THE REFLEX ARC AND REFLEX ACTION 519 



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 

 passing 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 horn 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 relation 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 cornua are 

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

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

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

 development and by its comparative anatomy. The pairs of nerves are 

 indicative of the component segments of the cord. The tracts of the cord are 

 in a sense connectives from segment to segment, connecting the cells of 

 both adjacent and of widely separated segments. The function of the cord 

 is comprised in the function of the segments and in the function of the tracts. 



From a physiological point of view, it may almost be considered as an 

 axiom that before a nerve cell can send out a nerve impulse it must first 

 receive a stimulus of some kind. This stimulus usually consists of an afferent 

 impulse from the periphery. Its effect upon the receiving cell may be insuf- 

 ficient to cause any response, or the response may be delayed for a long period 



