870 CENTRAL NERVOUS SYSTEM 



become grouped together in a single tract which is composed of second- 

 ary afferent fibers situated on the side of the cord opposite to that by 

 which they have entered. The impulses giving rise to the sensation of 

 heat, and those giving rise to the sensation of cold are also each col- 

 lected into separate tracts ascending through secondary afferent neurons 

 on the opposite side of the cord. 



All impulses concerned with the tactile aspect of sensation, including 

 the sensations involved in the recognition of position and passive move- 

 ment are grouped together in the cord irrespective of their origin from 

 cutaneous or deep lying receptors. These impulses, however, pass up the 

 cord in two distinct groups. Almost all impulses concerned with the recog- 

 nition of touch and pressure and the recognition of the location to which 

 these stimuli are applied eventually pass on to secondary afferent neurons 

 and cross to form a definite tract ascending the opposite side. But whereas 

 the impulses from painful stimuli, heat and cold cross within five or six 

 segments from their point of entrance into the cord, some of the tactile 

 impulses may not have crossed until the upper cervical region is reached. 



Impulses upon which depend the recognition of two dimensional local- 

 ization, the sense of position, and passive movement pass up the cord on 

 the side on which they have entered it, in the fibers of the primary affer- 

 ent neuron which lie in the dorsal f uniculas and terminate in the nucleus 

 gracilis and cuneatus of the medulla. 



As a result of this arrangement of the afferent fibers in the cord, cer- 

 tain characteristics are imposed upon the sensory disturbances which 

 result from spinal cord lesions. A lesion which destroys the greater part 

 of one half of the cord will obliterate the sensation of heat, cold, and 

 pain on the opposite side of the body in those parts whose nerves arise 

 below the level of the lesion. The ability to recognize touch and pres- 

 sure may be unimpaired by such a lesion, because the impulses on which 

 this depends cross over gradually as they ascend the cord, and conse- 

 quently there exists at any one level two paths over which such impulses 

 can travel. If the lesion is located high in the cervical region, a point 

 at which the crossing over of these tactile impulses is complete, this qual- 

 ity of sensation will also be lost from the opposite side of the body. Dis- 

 turbances in the recognition of the position of the limbs and of passive 

 movement and the distinctness of two points applied to the skin at once 

 (two dimensional localization) occur, in the case of a considerable uni- 

 lateral lesion, on the the same half of the body as the lesion, because the 

 ascending tracts for the impulses involved in these sensations are 

 uncrossed. On the contralateral half of the body dissociation occurs 

 between heat, cold and pain, which are obliterated, and the tactile sen- 

 sations which persist in part. On the homolateral half of the body the disso- 

 ciation is between the sensations of two and three dimensional localization, 



