868 



CENTRAL NERVOUS SYSTEM 



an area which is still sensitive to deep pressure and the resulting pain, 

 and in which the sense of position of the part remains normal. Such a 

 lesion is said to cause a dissociation of sensation. Dissociation between 

 cutaneous and deep sensibility occurs only in the case of peripheral nerve 

 lesions. Since the fibers of deep sensibility in muscles lie in the trunks 

 of motor nerves, a lesion affecting it is usually accompanied by certain 

 disturbances in motor function. 



Ascending Pathways in the Spinal Cord 



Our knowledge of the course of afferent impulses in the cord is gained 

 by determining which tracts degenerate when the dorsal roots are cut, 

 by destroying certain regions of the cord in animals and attempting to 

 correlate the resulting degeneration with such disturbances in sensation 

 as can be made out, and by studying the disturbances in sensation which 

 result from injuries or disease of the cord in man. The latter method 

 especially has been profitable because by it alone can accurate informa- 

 tion concerning sensation be acquired (Head and Thompson, 15 Holmes 16 ). 

 Unfortunately, however, an exact knowledge of the site of the lesion can- 

 not usually be had in these cases, so that at present the course of the 

 afferent impulses concerned with the various qualities of sensation from 

 different parts cannot be stated with the precision which will ultimately 

 be attained. On entering the spinal cord the course of the dorsal root fibers 

 branch. A few fibers pass posteriorly in the fasciculus inter fascicularis 

 of the dorsal column. The majority, however, extend upward in the 

 ascending tracts of the dorsal funiculiis. As these proceed upward their 

 number becomes less and less, because most of the fibers pass into the 

 gray matter and terminate within a few segments of their point of 

 entrance into the cord. As fibers enter the ascending tracts of the dorsal 

 funiculus from higher spinal nerves, they lie lateral to those which have 

 entered lower down, so that the funiculus takes on a laminated structure. 

 Upon entering the gray matter of the cord the afferent fibers undergo 

 synapse with association fibers which function as secondary afferent 

 neurons conducting the impulses either to the motor neurons within the 

 cord which are involved in spinal reflex acts, or leading to higher cen- 

 ters in the brain, some of which give rise to sensation. The fibers of 

 the secondary afferent neurons which are concerned with sensory im- 

 pulses cross to the opposite side of the spinal cord and pass upward to 

 the brain in the ascending tracts (dorsal and ventral spinothalmic) of the 

 spinal lemniscus. Studies of the sensory disturbances due to lesions of 

 the spinal cord in man indicate that on entering the cord the impulses 

 which give rise to the various qualities of sensation undergo a charac- 

 teristic grouping. All fibers conducting impulses which give rise to pain 

 whether from the cutaneous or deep distribution of the sensory nerves 



