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A TEXTBOOK OF PHYSIOLOGY 



and cuneate nuclei of the medulla through the postero-median and 

 postero-lateral columns. 



7. With other segments of the cord lower down by the comma 

 tract. 



The Spinal Arc. This arc, in its simplest plan, may be regarded 

 as being made up of the ingoing afferent fibre, which ends around 

 the anterior horn cell, of the same side, and the efferent fibre from 

 this cell. It is probable that the ingoing neuron ends around the 

 posterior horn cells of the same side, and a connecting neuron joins 

 up these cells to the anterior horn cells. 



The Cerebellar Arc To the Cerebellum. Impulses from the 

 posterior root neurons reach the cerebellum by several ways : ( 1 ) 

 On the same side by processes which end in Clarke's column, and 

 then by the neurons of this column, the axons of which form the 

 dorsal spino-cerebellar tract, Avhich goes to the superior verm is by 

 way of the inferior peduncle. (2) On the opposite side by means 

 of neurons of posterior horn cells, axons of which form the ventral 

 spino-cerebellar tract. This passes to the superior vermis by the 

 superior peduncle. (3) By the postero-median and postero-lateral 

 columns to the gracile and cuneate nuclei in the spinal bulb, and 

 thence by neurons the axons of Avhich pass to the cerebellum by the 

 inferior peduncle (the arcuate fibres, see pp. 689, 60). 



From the Cerebellum axons pass to Deiters' nucleus, which lies in 

 the upper part of the bulb, and is connected with the vestibular 

 branch of the auditory nerve (see p. 698); thence by the vestibulo- 

 spiual tract to the spinal cord. Connection with the spinal cord is 

 also made b} y way of the posterior longitudinal bundle. 



The Cerebral Arc To the Cerebrum. (1) The axons of the 

 posterior root neurons pass up to the gracile and cuneate nuclei of the 

 bulb by the postero-median and postero-lateral columns; (2) the 

 axons of the neurons (the intermediate neuron) of these nuclei pass 

 to the thalamus by a tract known as the mesial fillet; (3) the axons 

 of the third relay of neurons (the upper neuron) pass from thethalanms 

 to the cortex. 



From the Cerebrum by the cortico-spinal (p3'ramidal) fibres, crossed 

 and direct, to the anterior horn cells. 



Since the cerebellum is also connected with the cerebrum, it is 

 possible for impulses from the spinal cord to reach the cortex via 

 the cerebellum, and also to pass to the spinal cord from the cortex 

 through this organ. 



The Functions of the Spinal Cord. The chief functions of the spinal 

 cord are to act (1) as a reflex centre: (2) as a conductor of impulses 

 to and from the brain. 



The Spinal Cord as a Conductor. This function has been dealt 

 with in the description already given of the position of the various 

 tracts. It remains only to indicate that the impulses coming into 

 the spinal cord from the extero-ceptive and proprio-ceptive mechan- 

 isms undergo a redistribution according to their function and destina- 



