THE NERVOUS SYSTEM 439 



their extremes in the center of the plexus in connection with 

 these larger nerves, above and below which the nerves become 

 gradually less involved, until those are reached which have so 

 slight a connection with the plexus that they are included 

 within it by some authors and not by others. There is, in fact, 

 considerable individual variation in a given plexus, and a 

 debatable nerve may furnish a communicating branch in one 

 specimen which may be absent in another. 



The organization of a plexus may be best learned by actual 

 examples, for which the brachial plexuses of two amphibians, 

 two birds, and two mammals may be selected (Fig. 123). 

 From these it will be seen that not only is the number of nerves 

 involved a different one, but that the nerves themselves are 

 not the same, counting from the first. This latter fact is but 

 another way of saying that the girdles shift along the columns 

 in different animals, locating in all cases at the point where the 

 support will be the most effective, a fact brought out in 

 previous chapters in relation to the bones and muscles. It 

 shows clearly that homologies cannot rest upon definite body 

 metameres, since there is great variation, both in the total num- 

 ber of metameres and in the relative length of each subdivision 

 of the body; neck, trunk and tail. Thus in a frog, with a total 

 of but ten pairs of spinal nerves, the brachial plexus involves 

 the first three and the lumbo-sacral plexus the last four, leaving 

 but three pairs of spinal nerves not involved in plexus forma- 

 tion ; yet the metameres thus represented cannot be taken, meta- 

 mere for metamere, as the homologues of the first ten of other 

 animals, which, in some cases, as in most birds, for example, 

 would be included entirely in the neck ; it may rather be said 

 that the ten of the frog are homologous in a general way with 

 the total number of other animals, the two plexuses serving as 

 fixed points for comparison. 



In comparing the various forms of plexus with one another, 

 there are, in spite of the great diversity of combinations, cer- 

 tain points of similarity. In the first place, both plexuses are 

 alzvays formed entirely of the ventral divisions of the spinal 



