FUNCTIONS OF THE SPINAL CORD 869 



the lateral column on the left side caused marked degeneration in the 

 left direct cerebellar tract, the tract of Cowers, and the crossed pyram- 

 idal 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 impressions of pain and temperature, pass up in the antero-lateral 

 column, either in the tract of Cowers, 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 substi- 

 tuted for a severed path as a conductor of impulses which normally 

 traverse the latter. The rapidity with which sensation is restored 

 below the lesion after semisection of the cord in animals is an illustration 

 of this. Another difference, which can be explained in the same way, is 

 that a sharply-marked dissociation of sensations retention of tactile 

 sensibility, for example, with loss of sensibility to pain or to pain and 

 temperature changes either cannot be produced experimentally in 

 animals, or is very difficult to realize. 



The impulses which descend the cord give token of their arrival at the 

 periphery by causing either contraction of voluntary muscles, or con- 

 traction of the smooth muscular fibres of arteries, or secretion in glands. 

 They all pass down in the antero-lateral column, but the path of the 

 voluntary impulses in the pyramidal tracts is the best known and most 

 sharply defined. 



2. Modification of Impulses set up elsewhere (Reflex Action). 

 The spinal cord, although it is a conductor of nervous impulses 

 originating elsewhere, is by no means a mere conductor. Many of 

 the impulses which fall into the cord are interrupted in its grey 

 matter. Some of the efferent impulses proceeding from the brain 

 are perhaps modified in the cord, and thentransmitted to the muscles. 

 Some of the afferent impulses are modified, and then transmitted to 

 the brain ; some are modified, and deflected altogether into an efferent 

 path. These last are the impulses which give rise to reflex effects. 

 A reflex action has sometimes been defined as an action carried out 

 in the absence of consciousness; not necessarily, however, in the 



