STRUCTURE AND FUNCTIONS OF THE CEREBRUM 173 



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 chiefly 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. 



In the lateral margins of the spinal cord are tracts known as the 

 direct cerebellar tract and Gower's tract (Fig. 65), which consist of 

 the axons of association neurons that pass up to the brain stem and 

 directly through it by way of the peduncles to the cerebellum. The 

 cell-bodies of these axons are in the gray matter of the cord and 

 have synaptic connection with branches of sensory neurons, par- 

 ticularly neurons of muscle sense. These tracts are believed to 

 constitute the chief channels by which muscle sense exerts its 

 influence on the cerebellum in the mediation of locomotor reflexes. 



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

 are the head senses whose central connections are best known. 

 The central connections of the sense of smell are imperfectly 

 known; those of taste practically not at all. Axons conveying 

 visual impulses enter the midbrain by way of the optic nerves 

 and optic tracts and terminate for the most part in nuclei of the 

 midbrain, the external geniculates and superior colliculi; some 

 of them appear to terminate in basal nuclei of the cerebrum, the 

 optic thalami. In all these nuclei synaptic connection is made 

 with new neurons which carry the impulses into the cerebrum. 



