kinds of impressions in particular 

 tracts. One of the outstanding differences is that in animals it 

 seems to be easier for a still intact path to be substituted for a 

 severed path as a conductor of impulses which normally traverse 



