THE FUNCTIONS OF THE CORD 551 



the median set some fibers pass to Clarke's column, others pass by way 

 of the posterior commissure to the median cells of the other side. Others 

 pass through the gray matter to the anterior column cells of the same side, 

 figure 365. Besides this, they are connected through collaterals with the 

 intrinsic cells of the gray matter at different levels of the cord. One can 

 realize that each nerve root has, in this way, an effective grip upon a large 

 extent of the cord. This is seen well by studying figures 363 and 365. 



The Peculiarities of Different Regions of the Spinal Cord. The 

 outline of the gray matter and the relative proportion of the white matter 

 vary in different regions of the spinal cord, and it is, therefore, possible to 

 tell approximately from what region any given transverse section of the 

 spinal cord has been taken. The white matter increases in amount from 

 below upward. The amount of gray matter varies; it is greatest in the 

 cervical and lumbar enlargements, viz., at and about the 5th lumbar and the 

 6th cervical nerves, and least in the thoracic region. The greatest develop- 

 ment of gray matter corresponds with greatest number of nerve fibers pass- 

 ing from the cord. 



In the cervical enlargement the gray matter occupies a large proportion of 

 the section, the gray commissure is short and thick, the anterior column is 

 blunt, while the posterior is somewhat tapering. The anterior and posterior 

 roots run some distance through the white matter before they reach the 

 periphery. 



In the dorsal region the gray matter bears only a small proportion to the 

 white, and the posterior roots in particular run a long course through the 

 white matter before they leave the cord; the gray commissure is thinner and 

 narrower than in the cervical region. 



In the lumbar enlargement the gray matter again bears a very large propor- 

 tion to the whole size of the transverse section, but its posterior columns are 

 shorter and blunter than they are in the cervical region. The gray commis- 

 sure is short and extremely narrow. 



At the upper part of the conus medullaris, which is the portion of the cord 

 immediately below the lumbar enlargement, the gray substance occupies 

 nearly the whole of the transverse section, as it is invested only by a thin layer 

 of white substance. This thin layer is wanting in the neighborhood of the 

 posterior nerve roots. The gray commissure is relatively thick. 



At the level of the fifth sacral vertebra the gray matter is again in excess, and 

 the central canal is enlarged, appearing T-shaped in section; while in the 

 upper portion of the filum terminate the gray is uniform in shape without any 

 central canal. 



THE FUNCTIONS OF THE CORD. 



The Reflex Arc and Reflex Action. The spinal cord is morpho- 

 logically a segmental or metameric structure. This is shown both by its 



