THE GREAT AFFERENT SYSTEMS 313 



vague sensations of position and posture and of active and passive movements, 

 consists of a chain of at least three units. 



Neuron I. The cell bodies of the neurons of the first order belonging to this 

 system are located in the spinal ganglia. Their axons are myelinated and divide 

 into peripheral branches, running to specialized end organs within the muscles, 

 joints and tendons, and central branches directed through the medial division 

 of the dorsal root into the posterior funiculus of the spinal cord. Here they 

 divide; and their ascending branches run through the posterior funiculus to 

 terminate in the gracile and cuneate nuclei of the medulla oblongata, where they 

 enter into synaptic relations with neurons of the second order (Fig. 235). 



Neuron II. From cells located in the gracile and cuneate nuclei the axons 

 run as internal arcuate fibers across the median raphe in the medulla oblongata 

 and ascend by way of the medial lemniscus to end in the ventral part of the lateral 

 nucleus of the thalamus, where they form synapses with neurons of the third order. 



Neuron III. From cells in the lateral nucleus of the thalamus fibers pass by 

 way of the thalamic radiation through the posterior limb of the internal capsule 

 to the posterior central gyrus or somesthetic area of the cerebral cortex. 



SPINAL PROPRIOCEPTIVE PATHS TO THE CEREBELLUM 



Impulses from the muscles, joints, and tendons may reach the cerebellum by 

 three routes: 



A. By Way of the Dorsal External Arcuate Fibers : 



Neuron I of this chain is the same as in the path to the cerebral cortex just 

 described, the fibers from the dorsal root reaching the gracile and cuneate nuclei. 



Neuron II. From cells located in these nuclei axons run as posterior external 

 arcuate fibers to the restiform body of the same side, and thence through the 

 white center of the cerebellum, to end in the cerebellar cortex (Fig. 235, red). 



B. By Way of the Ventral Spinocerebellar Tract: 



Neuron I. The first neuron in this chain is similar to the primary neuron in 

 the two preceding paths. The impulses, however, travel over collateral and 

 terminal branches of the dorsal root fibers to reach the posterior gray column 

 and intermediate gray matter of the spinal cord. 



Neuron II. From cells located in the posterior gray column and intermediate 

 gray matter fibers run in the ventral spinocerebellar tracts of the same or 

 opposite side through the spinal cord, medulla oblongata and pons, bend around 

 the brachium conjunctivum, and then course back along the anterior medullary 

 velum to the cortex of the rostral part of the vermis (Fig. 235, blue). 



