THE SPINAL CORD AND ITS NERVES 137 



such injuries have taken place give the necessary control to con- 

 firm the proof that efferent fibers leave the spinal cord through 

 the ventral roots and afferent fibers enter through the dorsal 

 roots, for the loss of ventral roots results in a motor paralysis of 

 the muscles supplied by them, while the destruction of dorsal 

 roots results in the loss of superficial and deep sensibility in the 

 regions innervated, with no loss of motor function save for the 

 imperfect coordination resulting from the loss of the sensory 

 control through the proprioceptive system (ataxia). 



Turning now to the conduction paths between the spinal 

 cord and the brain, we notice first that the reactions involved 

 here may be performed either reflexly or consciously. In the 

 latter case a connection with the cerebral cortex is to be expected; 

 in the former case an infinite variety of reflex connections within 

 the brain stem is possible. 



The sensory or ascending fibers which pass between the 

 spinal cord and the brain may be classified as follows: 



I. Proprioceptive systems: 



1. To the cerebellum (unconscious). 



2. To the brain stem (unconscious). 



3. To the thalamus and cerebral cortex (sensations of posture and 



spatial adjustment). 



II. Exteroceptive systems: 



1. To the brain stem (unconscious). 



2. To the thalamus and cerebral cortex (sensations of touch, tempera- 



ture, and pain). 



I. Proprioceptive Systems. As soon as the afferent fibers of 

 the spinal nerves have entered the spinal cord they are im- 

 mediately segregated into proprioceptive and exteroceptive 

 groups, as suggested by the analysis above (see Figs. 63, 64, 81, 

 and 83). The proprioceptive fibers take quite different courses, 

 depending upon whether they are directed into the cerebellar 

 path or into the path to the brain stem and cerebral cortex. 

 Some terminals of this system end in the gray matter between the 

 dorsal and ventral columns (the nucleus dorsalis of Clarke, or 

 Clarke's column, and adjacent regions), whose neurons send their 

 axons into the dorsal and ventral spino-cerebellar tracts and 

 finally into the cerebellum. The cerebellum is the great center 

 of motor coordination, and these spino-cerebellar tracts are two 



