138 INTRODUCTION TO NEUROLOGY 



only out of a larger number of paths by which afferent spinal 

 impulses may be discharged into it (see p. 188). 



The remaining proprioceptive fibers of the spinal roots are 

 directed upward in the dorsal funiculus, of which they form the 

 larger part. At the point where the spinal cord passes over 

 into the medulla oblongata they terminate, and after a synapse 

 here the neurons of the second order carry the impulse across to 

 the opposite side of the brain and upward toward the thalamus 

 in a tract known as the medial lemniscus or fillet (Fig. 64). 

 After another synapse here, a final neuron may carry the nervous 

 impulse forward to the cerebral cortex. This medial lemniscus 

 system is largely concerned with unconscious motor adjustments 

 involving the muscles of the trunk and limbs. Disturbance of 

 its functions produces motor incoordination (ataxia), but not 

 necessarily any great loss of exteroceptive sensations. So far as 

 its functions come into consciousness, they are recognized as sen- 

 sations of position, spatial localization, and motor control. 



II. Exteroceptive Systems. The central course of the extero- 

 ceptive fibers of the spinal nerves is quite different from that 

 just described. Almost immediately after entering the spinal 

 cord these fibers terminate among the neurons of the dorsal 

 gray column. After a synapse here the fibers of the second order 

 cross to the opposite side of the spinal cord, and here turn and 

 ascend in the white matter of the lateral and ventral funiculi, 

 where they form the spinal lemniscus, or tractus spino-thalamicus. 

 Some fibers of the spinal lemniscus ascend throughout the entire 

 length of the spinal cord, medulla oblongata, and midbrain, to 

 end in the thalamus. In the upper part of their course these 

 fibers accompany those of the medial lemniscus already des- 

 cribed. 



Collateral connections are effected between the ascending 

 fibers of the spinal lemniscus and the various motor nuclei of the 

 brain for different cranial reflexes, such as turning the eyes in 

 response to a cutaneous stimulation on the hand. But their 

 final terminus is in the thalamus, and after a synapse here the 

 nervous impulse may be carried forward to the cerebral cortex 

 by neurons of the third order. The spinal lemniscus system is 

 the chief ascending pathway for nervous impulses giving rise to 

 consciousness of touch, temperature, and pain from the trunk 



