STRUCTURE OF THE VERTEBRATES 281 



tj'pes of impulse are grouped into tracts. The sensory fibers 

 tend to be concentrated in the dorsal part of the cord, and the 

 motor fibers in the ventral half. The division of the tracts is 

 carried even further, particularly in regard to the sensory 

 fibers. Fibers carrying specialized impulses (as tactile, pain, or 

 kinesthetic sensations) are congregated into smaller tracts on 



A. Cervical 



B. Thoracic 



C. Lunibo-sacral 



D. Sacral 



EiG. 160. Sections of the Cord at Four Levels, showing the increase of 

 fibers in the cervical region. The sacral enlargement controls the pelvis 



and pelvic limbs. 



either side of the cord. The triangular region bounded dorsally 

 by the periphery of the cord, and laterally by the entrance of 

 the dorsal roots of the spinal nerves, is filled with the sensory 

 fibers from the muscles carrying kinesthetic sensations. These 

 fibers convey sensations of muscle position to the brain, and 

 their loss causes a lack of coordination in the affected part of 

 the body. The tactile and pain sensory fibers are located 



