

COMPOSITION AND CENTRAL CONNECTIONS OF SPINAL NERVES 851 



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up and down or in both directions giving off collaterals and finally terminating in 

 the gray matter of the same or the opposite side. The shortest fibers of the proper 

 asciculi lie close to the gray matter, the longest ones are nearer the periphery of 

 he proper fasciculi and are more or less intermingled with the long ascending and 

 escending fasciculi which occupy the more marginal regions of the spinal cord. 

 Each sensory neuron, with its ascending and descending branches, giving off as 

 it does many collaterals into the gray matter, each one of which may form a synapse 

 with one or several correlation neurons, is thus brought into relation with many 

 correlation neurons and each one of these in turn, with its ascending and descending 

 branches and their numerous collaterals, is brought into relation, either directly 

 or through the intercalation of additional correlation neurons, with great numbers 

 of motor cells in the anterior column. The great complexity of these so-called 

 simple reflex mechanisms, in the least complex portion of the nervous system the 

 spinal cord, renders them extremely difficult of exact analysis. 



The association or correlation neurons are concerned not only with the reflex 

 mechanisms of the spinal cord but play an equally important role in the trans- 

 mission of impulses from the higher centers in the brain to the motor neurons of the 

 spinal cord. 



The complex mechanisms just described are probably concerned not so much in 

 the contraction of individual muscles as in the complicated action of groups of 

 muscles concerned in the enormous number of movements, which the limbs and 

 trunk exhibit in the course of our daily life. 



Sensory Pathways from the Spinal Cord to the Brain. The posterior root fibers 

 conducting the impulses of conscious muscle sense, tendon sense and joint sense, 

 those impulses which have to do with the coordination and adjustment of muscular 

 movements, ascend in the fasciculus gracilis and fasciculus cuneatus to the nucleus 



gracilis and nucleus cuneatus in the medulla oblongata (Fig. 759). 

 In the nucleus gracilis and nucleus cuneatus synaptic relations are found with 

 neurons whose cell bodies are located in these nuclei and whose axons pass by way 

 of the internal arcuate fibers, cross in the raphe to the opposite side in the region 

 between the olives and turn abruptly upward to form the medial lemniscus or medial 

 fillet. The medial fillet passes upward in the ventral part of the formatio reticularis 

 through the medulla oblongata, pons and mid-brain to the principal sensory nucleus 

 of the ventro-lateral region of the thalamus. Here the terminals form synapses 

 with neurons of the third order whose axons pass through the internal capsule and 

 corona radiata to the somatic sensory area of the cortex in the post-central gyrus. 

 Fibers conducting the impulses of unconscious muscle sense pass to the cerebellum 

 partly by way of the fasciculus gracilis and fasciculus cuneatus to the nucleus 

 gracilis and nucleus cuneatus, thence neurons of the second order convey the 

 impulses either via the dorsal external arcuate fibers directly into the inferior 

 peduncle of the cerebellum or via the ventral external arcuate fibers which are 

 continued from the internal arcuate fibers through the ventral part of the raphe 

 and after crossing the midline emerge on the surface of the medulla in the ventral 

 sulcus between the pyramids or in the groove between the pyramid and the olive. 

 They pass over the lateral surface of the medulla and olive to reach the inferior 

 peduncle through which they pass to the cerebellum. 



Other fibers conducting impulses of unconscious muscle sense pass upward in the 

 orsal spinocerebellar fasciculus, which arises from cells in the nucleus dorsalis. 

 The posterior root fibers conducting these impulses pass into the fasciculus cuneatus 

 and the collaterals from them to the nucleus dorsalis are said to come almost 

 exclusively from the middle area of the fasciculus cuneatus. They form by their 

 multiple division baskets about the individual cells of the nucleus dorsalis, each 

 fiber coming in relation with the bodies and dendrites of several cells. The axons 

 >f the second order pass into the dorsal spinocerebellar fasciculus of the same side 



