144 THE HUMAN BODY 



Tracts of Body Sense. Sensory neurons of body sense enter 

 the spinal cord all along its length. Afferent paths within the 

 cord begin, therefore, at its extreme end. These are to be looked 

 for, as previously stated, in the columns of white matter which 

 make up the greater part of the substance of the cord. Two dis- 

 tinct regions of white matter in each half of the cord have been 

 shown to consist chiefly of afferent neurons leading toward the 

 cerebrum. These are : first, the dorsal columns, each of which con- 

 sists of two rather well-marked bundles of axons, the so-called fas- 

 ciculus gracilis (Column of Goll) next the dorsal fissure, and the 

 fasciculus cuneatus (Column of Burdach) next to the dorsal horn 

 of gray matter; second, the ventrolateral tracts which lie next to 

 the ventral horns of gray matter, surrounding them on the sides 

 and below (Fig. 66) . It is thought that the dorsal columns con- 

 sists chiefly if not wholly of the axons of sensory neurons which, 

 entering the cord by the dorsal roots of spinal nerves, extend for- 

 ward within the dorsal columns, giving off collaterals into the gray 

 matter at various levels. Only a part of the sensory axons which 

 enter the dorsal columns continue along them as far as the medulla; 

 the others after extending a short distance plunge into the gray 

 matter and terminate in synaptic connection with association 

 neurons. The ventrolateral afferent columns consist wholly of 

 association neurons which communicate, presumably, with those 

 sensory neurons which do not themselves extend all the way to 

 the medulla; these columns serve, therefore, to afford cerebral 

 communication to those sensory neurons which terminate within 

 the gray matter of the cord. 



None of the afferent axons coming up the cord by the tracts 

 just described extend further than the medulla; they all termi- 

 nate there in masses of gray matter known as the gracile and 

 cuneate nuclei; here they form synaptic connections with a new 

 set of association neurons which continue the path toward the 

 cerebrum. These tracts, which from their ribbon-like appearance 

 have been named the fillets, cross the mid-line at a point in the 

 medulla known as the sensory decussation; so that sensory stimuli 

 from the right half of the Body are carried to the left cerebral 

 hemisphere, and those from the left half of the Body to the right 

 hemisphere. 



Tracts of the Head Senses. The senses of sight and hearing 



