246 SUMMARY. 



arise from ganglia. A plexus may associate several branches of the same or of 

 different nerves, but it does not prevent each separate nerve from actuating any part 

 for a single purpose, whilst it combines the whole for a more general and complicated 

 operation. 



Plexuses are formed in the spinal nerves for associating in action the several sets 

 of muscles, as the axillary in the upper extremity, and the sciatic in the lower ; 

 sometimes, however, a single nerve exists instead of the axillary plexus. They 

 also associate cutaneous nerves in the pelican. There is a necessary order observed 

 with respect to the nerves entering the plexus, inasmuch as all of them do not 

 communicate equally with each other, but only so as to produce particular associations 

 in parts. 



Plexuses are not necessary when there are many legs, as in the centipede, but 

 only when there are many muscles for a single limb, which has extensive and compli- 

 cated actions. The axillary plexus is much less complicated in the lion, whose muscles 

 have simple uses, than in many other animals, which have heavier fore-quarters to 

 sustain. 



The first dawn of life is excited into action either through some provision in the 

 body of the parent, or some external agent, as heat applied to the egg. The excited 

 living power produces the first evolution of organic structure, and shortly afterwards 

 the heart becomes apparent, which soon and ever after takes a prominent part in 

 perfecting the functions of the whole body, but most particularly in those of the nervous 

 system ; its influence is not, however, equal in all animals, as so much also depends 

 upon the warmth and coldness of the atmosphere and other circumstances ; nevertheless 

 the heart generally, when it has been once brought into action, continues so until 

 death ; and although life may persist without its influence a longer time in some 

 instances, yet in others it is very speedily extinguished. It therefore appears that the 

 essential part of the nervous system may lie dormant in the egg, also in the more 

 mature bodies of some animals, but that generally, when it has been once excited, it 

 requires a perpetual stimulus for keeping it in connexion with the body. 



The functions of the nervous system are promoted by their contiguity to moving 



