350 THE SPINAL CORD. 



The gray commissure of the spinal cord (the gray anterior, 

 and the posterior commissure) is the vertical, transverse sheet of 

 gray substance connecting the two crescents together (Fig. 101). 

 This commissure (commissura grisea) completes the gray matter 

 of the cord. It unites the gray crescents together a little in front 

 of their center, except in the lumbar region where it joins their 

 centers. It forms the floor of the posterior median fissure; and, 

 in front, is in relation with the white anterior commissure. It 

 is pierced longitudinally by the central canal of the spinal cord, 

 which is surrounded by a thick envelope of substantia gelatinosa. 

 This canal, the sixth ventricle, divides the commissure into two 

 parts. That part of the commissure in front of the canal is the 

 gray anter'ior commissure (commissura anterior grisea) and that 

 behind it is the posterior commissure (commissura posterior, Fig. 

 101). The gray commissure, comprising both these divisions, 

 is composed of spongy and gelatinous substance in which there 

 are imbedded the bodies of many nerve cells and a large number 

 of medullated fibers. The medullated fibers are derived from 

 the posterior roots of the spinal nerves, and from intrinsic neu- 

 rones of the cord, whose centers are situated chiefly in the commis- 

 sure and in the posterior columna. The posterior commissure 

 is said to contain a long sensory tract, between the ventricle and 

 dorsal surface (Ciaglinski). This long sensory tract is found in 

 the thoracic portion of the cord and the discoverer believes it to 

 be made up of ascending root-fibers which conduct pain and 

 temperature impulses. It is in need of further investigation 

 (Barker). 



Lesions of the gray substance, as in syringomyelia, completely 

 abolish the pain and temperature senses at the level of the lesion, 

 while the muscular and tactile senses are preserved. The dis- 

 sociation of sensations is most complete and bilateral when the 

 lesion destroys the posterior commissure; if the lesion be limited 

 to one crescent, the pain and temperature senses are affected on 

 the side opposite to the lesion. These facts show that the pain 

 and temperature paths to the cerebral cortex run through the gray 

 substance of the cord and that they decussate, through it imme- 

 diately upon entering the cord. It also shows that muscular and 



