708 FUNCTIONS OF THE SPINAL CORD [CH. XL1X. 



so reach the gracile and cuneate nuclei of the bulb, and it is the 

 fibres which arise from the cells of these nuclei which cross in the 

 decussation of the fillet. 



The rapidity with which the sensory impulses cross to the 

 opposite side varies greatly. Some, such as those associated with 

 pain, heat, and cold, cross over in the space of five or six spinal 

 segments. With tactile impulses the crossing is evidently less rapid ; 

 but until the crossing is completed there will obviously be two 

 channels, one on each side of the cord, open for tactile impulses ; 

 and such a double path will be shorter for the impulses which cross 

 more rapidly; and finally, as just stated, the impulses associated 

 with position, movement, and tactile discrimination have only one 

 path in the cord, as the decussation does not take place until the 

 bulb is reached ; hence a hemisection of the cord, or of one posterior 

 column, will abolish these forms of sensibility from the parts below 

 the lesion, on the same side of the body as the lesion. 



Painful impulses from the skin arriving in the cord from proto- 

 pathic fibres pass into the second level at the point of entry, and 

 rapidly cross over to the other side. Fibres of the deep system 

 running with the muscular nerves and carrying impulses also of a 

 painful kind from the same part of the body do not necessarily enter 

 the cord by the same posterior roots as those carrying cutaneous 

 painful stimuli. Thus more than one segment of the cord is 

 required before all painful impulses from any one part of the body 

 can be gathered together and recombined. This is the reason why, 

 in a local lesion in the cord, there may be a want of correspond- 

 ence between the extent of the cutaneous and deep analgesia (loss 

 of sensation to pain). 



Up to this point we have only considered the sorting out of those 

 impulses which reach the cerebrum and thus rise into consciousness. 

 In addition to this there is another group of impulses which never 

 rise into consciousness at all, and although these are afferent they 

 are therefore not sensory. 



Our previous illustration of the correspondence of a busy man 

 may help us again in understanding this. His clerks sort his letters, 

 and those of a certain kind (circulars and the like) will probably never 

 reach him at all. So it is with afferent impulses; the primary 

 sorting is into sensory and non-sensory ; the sensory impulses are 

 again sorted into those of touch, pain, and temperature; the non- 

 sensory impulses are those mainly destined for the cerebellum, and 

 reach it by the cerebellar tracts. These travel up the cord on the 

 side of entry, and reach the same side of the cerebellum. This 

 explains the delay in the crossing of the sensory impulses which 

 subserve the sense of position and movement and tactile discrimina- 

 tion, a crossing which, as we have seen, does not occur until the bulb 



