442 HISTORY OF THE HUMAN BODY 



Gymnophiona and in the lost hind limbs of the urodele Siren, 

 not only have the limbs and their girdles utterly vanished, but 

 there is also no trace of a plexus, showing that since the reduc- 

 tion of the limbs a much longer time has elapsed than in the 

 former case, a conclusion in full accord with the relative place 

 of these animals in the system and in their geological appear- 

 ance. 



It is not probable, however, that all the changes in a plexus 

 have a historic significance, since another factor must be taken 

 into consideration, one that is the cause of certain changes, 

 especially those of an individual character. This factor is 

 found in the evident tendency, of certain forms at least, to 

 shift the position of their girdles. This tendency is shown in 

 individual cases by an increase in the size of certain of the 

 nerves involved and a corresponding diminution in that of those 

 either anterior or posterior to them ; and, in certain species, by 

 making careful counts of the separate fibers of the main nerves 

 in a large number of individuals of a given species, the direc- 

 tion in which the girdle is migrating has been definitely estab- 

 lished. Thus in the common toad there is shown a tendency 

 to push the shoulder girdle still further anteriorly, and as its 

 present position is extremely cephalic, the continued tendency 

 must be an instance of the inertia of variation through which 

 a line of development, once started, is often carried far beyond 

 the point of greatest efficiency. This procedure involves more 

 generally the posterior than the anterior girdle, and hence the 

 lumbo-sacral plexus is more apt to vary individually. This 

 migratory tendency may result in the establishment in a given 

 species of two or three types of plexus, to which all individual 

 variations may be referred, as has been established in the case 

 of the urodele Necturus, well known also for its variability in 

 pelvic attachment. 



The early anatomists, by a careful count of the nerve roots 

 as they were found proceeding from the brain in the human 

 subject, enumerated the following twelve pairs of cranial 

 nerves, that is, of nerves which originate within the cranial 

 cavity and escape through foramina in the bone: 



