100 INFLUENCE OF NERVES 



the simultaneousness of their excitement to the con- 

 nexion established between each of them by these 

 filaments. Wherever many muscles combine to 

 execute an important movement, they are uniformly 

 found to be provided with, and connected by, 

 branches from the same system of nerves ; as, with- 

 out this means, simultaneousness of action could not 

 be ensured. Thus the muscles which cover the 

 upper part of the chest co-operate in the voluntary 

 movements of the arm, and at the same time in the 

 respiratory movements of the chest ; but these, 

 being two distinct purposes, require different com- 

 binations of the muscles among themselves. To 

 effect these combinations, two sets of nerves are 

 provided, as has been shown by Sir Charles Bell ; 

 the one regulating the respiratory,, and the other the 

 purely voluntary movements of the muscles. This 

 is the true reason why the same muscle sometimes 

 receives nerves from two or three different quarters ; 

 a circumstance which, before the principle was dis- 

 covered, and when all nerves were considered alike, 

 was altogether inexplicable, and seemed a work of 

 mere supererogation. 



The influence of the nervous agency may be still 

 further illustrated. When the trunk of a muscular 

 nerve is irritated by the contact of an external body, 

 or by the electric spark, the muscles which it supplies 

 instantly contract, but without either harmony or 

 permanency of action: the contraction is like the 

 violent and ill-regulated start of convulsion. It is 

 the influence of the brain and mind in the equal dif- 

 fusion of the required stimulus to each muscle, in 

 the exact proportion needful, that characterizes 

 healthy and sustained voluntary motion, as opposed 

 to the irregular convulsive start. Nothing can be 

 more wonderful than the accuracy with which, in 

 the most delicate movements, this stimulus is ad- 

 justed and apportioned to such a variety of parts, 

 particularly where practice, or, in other words, edu- 



