438 HISTORY OF THE HUMAN BODY 



The second principle, that of the permanence of nerve dis- 

 tribution, has already been referred to in several places, since 

 many of our safest and surest conclusions concerning homolo- 

 gies are based upon it. This principle, more fully expressed, 

 affirms that a part never changes its nerve supply, and that a 

 given nerve, once associated with a certain organ or complex 

 \of organs, will follow it through all its subsequent transforma- 

 tions and even migrations. A good illustration of this is seen 

 in the history of the stapedius muscle of the middle ear, which 

 is supplied by a branch of the facial nerve. This supply is by 

 no means the most convenient, and is reached only through 

 overcoming a series of mechanical disadvantages, yet it is 

 rendered necessary by the fact that the muscle in question was 

 once a part of the digastricus (the posterior belly of the mam- 

 malian muscle of the same name), and as such was supplied 

 by the Facialis. Through the application of this inviolable 

 principle numerous homologies have been established, and 

 others, long believed in, have been disproven. 



Of undoubted connection with this close correspondence be- 

 tween peripheral nerves and the organs to which they are dis- 

 tributed, as enunciated in the above principles, is the singular 

 phenomenon of plexus formation, seen in the nerves which 

 supply the limbs. These plexuses consist of a more or less 

 intricate set of intercommunications between the spinal nerves 

 that are distributed to the limbs, and are hence two in number, 

 plexus brachialis and plexus lumbo-sacralis, involving the 

 nerves which supply the anterior and posterior limbs respec- 

 tively. The number of nerves involved in each plexus differs 

 considerably, and reaches a large number in certain fishes, in 

 which the fins are associated with a large number of myotomes, 

 but in animals with the hand form of limb (chiridia) the num- 

 ber varies between two and seven. Of this series one or two, 

 usually the central ones, perform the greater part of the task 

 of supplying the limb, and are consequently the largest; the 

 others grade off above and below to those of normal size. The 

 number and complexity of the intercommunications also reach 



