300 ABRAM T. KERR 



whether there is a relation in size between these two nerves when 

 they both enter the plexus. The few cases that I have examined 

 seem to indicate an expansion and contraction of the plexus rather 

 than a shifting of it along the cord. I hope that soon we may have 

 sufficient records so as to be able to determine this more definitely. 

 We must bear in mind that in the spinal cord there are rows or 

 columns of cells extending its whole length from which the nerve 

 fibers take origin or around which they end. The cells are not, 

 so far as we know, arranged in groups corresponding to the 

 groups of fibers in the root fila or in the nerves. The nerve 

 fibers in the case of motor nerves extend from the cells in the 

 gray matter of the cord towards its periphery and then break 

 through the periphery and extend beyond the periphery in groups, 

 the root fila. In the case of the sensory nerves they extend in 

 the opposite direction, that is centrad. The fila radicularia are 

 arranged in continuous rows separated from one another, as a 

 rule, by slight intervals, which are generally somewhat greater 

 between the fila of one nerve and those of the one next cephalad 

 or caudad. As they pass laterad, groups of the fila converge 

 and are joined together into the dorsal and ventral nerve roots. 

 The segmental subdivision of the adult spinal cord in based en- 

 tirely upon the points of attachment to the cord of the groups of 

 root fila that converge to join a single pair of nerves. Whether 

 or not the same number of nerve fibers enter the same filum in 

 different individuals is not known but it is altogether probable 

 that there is a variation in this respect. We do know that there 

 is a variation in the number of fila which join to form a given 

 ventral root and dorsal root. It is easy to understand how either 

 the appearance of shifting of the plexus along the spinal cord or 

 of its expansion might be produced by variation in the grouping 

 of the fila as they converge to form the nerves, the number of 

 nerve fibers remaining the same. 



PREFIXED AND POSTFIXED PLEXUSES. RELATIVE SIZE OF THE 



NERVES 



As already noted some anatomists divide the brachial plexus 

 into prefixed and postfixed groups based upon the position in the 



