CHAPTER IV. 



Linear Series — continued. 

 Spinal Nerves. 



The spinal nerves compose a Meristic Series in many respects 

 similar to that of the vertebrae. As between the vertebra*, so 

 between the spinal nerves, there is differentiation according to the 

 ordinal succession of the members, certain distributions and func- 

 tions being proper to nerves in certain ordinal positions. The study 

 of the way in which Variation occurs in this series is one of great 

 interest, but unfortunately it is extremely complicated. For while 

 as regards vertebrae the distribution of structural differentiation 

 can be recognized on inspection, in the spinal nerves to obtain a 

 true knowledge of the arrangement in any one case physiological 

 investigation or at least elaborate and special methods of dissection 

 are needed. Though it is therefore impossible to introduce any 

 account which should at all adequately represent the great diver- 

 sity of possible arrangements, it is nevertheless necessary to refer 

 briefly to the chief results attained by these methods and to the 

 principles which have been detected in the Variation of the nerves. 

 It must of course be foreign to our purposes to examine the many 

 diversities of pattern produced by the divisions and anastomoses of 

 nerve-cords in the formation of plexuses, &c, and we must confine 

 our consideration to cases of Variation in the distribution of dif- 

 ferentiation among the spinal nerves, that is to say, in the segmen- 

 tation of the nervous system in so far as it may be judged from 

 the arrangement of spinal nerves. 



Some conception of the magnitude and range of Variation found 

 in single species of Birds may be gained by reference to the beau- 

 tiful researches of Furbringer 1 . A table is given by Fiirbringer, 

 shewing the number and serial position of the spinal nerves which 

 take part in the formation of the brachial plexus in 07 species of 



1 Fiirbringer's memoirs are of such magnitude and completeness that I have felt 

 it to be somewhat of an impertinence to attempt to make selection from them : and 

 it must be remembered that from the isolated and typical cases here given, only 

 a distorted view of the evidence can be gained. As regards this subject, therefore, 

 reference to the original work is especially needed. 



B. 9 



